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Sailing in tide..quiz.

Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Technique
Forum Discription: 'How to' section for dinghy questions and answers
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=12871
Printed Date: 14 Dec 17 at 4:19am
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Topic: Sailing in tide..quiz.
Posted By: iGRF
Subject: Sailing in tide..quiz.
Date Posted: 02 Oct 17 at 5:16pm
OK here's my first case study, taken from the Gate start thread, this was a race in the late 90's, long distance across the milford haven entrance channel off Dale, where all week the racing had taken place just off Dale in the confines of the bay and the cliffs of Queen Annes head had been offering favorable shifts to the right all week.

So my question what's your strategy for the race it's a straight windward course round the island then a wing mark (boat) and finish at the start line beach. The wind is pretty clean maybe 5 degree oscillating certainly no more than 10, the tide is flooding indeed they had tried but failed to get the start off before it fully flooded.

The line is pretty square but slightly favoured favoured the committee boat (starboard) end as the wind swung slightly that way during the 10 -5 countdown.



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Replies:
Posted By: Pierre
Date Posted: 02 Oct 17 at 8:20pm
Ha haaa... They've all had a squeaky bum moment Graeme. I thought the knowledgeable forumites would have sorted this out in a trice. It would seem not. 


Posted By: Rupert
Date Posted: 02 Oct 17 at 8:27pm
I learned to sail on a river. Suspect others cannot be bothered feeding the troll.

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Firefly 2324, Lightning 130, Puffin 229, Leader, Topper 44496, yellow Minisail


Posted By: iGRF
Date Posted: 02 Oct 17 at 8:28pm
Three hours and not a peep, lol, it's one thing regurgitating something you've read in a text book, quite something else when someone calls you on it..

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Posted By: KazRob
Date Posted: 02 Oct 17 at 8:46pm
I grew up in a place with 0.1knt of tide and wind that shifted once a week so cr*p in tricky tide but will give it a bash. I assume there are no shoals or anything that would affect the current in the channel
Head right off the line to pick up slacker tide or even back eddy off the headland and short tack up there until the channel. Then keep heading to the right and into the tide and try to cross the tide to the other side and come in to the with tide under me rather than coming from left and getting past the island in the lee of the shore. Reaches should be simpler with just crabbing enough to keep the track.
If nothing else it will give iGRF something to chew on


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OK 2122 & 2148


Posted By: mozzy
Date Posted: 02 Oct 17 at 8:58pm
Sorry, don't really read technique.

One thing I don't get, in the same sentence you say the tide is flooding in and they tried and failed to get the race off before it fully flooded. By 'fully flooded' do you mean 'high tide', or is 'fully flooded' max current? I guess the latter?

Also, no depth chart, so I'll assume there's no funky bathymetry or banks?

I'd start middle line. Against the tide (does it run that far down?) there will likely be sag, and if you are expecting the wind to go back left you don't want to be on the right of fleet. The shift is right phase so you'd want to sail starboard first if you are expecting it to go back left. 

I'd also go left up the beat to get in to the shore away from tide which is pretty much with the wind. There may also be a left shift with convergent wind on the left shore to add to the oscillation left you are expecting.

Coming in to the windward mark you'd experience a tidal lift in apparent wind too. You'd have to be careful not to over stand the windward mark though.

You'd want to get a good transit on the reach and crab down as the traditional fleet windward arc will be even more pronounced with the tide pushing people high. 

Not much to do on the bottom reach unless there is a tide line where the tide splits which you'd want to get the correct side of. 

Ultimately, only you were there, so you could say anything worked, and we'd have no recourse. 


Posted By: iGRF
Date Posted: 02 Oct 17 at 9:00pm
Pretty much exactly what the coach suggested, he drew it on a board, had he not done so I might not have even bothered, but it was too good an opportunity for an I told you so moment.

The temptation for the cliffs, and yes maybe a back eddy swirl around St Annes head had almost the entire fleet doing as you describe, but the other route, bang over to the left where there was also a bit of a lift off the land, sufficient to get the nose into the tide and give a fair run out into the bay where the full blast of it not only lifted the nose but put the board on the plane, turned the the 8 - 10 knots into 10 to 12 knots which is marginal planing for a race board which reached the island whilst the fleet were still tacking up the coast in slacker air (which also lifts up and over those cliffs.

But fundamentally it was tide initially on the nose then full on the lee bow which did it.

So much so, that by the island the board had to be sailed free onto a reach.

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Posted By: sargesail
Date Posted: 02 Oct 17 at 9:04pm
No Graeme just busy working....but looking forward to hearing exactly how you think you won it.

So my strategy would have 3 parts.

1.  Take the 'free' southerly uptide gain behind the promontory immediately to the south of the start line.  I would want to be careful of any potential race on the headland.  I'd reckon that right would probably pay, depending on the exact shape of the land and the feel of the breeze.  Hopefully I'd have some shifts to help me get there efficiently.

2.  I think I'd then sail a short starboard leg until I reached the port layline.  This would come quite early because of the east going tide in the east side of the bay.  I'd be worried about what sort of tidal compression I'd experience around the island - if it's strong then I would have made the wrong call.

3.  For the rest of the race I'd make ground up tide when in less tidal flow, for example in the tidal lee of the island.  That is to say I'd emerge as far west as I could without overly compromising my speed.

If I thought there was going to be a significant increase in north going tide (a race) to the immediate west of the island then I would execute a different strategy.  I would sail a long port tack first and flick on to starboard when I got to the layline for the mark, or, depending exactly on my tacking angle, before that, in order to take advantage of the maximum flow in the main channel before being tidally sheltered behind the southernmost headlands.  But I'd still look to flick back on starboard and approach from uptide - remembering that this is predicated on a strong race off the island.






Posted By: KazRob
Date Posted: 02 Oct 17 at 9:06pm
Sorry - I don't understand what you mean. If you're sailing out into the tide you are also going backwards with the tide relative to the wind so the 8-10knts you talk about would be 3-4knts slower (or whatever the tide was). If you're moving backwards relative to the wind I also can't see how the board got more air under the nose to help it lift either. I spent many years on boards and have not long bought a big ol' raceboard again, so sailing a board v a dinghy is not completely alien to me.
Explain again please? Perhaps it was just plain old windier over there which isn't the same as some magic tide trick


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OK 2122 & 2148


Posted By: mozzy
Date Posted: 02 Oct 17 at 9:22pm
Originally posted by sargesail

1.  Take the 'free' southerly uptide gain behind the promontory immediately to the south of the start line.  I would want to be careful of any potential race on the headland.  I'd reckon that right would probably pay, depending on the exact shape of the land and the feel of the breeze.  Hopefully I'd have some shifts to help me get there efficiently.
I don't see that there is a uptide gain south of the line, the way iGRF has shown it, the tide just runs straight into that arm of the harbour. Even still, you'd have to sail a header off the line on port to get there. When you do get there it will be a headed shore (divergent breeze) anyway. 

If there was a strong eddy you may gain, but all that effort to get over that way would be swept away when you hit the tide coming in northerly and north easterly. That tide would also give you another apparent header. 

Originally posted by sargesail

2.  I think I'd then sail a short starboard leg until I reached the port layline.
Coming up to the port layline late would feel pretty awful. Yes, the tide would be on your transom, but it would also be giving you a header. 

Originally posted by sargesail

 This would come quite early because of the east going tide in the east side of the bay.  
Damn straight there. 


Posted By: iGRF
Date Posted: 02 Oct 17 at 9:28pm


So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

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Posted By: mozzy
Date Posted: 02 Oct 17 at 9:37pm
So do I get any brownie points for getting it right?

You'd also imagine the boats coming out of the headland on port would get a tidal lift, of course as they went further right they'd know they'd have to come back in the corresponding header (I just think he tacking angle out of the headland on port would be better). 

But the above scenario is just the tide changing the apparent wind. I want you to explain how changing mode in tide can lead to a better VMG.


Posted By: KazRob
Date Posted: 02 Oct 17 at 9:43pm
Are you sure you didn't just get lucky taking a flier?
Still don't understand your tide assisted speed or your lee bow stuff. When you head back out from the shore the tide is heading NE-ish so you should get pushed that way and in a reduced wind strength. I can see if you made it to where the tide runs west you will get more breeze but your still being pushed away from where you want to go.


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OK 2122 & 2148


Posted By: sargesail
Date Posted: 02 Oct 17 at 9:51pm
Originally posted by mozzy

Originally posted by sargesail

1.  Take the 'free' southerly uptide gain behind the promontory immediately to the south of the start line.  I would want to be careful of any potential race on the headland.  I'd reckon that right would probably pay, depending on the exact shape of the land and the feel of the breeze.  Hopefully I'd have some shifts to help me get there efficiently.
I don't see that there is a uptide gain south of the line, the way iGRF has shown it, the tide just runs straight into that arm of the harbour. Even still, you'd have to sail a header off the line on port to get there. When you do get there it will be a headed shore (divergent breeze) anyway. 

If there was a strong eddy you may gain, but all that effort to get over that way would be swept away when you hit the tide coming in northerly and north easterly. That tide would also give you another apparent header.

I'm talking about making southing before you meet the north going tide.  You always have to make southing somewhere because the tide always sets you north somewhere on the track.  I agree with GRF that making southing before coming out from behind the headland (in the context of the big main flow running north) makes sense.  Probably less west going tide inshore.

Originally posted by sargesail

2.  I think I'd then sail a short starboard leg until I reached the port layline.
Coming up to the port layline late would feel pretty awful. Yes, the tide would be on your transom, but it would also be giving you a header.

But the effect of the push to weather is always more than the header....

Originally posted by sargesail

 This would come quite early because of the east going tide in the east side of the bay.
Damn straight there. 
 

Exactly - so I'm not worried about the header!


Posted By: sargesail
Date Posted: 02 Oct 17 at 9:54pm
Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

So tell me why it wouldn't have been faster for blue to do a hybrid of the two: short tack like red and then do a starboard leg from the end of the headland until meeting the port layline drawn?


Posted By: mozzy
Date Posted: 02 Oct 17 at 9:54pm
When he tacks out from the left shore the tide is first mostly on the head, but becomes increasing cross and even slightly against the wind. In teh boat it would feel like the breeze was increasing and lifting as you come back in on port. 

Looking at the vectors (3.5 knot tide, 9 knot true wind) this would feel like a 10-20 degree lift (assuming tide was slack or against on the north shore (far left of the beat). 

The component of the tide which is running in to the wind obviously increases the apparent wind strength for both port and starboard boats; however, the cross tide component would lift the port boats (and head equally the starboard). 

Basically, as the boats on the right come out from the head land, back toward the middle, they would feel the breeze increase as the tide gets on their stern, but also get a header. 




Posted By: iGRF
Date Posted: 02 Oct 17 at 9:57pm
No it wasn't a flier, it gets back to my explanation on the Gate start thread, we are using the energy from three sources instead of two.

1) Energy from the true wind drives the boat/board, this generates, then combines with
2)The created wind of the board boat moving OVER THE GROUND
3)The third force is the water which as a fluid also moving over the ground, like the wind can either combine with the wind to act in concert (the force is greater, the more angled the lower foil is (think sheeting it in)) or it can act against and reduce the energy from the true wind.

That's it really.

By harnessing an extra 4 knots on the centreboard, it combines with the 8 knots in the sail to drive the craft faster, the fact the water is also moving like a conveyor belt is irrelevant, what is important is the board/boat speed over the ground and relative to everyone else.

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Posted By: sargesail
Date Posted: 02 Oct 17 at 9:58pm
Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

OK - so talk me through the curved laylines.  Why does blue gradually get lifted on port?

Why does red gradually get headed?

And where can I get one of these windsurfers which point so high that the tacking angle looks like 40 degrees?


Posted By: mozzy
Date Posted: 02 Oct 17 at 10:00pm
Originally posted by KazRob

When you head back out from the shore the tide is heading NE-ish so you should get pushed that way and in a reduced wind strength.
Getting pushing north east is pushing you in to and accross the wind. You will feel an apparent increase in wind and apparent backing (lift on port).

Originally posted by KazRob

 I can see if you made it to where the tide runs west you will get more breeze but your still being pushed away from where you want to go.
Do you mean heading west running tide. The only place the tide is running west is at the start?


Posted By: sargesail
Date Posted: 02 Oct 17 at 10:04pm
Originally posted by mozzy

When he tacks out from the left shore the tide is first mostly on the head, but becomes increasing cross and even slightly against the wind. In teh boat it would feel like the breeze was increasing and lifting as you come back in on port. 

Looking at the vectors (3.5 knot tide, 9 knot true wind) this would feel like a 10-20 degree lift (assuming tide was slack or against on the north shore (far left of the beat). 

The component of the tide which is running in to the wind obviously increases the apparent wind strength for both port and starboard boats; however, the cross tide component would lift the port boats (and head equally the starboard). 

Basically, as the boats on the right come out from the head land, back toward the middle, they would feel the breeze increase as the tide gets on their stern, but also get a header. 



Yes - I know that.  But what I'm getting at is that blue still has to make to the south into a north or north-west going tide.  Even if the diagram is not as drawn and the east going flood starts further west (unlikely) then it still starts as a north-easterly flow and turns east as you get east.  So going south before you are in north going flow is always a gain (other things being equal - which they're not!).


Posted By: mozzy
Date Posted: 02 Oct 17 at 10:05pm
Originally posted by iGRF


1) Energy from the true wind drives the boat/board, this generates, then combines with
2)The created wind of the board boat moving OVER THE GROUND
3)The third force is the water which as a fluid also moving over the ground, like the wind can either combine with the wind to act in concert (the force is greater, the more angled the lower foil is (think sheeting it in)) or it can act against and reduce the energy from the true wind.

In the scenario you describe, I think you made the right choice. But those three point above make no sense. 

You sail on the difference between air and water. Two things. Over the ground is only relevant in that as the tide is moving it makes the apparent wind different on water compared to land. Its not a separate for to true wind. 

Your third point is crap. Leeway over foils is about 3-4 degrees. This doesn't change if you are in tide or not. 


Posted By: KazRob
Date Posted: 02 Oct 17 at 10:08pm
Oops - got my tide directions mixed up. For NE read NW and for West read East. That's basically why I'm cr*p in tides I guess

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OK 2122 & 2148


Posted By: iGRF
Date Posted: 02 Oct 17 at 10:10pm
Originally posted by sargesail


Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

So tell me why it wouldn't have been faster for blue to do a hybrid of the two: short tack like red and then do a starboard leg from the end of the headland until meeting the port layline drawn?


Pretty much what we'd been doing earlier in the week when the windward mark was sited just at the tip of that left arrow. The main reason to go left (as also pointed out by Mozzie, credit where it's due) was the likelihood there would be a better lift off the opposite cliffs, which would get the nose into the tide earlier than rounding the headland where the lift off the land petered out, quite why they didn't harden up to head out to sea I'll never know, they were not stupid, some were Hayling sailors so not tidal neophytes, but then again who knows what squads have to do when their coach is dictating the route.

I didn't win the race, I spent too long berating the coaching team who were moored at the island, about listening to lectures on tide and not arguing the point when I bother to drive down to weymouth at my own expense to try and help them, but then it's not in a paid professional coach's interest to have anyone around with superior tactical advice is it? Then I always was naive..

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Posted By: mozzy
Date Posted: 02 Oct 17 at 10:15pm
Originally posted by sargesail

Originally posted by mozzy

When he tacks out from the left shore the tide is first mostly on the head, but becomes increasing cross and even slightly against the wind. In teh boat it would feel like the breeze was increasing and lifting as you come back in on port. 

Looking at the vectors (3.5 knot tide, 9 knot true wind) this would feel like a 10-20 degree lift (assuming tide was slack or against on the north shore (far left of the beat). 

The component of the tide which is running in to the wind obviously increases the apparent wind strength for both port and starboard boats; however, the cross tide component would lift the port boats (and head equally the starboard). 

Basically, as the boats on the right come out from the head land, back toward the middle, they would feel the breeze increase as the tide gets on their stern, but also get a header. 

Yes - I know that.  But what I'm getting at is that blue still has to make to the south into a north or north-west going tide.  Even if the diagram is not as drawn and the east going flood starts further west (unlikely) then it still starts as a north-easterly flow and turns east as you get east.  So going south before you are in north going flow is always a gain (other things being equal - which they're not!).

I think iGRF has drawn the lines a bit wrong. The way he's shown it both tacks are lifted in to the windward mark, which is impossible. 

As I said in another post  the right would look okay, until they came out from the shore. They would get a lift on port from the incoming tide, but when you're the furthest boat right on the course, the last thing you want is a lift on port! They, would have to get out of that right hand side at the top of the beat by sailing on starboard. Starboard tack will be feel an apparent header. 

Also, I wouldn't see going right as that much of a gain at the start either (unless there is a big back eddy which isn't shown). The tidal stream will be diverging in the middle as it splits (divergent) to two channels so the north bank probably won't see strong adverse tide, especially mid way up the beat. 

Then on top of that, iGRF does say the wind was in the right phase at the start, so you'd have to sail a ten degree header to get to that right shore. 


Posted By: iGRF
Date Posted: 02 Oct 17 at 10:21pm
Originally posted by mozzy


]You sail on the difference between air and water. Two things. Over the ground is only relevant in that as the tide is moving it makes the apparent wind different on water compared to land. Its not a separate for to true wind. 
Your third point is crap. Leeway over foils is about 3-4 degrees. This doesn't change if you are in tide or not. 



You only move because the water provides resistance to the wind.

That then enables created wind which also combines against the static resistance of the water to enable forward motion.

If the water moves away from the wind at the same pace, no forward movement of the boat will take place.

If the water were moving toward the wind at the same speed then the boat will move forward as if the wind speed were double.

I'm sorry but whoever told you all that doesn't entirely understand what is going on, where the Energy is coming from.

Three elements, not two.

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Posted By: mozzy
Date Posted: 02 Oct 17 at 10:29pm
Okay, I get what you're saying. But I don't think you're phrasing it very well. The true wind over ground, plus the tidal vector = the wind you feel if you were floating stationary. Depending on direction the tide will shift, add to, or decrease the true wind (from what is experienced on land (or moored up). 

But, in the boat, you can't feel that. the boat can't 'feel' that. All the energy comes from the sheer between air and water. Those two things, nothing else. Whether it's just the air that's moving (lake) or air and water (sea) it is still just the sheer between the two.  

Also, you'd still not explained the 3rd point of tide hitting the foils etc etc. 


Posted By: sargesail
Date Posted: 02 Oct 17 at 10:30pm
Originally posted by iGRF

Originally posted by sargesail


Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

So tell me why it wouldn't have been faster for blue to do a hybrid of the two: short tack like red and then do a starboard leg from the end of the headland until meeting the port layline drawn?


Pretty much what we'd been doing earlier in the week when the windward mark was sited just at the tip of that left arrow. The main reason to go left (as also pointed out by Mozzie, credit where it's due) was the likelihood there would be a better lift off the opposite cliffs, which would get the nose into the tide earlier than rounding the headland where the lift off the land petered out, quite why they didn't harden up to head out to sea I'll never know, they were not stupid, some were Hayling sailors so not tidal neophytes, but then again who knows what squads have to do when their coach is dictating the route.

I didn't win the race, I spent too long berating the coaching team who were moored at the island, about listening to lectures on tide and not arguing the point when I bother to drive down to weymouth at my own expense to try and help them, but then it's not in a paid professional coach's interest to have anyone around with superior tactical advice is it? Then I always was naive..

I feel your pain in terms of the interaction with the coaches.....

But I also put it to you that the lead you established that day was due to the lift off the northern cliffs, and not some inexplicable tidal magic?


Posted By: iGRF
Date Posted: 02 Oct 17 at 10:35pm
Originally posted by sargesail


Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

OK - so talk me through the curved laylines.  Why does blue gradually get lifted on port?
Why does red gradually get headed?
And where can I get one of these windsurfers which point so high that the tacking angle looks like 40 degrees?


The curve indicates the strength of the tide 'lifting' the board higher, tidal lee bow, as classic as that, is twofold, not only does the board physically go faster thanks to the energy provided by the tidal flow, but the direction in which the tide is headed lifts the craft upwind.

The reds stayed out of the tide.

You can buy race boards for next to nothing on eBay and I'd strongly recommend any sailor trying one out if for no other reason, than to actually feel all that knowledge you obviously have, in the palms of your hands, you'd find it exhilarating.

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Posted By: sargesail
Date Posted: 02 Oct 17 at 10:39pm
Originally posted by mozzy

Okay, I get what you're saying. But I don't think you're phrasing it very well. The true wind over ground, plus the tidal vector = the wind you feel if you were floating stationary. Depending on direction the tide will shift, add to, or decrease the true wind (from what is experienced on land (or moored up). 

But, in the boat, you can't feel that. the boat can't 'feel' that. All the energy comes from the sheer between air and water. Those two things, nothing else. Whether it's just the air that's moving (lake) or air and water (sea) it is still just the sheer between the two.  

Also, you'd still not explained the 3rd point of tide hitting the foils etc etc. 

Ahhh Mozzy - I've been here with GRF before.  I can't wait to see what he comes up with this time.  I've been waiting to hear about the wonder race that would prove his theory for years.  And there it was.....and I see nothing in it that remotely proves his theory.  In fact the decisive factor appears to have had nothing to do with the tide at all....

So either I'm still as dumb as I used to be for letting him troll me again....or I'm not the dumb one....


Posted By: KazRob
Date Posted: 02 Oct 17 at 10:44pm
All that talk about energies makes me think that he'll be invoking crystals and vibrations on a higher plane next!
But seriously as Mozzy points out, the board/boat doesn'tt know about the ground (unless it's anchored) in the same way it doesn't know it's spinning through space at thousands of miles an hour. If your anchored you'll get 'lift' off the tidal stream but floating free your foils are moving with and relative to the water. Transpose the scenario si there's no no land present and see if it still makes sense

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OK 2122 & 2148


Posted By: sargesail
Date Posted: 02 Oct 17 at 10:45pm
Originally posted by iGRF

Originally posted by sargesail


Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

OK - so talk me through the curved laylines.  Why does blue gradually get lifted on port?
Why does red gradually get headed?
And where can I get one of these windsurfers which point so high that the tacking angle looks like 40 degrees?


The curve indicates the strength of the tide 'lifting' the board higher, tidal lee bow, as classic as that, is twofold, not only does the board physically go faster thanks to the energy provided by the tidal flow, but the direction in which the tide is headed lifts the craft upwind.

The reds stayed out of the tide.

You can buy race boards for next to nothing on eBay and I'd strongly recommend any sailor trying one out if for no other reason, than to actually feel all that knowledge you obviously have, in the palms of your hands, you'd find it exhilarating.

OK so blue, once on port, gets an increasing shove to the east from the tide, which has a greater east going component as she heads south.

What about red?  I'm not sure I understand how they stayed out of the tide.  Surely they crossed the main flow on starboard.  And what they experienced on the west coast they should also experience on the east?


Posted By: Time Lord
Date Posted: 02 Oct 17 at 10:50pm
Hasn't anyone accounted for the rotation of the earth yet? Must have an effect surely.   

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Merlin Rocket 3609


Posted By: iGRF
Date Posted: 02 Oct 17 at 10:58pm
I'm not sure why he (the coach) sent them so far along the coast, I often wonder wether he might have had a nightmare experience at Portishead (Windsurfers used to sail there every year) and if you want a truly aw (ful or some) tidal experience, try sailing there, I swear to God at some stages of the tide if you lassoed a fixed mark you could waterski behind it.

But one of the things about Portishead they used to set a wing mark right out in the channel and if you didn't beat far enough up the bank and set out for it too soon, you'd never be seen again.

Getting back to the energy thing again, something back there someone said was wrong, inland on a lake, there are only the two components, the true wind and the created wind which only happens once the board/boat moves and they combine, so there is nothing else for the boat and it's foils and the wind to 'know' other than the static water we sail on.

The boat or it's foils don't 'know' anything on the sea either, the thing if anything that 'knows' the difference is the true wind which reacts against increasing or decreasing resistance from the water because it also is flowing either with or against the strength of the wind and so delivers more or less energy to the foils.

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Posted By: iGRF
Date Posted: 02 Oct 17 at 11:02pm
Originally posted by Time Lord

Hasn't anyone accounted for the rotation of the earth yet? Must have an effect surely.   


Coriolis effect? Come on, just as i was beginning to make some progress...

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Posted By: mozzy
Date Posted: 03 Oct 17 at 10:52am
So, simple tidal question. In the scenario above, which end of the line is favoured and by how many degrees (upwind start)?  


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 12:05pm
Port end, on Starboard tack, CBA doing the maths but around 10%? The apparent wind caused by drift (i.e. boat stationary in the water but moving 1.5kn at 090 over the ground) would be around 010 and slightly more than 9 knots so a good lift in heading plus the drift would take you towards the windward mark. You would, in fact, be 'lee bowing' the tide Clap

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Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: mozzy
Date Posted: 03 Oct 17 at 12:07pm
Almost, but you've called the wrong end of the line. 

Maybe you're accounting for the inevitable pile up at the committee boat; but I didn't actually say there were any other boats, just simply which end was favoured. 


Posted By: mozzy
Date Posted: 03 Oct 17 at 12:30pm
The tide will veer the true wind by 9.46 degrees, lifting starboard tack. And creating a starboard end bias of almost 10 degrees. 
The race officer (moored up, so not moving in current), will measure true wind direction and believe their line is perfectly fair. If a fleet start the result will be a pile up at the committee boat as not only will people be aiming to start at the starboard end because of the bias, but will also be swept toward that end as they rack up. 
Bonus points if you also calculated the 0.12 knot increase in wind speed! 
See below vector calculation. A is true wind speed, and B is current induced apparent.  Ignore the angles (they’re different because of the way we describe wind coming from and tide going to then tide creating apparent from the direction it is pushing you towards). You can see the grey resultant vector veered from the true wind. The angle change is highlighted at the bottom.



Posted By: iGRF
Date Posted: 03 Oct 17 at 12:40pm
That's a very interesting calculation and perfectly logical and in all my time I've never seen it explained like that and which goes some way for me to understand why you don't 'get' my explanations, but...

Real world

You haven't quantified the length of the beat, therefor the duration, one of the other reasons for choosing the port end, is the liability that going right will cause possible overstand scenarios at the windward mark.

Questions, love to see the maths on your vector calculations, also where that chart thing comes from? Is this part of the same deal that gives those 'polar' things you guys often crack on about.

We (windsurfers) obviously never have or had access to all this, so it's actually cool to see it all written down and explained logically, thanks, lets have some more..


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Posted By: mozzy
Date Posted: 03 Oct 17 at 12:55pm
I didn't include length of beat, length of line or any other details as they are not needed to say which end is favoured. I thought it would complicate things adding gain factor which would come in to play later in the race. I understand there are valid reason not to start at the favoured end if there is a bigger gain to be made by course positioning later up the beat. 

I just wanted to show that when the race officer sets this perfect line, the sailors will feel an apparent (and very real for them) bias of almost 10 degrees.  If you want to lead this race from the start, you need to be at the starboard end. 

What's also interesting, is the above information is pretty much redundant to sailors on the water. The bias will be apparent to them. Whatever technique they use to assess bias (crudely sticking the boat in irons or sailing the line on each tack and subtracting your tacking angles) will tell them the starboard end is favoured. 

The only person likely to be fooled by this is the race officer. Measuring his line against true wind, for boats sailing in apparent he will be unaware of the 10 degree starboard bias. He may even get cranky and go in to black flags when the resultant pile ups and general recalls ensue. 

A smart race officer will set a true wind port bias to account. He will also offset the windward right, to account for the apparent right shift and the fleet being swept right as they progress up the beat. Offsetting the windward will off course offset the run as well, so it's a balancing act. 


Posted By: iGRF
Date Posted: 03 Oct 17 at 1:16pm
This race officer? The one that takes all that into account, love to meet her.


Edit, so the next thing we need to use your magic tool for is that, say it's a lay line approach with the adverse tide just a degree to weather of it. Then Boat A sails a normal course, but Boat B squeezes two degrees up to nose into or slightly present the leeside of the nose into it.

What happens then?

There lyeth the crux to earlier discussion

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Posted By: mozzy
Date Posted: 03 Oct 17 at 1:42pm
By 'normal course' do you mean the fastest angle of sail for making good VMG? 

If pinching was faster you'd do it all the time, and it would become the normal course. So, by pinching, I take it to mean a higher and slower course which produces an overall slower VMG. 

The only time pinching for a mark is quicker than sailing your normal (fastest) course is if the gain from not having to double tack outweighs the gain of sailing you faster normal VMG angle. 

Basically, if you are on a layline with a weather tide and are struggling to make the mark, then you have understood. It is no different to underlaying a mark on a lake. So the decision making process is the same. 

How much will I loose by pinching (slower VMG mode) versus how much will I loose by double tacking. 

If you are far out from the mark, a two tacks is usually better. You will have plenty of time to complete the tacks (and could even do them on a little shift, or to get out of dirty air) and pinching from a long way out will mean sailing an inefficient mode for a long time. 

If you are very close to the mark, a double tack in such a short distance will usually be worse than just pinching "or 'shooting the mark". You'll also be on sticky ground rules wise if within 3 boat lengths. 

A classic  mistake, and it gets worse in big fleets is trying to pinch to lay a mark you have underlain from far out. You see it often in junior fleets in the very situation you describe. The sailors would do better if they accepted they had understood and sailed their normal mode and complete the extra tacks. 

Usually it is better in a fleet scenario to complete the first tack early, then you're not sat in the procession of pinching boats getting dirty air as well. 

If you are not in a fleet (or clear ahead Wink) I would do the extra tacks nearer the mark to make judging the true layline easier. 

I can draw out the vectors, but it will require knowing the speed penalty for pinching versus normal mode and the speed penalty of tacking; both will be class specific.  


Posted By: iGRF
Date Posted: 03 Oct 17 at 1:55pm
Well what normally happens, you make the mark and the boat/board that doesn't react has to double tack and if as it so often happens, it's a starboard layline then he's screwed. Double screwed because he continues to get knocked and you get lifted but then it doesn't work so could never happen according to your logic.

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Posted By: mozzy
Date Posted: 03 Oct 17 at 2:04pm
It would be silly to double tack late if you are under pressure from starboard boats behind. Like I said in my post, if you are in a big fleet tack out early then sail your normal faster VMG angles.

But, you're point above is nothing to do with lee bow. It's just someone who's under stood a mark, was slow to react and got himself pinned out by a procession of starboard tackers on the layline.

Tell me this, if pinching is faster VMG than the normal mode, why do you ever sail the normal mode? If that high mode was best, then surely it would become your normal mode, because you'd always want the best VMG?

Makes no sense. And you see it in fleets all the time; if I just squeeze the bow up a bit further I'll make this mark. And they rack up from 100ms out. Then finally a boat doesn't make it, they've not left enough time to tack out, or even gybe out and the pile up starts. 

You're faster angle is your fastest angle. The only time I'd sail a slower mode is to avoid a late double tack. In a reasonably good tacking boat I'd never sail a pinching mode for more than 10-15 seconds. 

Same goes for gybing in to a leeward mark. Sometime I will sail low and slow for the last 10-15 seconds to avoid the dreaded double-gybe drop. Any longer than that and I'll just sail my fastest mode as I always would; because it's faster!  


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 2:22pm
Originally posted by mozzy

Almost, but you've called the wrong end of the line. 

Maybe you're accounting for the inevitable pile up at the committee boat; but I didn't actually say there were any other boats, just simply which end was favoured. 

Got that, and correction accepted Smile


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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: mozzy
Date Posted: 03 Oct 17 at 2:24pm
Originally posted by iGRF


one of the other reasons for choosing the port end, is the liability that going right will cause possible overstand scenarios at the windward mark.
Just seen this. So your answer to over standing the windward mark is to start further left? 

How would starting on the left possibly make calling the layline easier? 

The answer to that may be, that if you start port end you won't be first to the layline, so you can use the leading boats to judge when you should tack. Not going to win many races that way though!


Posted By: iGRF
Date Posted: 03 Oct 17 at 2:36pm
Originally posted by mozzy



Originally posted by iGRF

one of the other reasons for choosing the port end, is the liability that going right will cause possible overstand scenarios at the windward mark.

Just seen this. So your answer to over standing the windward mark is to start further left? 
How would starting on the left possibly make calling the layline easier? 
The answer to that may be, that if you start port end you won't be first to the layline, so you can use the leading boats to judge when you should tack. Not going to win many races that way though!



As I said, it's not a complete scenario, you haven't quantified the line length, nor length of the beat, but assuming it's a big fleet with a long line and say half mile beat, then the left has more options than the right unless you somehow manage to blast clean from the pack off the line which would be very unlikely from the starboard end for all the reasons you cite.

Mainly because the left will over the course of the race become the middle.

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Posted By: mozzy
Date Posted: 03 Oct 17 at 3:00pm
Knowing no information about an upwind leg, you are able to decide that starting at the unfavourable end would give you more options? Interesting Thumbs Up

I agree, you'd have to be weary of being swept past the starboard lay. If the race officer laid the course to true wind the beat would be feel offset and you'd have to spend more time on starboard than port. But starting on the left doesn't help you judge either of those factors any better.

The fact is, when a boat starting at the starboard end and a boat starting at port cross, the starboard boat will be clear ahead. 

The most likely fate of the port end starters is that, starting down wind of the starboard end they are more likely to be rolled by the boat to windward of them (as they will have started slightly ahead). The option for the port end boats is then to sail over to left in dirty air, or tack out right. 

Boats starting at the starboard end are more likely to have a clear lane and a choice of where they position themselves on the course. Starboard end starters have more options as they are ahead.

I would only start at the pin if there was a massive gain feature out left that would outweigh the 10 degree bias and I could get there before the starboard end boats rolled over me. But I deliberately didn't mention any gain features in my example, as it over complicates things. 


 


Posted By: iGRF
Date Posted: 03 Oct 17 at 4:55pm
OK I've now given this considerable thought,

First I'm going to concede in your circumstance (If I'm right you're quite a handy RS200 jockey with at least one championship win to your tally and you're a Hayling man, so not entirely without tidal credentials) which in that particular boat (In which I've only had very limited experience, but enough to tell me that going too high would slow the boat significantly more than any tidal gain would exacerbate, which isn't the case with an 18 kilo board, or even a 50 kilo single hander, so forget the pinching it's irrelevant to you and you'd never have experienced it.

But,

Second I'm going to call you on your assumptions regarding the tidal lift off that line and in this instance I'm going to quote back at you the conveyor belt assumption which will lift all the boats on starboard off the line, but not in the same way an actual wind shift would, i.e the relation of the boat sailing angles will not alter. They'll still point as they would in relation to the true wind angle but the inter boat relationship angle would not change as it would in a wind shift, so a faster boat off the Port end could still make enough ground to tack across slower starboard boats from the starboard end. The vector they all make good however would follow your assumptions.

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Posted By: Cirrus
Date Posted: 03 Oct 17 at 5:07pm
Whatever next .. a 'Brexit' debate ?   But it might be a tad easier to shift the opinions held on 'the other side' mind you ..... LOL   


Posted By: mozzy
Date Posted: 03 Oct 17 at 6:43pm
The bias is very real. It won't feel like a shift though, it will just feel like committee boat is favoured. 

Say the line was 100m. With a bias of 9.46 degrees the committee boat end starters would be 16m 'upwind' of the pin end starters. 

The conveyor belt analogy is correct. All boats will be lifted the same amount on starboard, and headed the same on port (from the true wind). If the conveyor belt keeps on moving at the same speed, nothing changes and the boat will finish the beat with same 16m difference they started it.

Now, if the conveyor belt changes speed... then you have yourself the next question...

Take the same scenario as I've posted already (9 knots true wind, 1.5 knot cross tide at the start, line set square to the true wind).  The line is 100m long (so I've already told you the bias in meters). Boat A starts at the CB, Boat B at the pin. 

Both boats are perfectly matched in angle and speed. There are no shifts in the true wind. 

Both boats sail on starboard tack, as they progress the tide slackens to 0.1 knots then B tacks. When they cross which boat is ahead and by how far?

 


Posted By: iGRF
Date Posted: 03 Oct 17 at 8:39pm
Impossible scenario, both boats start on port?

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Posted By: Dougaldog
Date Posted: 03 Oct 17 at 8:41pm
I'm sorry to spoil the earlier comments about how the line was laid, but any RO worth the title takes his wind readings from a 'free floating' boat and not one that is moored. At the OK's a fortnight okay, sailed over the highest springs of the year, we delayed final position whilst taking wind readings when drifting. Once we anchored the Committee Boat the course looked 'orrible and skewed - but as soon as the boats came out to start sailing and you could see them close hauled on both tacks, the beat looked spot on.
D


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Dougal H


Posted By: iGRF
Date Posted: 03 Oct 17 at 8:44pm
Any race officer b'stard I knew would have skewed the line to make sure the tide was backing them off..

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Posted By: sargesail
Date Posted: 03 Oct 17 at 9:12pm
Originally posted by Dougaldog

I'm sorry to spoil the earlier comments about how the line was laid, but any RO worth the title takes his wind readings from a 'free floating' boat and not one that is moored. At the OK's a fortnight okay, sailed over the highest springs of the year, we delayed final position whilst taking wind readings when drifting. Once we anchored the Committee Boat the course looked 'orrible and skewed - but as soon as the boats came out to start sailing and you could see them close hauled on both tacks, the beat looked spot on.
D

And there are too many that aren't worth the title....the number of times I see an RO with a compass and a bit of string at anchor.  As I moaned about it at the weekend one of the group commented that a friend of his used tidal stream data in a computer to work out where his marks should be....and that's fine.  But all too often you end up with a WM where the string was pointing!

Used to love Ken Falcon as RO - would jump in a RIB and then 'borrow' a boat to get a feel for conditions.

Did you up hook routinely between races to test that your feel for the tidal apparent was still in date?


Posted By: mozzy
Date Posted: 03 Oct 17 at 9:15pm
Originally posted by iGRF

Impossible scenario, both boats start on port?

You got me! I've edited to starboard. Both boats start on starboard and sail on starboard until the tide slackens and B tacks. 


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 9:23pm
Assuming the tide slackens across the whole course? No windshifts? And they were sailing at exactly the same speed? B would have to take avoiding action to avoid hitting A (still on Starboard)?

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: Dougaldog
Date Posted: 03 Oct 17 at 9:30pm
Sam - as soon as the line was closed we were off....Of course, taking readings from a moored boat works fine on a pond....

D


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Dougal H


Posted By: mozzy
Date Posted: 03 Oct 17 at 9:46pm
Dougaldog, I love your optimism and faith in race officers! But the scenario is only a bit of fun to illustrate a point. Out of all the unrealistic things about it (no shifts, same boat speed, no gust etc etc) a race officer setting an unintentionally bias line is about the most realistic. 

Sounds like you did an excellent job at the event you ran though. 

When I'm race officer at my club I just guess the course by eye. As long as they can cross the line it's all fair; everyone has the same opportunity to choose the correct end. 

Once I did lay a finish mark that dried out... oops, I did get a fair bit of stick for that. But there is no rule that the course has to be on the water!

I doubt anyone takes floating wind readings for club racing, for instance, and lots of club will be working off fixed lines anyway!


Posted By: mozzy
Date Posted: 03 Oct 17 at 9:49pm
Originally posted by Sam.Spoons

Assuming the tide slackens across the whole course? No windshifts? And they were sailing at exactly the same speed? B would have to take avoiding action to avoid hitting A (still on Starboard)?

Spot on! A would have a one meter lead and B would have to avoid. 


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 9:55pm
Sorry Doug, I was trying to answer mozzy's 'supplementary question' Smile Not sure I have the answer right but based on 'conveyer belt' theory?

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 10:04pm
Originally posted by mozzy

Once I did lay a finish mark that dried out... oops, I did get a fair bit of stick for that. But there is no rule that the course has to be on the water!

I remember Raceboards Open Meetings at Morecambe many years ago, I got there early one year and helped set the course marks with the aid of a wheelbarrow (and, incidentally, collect them the same way after the event) Tongue happy days......


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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: iGRF
Date Posted: 03 Oct 17 at 10:04pm
Originally posted by mozzy


Originally posted by Sam.Spoons

Assuming the tide slackens across the whole course? No windshifts? And they were sailing at exactly the same speed? B would have to take avoiding action to avoid hitting A (still on Starboard)?

Spot on! A would hold a 1 metre is lead and B would have to avoid. 


Agreed, but then B could still short tack, onto the starboard layline and force A behind and round first

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Posted By: A2Z
Date Posted: 03 Oct 17 at 10:14pm
Originally posted by Sam.Spoons

Sorry Doug, I was trying to answer mozzy's 'supplementary question' Smile Not sure I have the answer right but based on 'conveyer belt' theory?
This conveyor belt theory. Why is it assumed all boats drift downtide at the speed of the current?  Isn't that like assuming all boats can run downwind at windspeed?


Posted By: mozzy
Date Posted: 03 Oct 17 at 10:22pm
Originally posted by A2Z

Originally posted by Sam.Spoons

Sorry Doug, I was trying to answer mozzy's 'supplementary question' Smile Not sure I have the answer right but based on 'conveyer belt' theory?
This conveyor belt theory. Why is it assumed all boats drift downtide at the speed of the current?  Isn't that like assuming all boats can run downwind at windspeed?
All boats do drift down tide at the speed of the current. Unless you're sailing them through the water. In which case they will move in a direction over land which is the sum of those two vectors (the tide and the sailing).

Trust me, when the wind drops to nothing, and the tide is flushing out the harbour, all the boats, 49er to oppie, all drift out the harbour with equal hopelessness! 


Posted By: Sam.Spoons
Date Posted: 03 Oct 17 at 10:38pm
In a flat calm that must the case (well actually won't they drift slightly slower than the tide due to air resistance)? TBF that might slow them by a fraction of a knot........ "Pedant's rule OK?"

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: A2Z
Date Posted: 03 Oct 17 at 10:51pm
As a pondie I'm clueless but intrigued.
Boats aren't rigidly attached to the water, the water flows past and hydrodynamic forces are imparted.  I can't see why they should be the same regardless of the type of boat, in which case I can't see why they drift off at the same (current) speed. Therefore, if not all boats slip at 1.5kts in the example above, the winning tactics might not always be the same?


Posted By: 423zero
Date Posted: 04 Oct 17 at 7:25am
They don't, amount of boat in water, plus wind resistance, handicap fleet, different boats react differently.


Posted By: sargesail
Date Posted: 04 Oct 17 at 7:41am
But the wind resistance is just the gaseous fluid motion bit of the vector.

In a flat calm the rate of drift is the same.  Seen it in the Cherbourg Race couple of years ago.  Heaps of different boats drifting together.  Possibly you could have assessed that the much heavier ones had momentum and took longer to start drifting the other way when the tide turned.

Or that might have been the beer we were drinking for breakfast..... 


Posted By: Sam.Spoons
Date Posted: 04 Oct 17 at 8:49am
Surely, in a flat calm, as soon as the boat starts to drift with the current an apparent wind equal to the speed of the current will be generated? E.g. in a 3 knot tide with no actual wind (as measured by an anchored boat) any boat drifting with the current will feel a 3 knot breeze?

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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: mozzy
Date Posted: 04 Oct 17 at 9:35am
Sam, I take your point, but if you really want to be a pedant, then wind resistance moving a boat through the water = sailing. To be totally becalmed in tide, the true wind would need to have an equal vector to the tide, to cancel out (i.e same direction and strength). I.e wind felt on the water is zero = flat calm. 

All boats are affected the same by tide, here is why:
A2Z; ask yourself this. When it's a flat calm on the lake, do the boats move through the water? The boats aren't rigidly attached to the water on a lake, but if they are becalmed they don't move. You don't see water flows past and hydrodynamic forces moving boat through the water on a lake do you? 

So if you accept all boats becalmed on a lake sit still,  then why would they move through water when becalmed in a current? If there is no force to propel the boat through the water they will sit still, apparently attached to the water and from the shore the boat will appear to move with the tide. 

Now, add in wind. All boats are still affected just the same by tide (this is where iGRF is wrong about boards being more or less effected by being lighter or having less hull in the water or whatever). The tidal vector is the same for all boats. 
Now; when boats move faster, their vector through the water will be greater but crucially tidal vector remains the same. So it's more accurate to say: "all boats fast or slow are affected the same by current over time, however faster boats which cover more ground in a set time are less affected by current over a set distance". 

All boats are affected the same by tide over time, but faster boats spend less time being affected...
Some boats may move faster through the water, but not because they are 'cutting through the tide better', but simply because they are cutting through the water better. They are just a faster boat. 

Now, this is where things do get interesting, if you're still following me. We race around marks which are fixed to the ground. If all boats raced against the tide for 60 minutes all boats would experience 60 minutes of tide, regardless of how fast they are: simple. If there was 1 knot of tide, all boats, no matter how fast or slow, will be moved 1 nautical mile by the tide in those 60 minutes.  

However, we don't race for set times, we race over set distances fixed to the land. Faster boats spend less time going around the course, so spend less time being affected by the tide. 

This phenomena is called the 'tidal gate'. If you go from a favourable tide to an unfavourable tide the time between boats on the water will reduce. The opposite is true is you go from unfavourable to favourable.  You will notice this most when beating against the tide. The lead boats round the windward mark and accelerate away stretching out a large lead over the water. When they head up round the leeward mark, the pack appear to catch them back up as the leader hits the unfavourable tide.

But, if faster boats spend less time in the unfavourable tide, they'll also spend less time in favourable tide, so it will even out won't it? No, it doesn't and here's why: 
The net tide is always unfavourable, if you finish roughly where you started. This is because when the tide is against you it will make the leg feel longer, and when it is with you it will make the leg shorter. So around the whole course, you'll spend more time in unfavourable tide. 
This is why HISC adjusts handicaps, as on any given day faster PY boats, over a set distance course, experience less tide (and tide is always net unfavourable) giving faster boats an advantage. However, as the tide is always different (either itself or compared to the wind) it means the magnitude of the advantage for faster boats speed is constantly changing. Therefore there can never can be one fixed PY. 
Also, slow boats aren't equally slow / fast on all legs. Some boats are better at reaching, or beating, or fetching. The PY would also need to account for which legs the tide was unfavourable, and how this matched against the boats strength on these legs. 

Complicated enough for you? 


Posted By: Sam.Spoons
Date Posted: 04 Oct 17 at 9:54am
Originally posted by mozzy

Sam, I take your point, but if you really want to be a pedant, then wind resistance moving a boat through the water = sailing. To be totally becalmed in tide, the true wind would need to have an equal vector to the tide, to cancel out (i.e same direction and strength). I.e wind felt on the water is zero = flat calm.

I very nearly added the following to my previous post the decided it was stating the obvious :-

"And most boats will sail pretty effectively in 3 knots of wind"  LOL




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Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Supernova 395 "dolly the sheep"


Posted By: iGRF
Date Posted: 04 Oct 17 at 9:57am
Originally posted by mozzy




]Now, add in wind. all boat are still effected just the same by tide (this is where iGRF is wrong about board being more or less effected by being lighter or having less hull in the water or whatever). The tidal vector is the same for all boats.



Now now, don't go getting above yourself, some of what you say here is true, but your start line scenario was wrong, and so is your assertion that boards and boats being more or less affected by the tide.

I pointed out earlier, a cross tidal vector can induce a board onto the plane, the same vector would never induce planing in something like your RS200, ergo the lighter board would seek out sectors of a course where tidal lee bow were stronger.


I have another scenario for you to consider, in your {doesn't make any difference which angle the boat is to the tide) assumptions.

Lets take the typical triangle with a cross tide, but the 1st leg has been set broad and the return leg of the triangle is tight.

The advantageous cross tide assists on the 1st leg, but is disadvantageous on the 2nd.

It is still a reach though but the wind is failing

Boat A gybes and sails for the mark with the tide still on the weather side of the foils.

Boat B gybes higher and presents the bow directly into the tidal flow.

Who gets to the lee mark first?

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Posted By: mozzy
Date Posted: 04 Oct 17 at 10:04am
Originally posted by iGRF

Now now, don't go getting above yourself, some of what you say here is true, but your start line scenario was wrong, and so is your assertion that boards and boats being more or less affected by the tide.

I pointed out earlier, a cross tidal vector can induce a board onto the plane, the same vector would never induce planing in something like your RS200, ergo the lighter board would seek out sectors of a course where tidal lee bow were stronger.

The start line scenario isn't wrong. I clearly showed it was true and backed it up withe maths. That you still argue otherwise or can't comprehend it is another thing. 

Yes, a tidal increase in wind can lift a board on to the plane. And it can and does an RS200 as well. 

What you fail to realise is that when the tide shifts the apparent wind it shifts it in one direction (which should be obvious, it can't lift both tacks, or head both tack). This will favour one tack or the other. When the tide increases the apparent wind it increases it does so for both tacks. 

So in the board race you described, where the current lifted you and increased the apparent wind allowing you to plane. That's fine. But, the increase in apparent would have also been enough for the board on starboard to plane. It's just that they would be on a header and you were on a lift. 


Posted By: iGRF
Date Posted: 04 Oct 17 at 10:41am
Tide can and does lift on both tacks.

It just doesn't lift in the same manner as a windshift lift which is what you and your maths fail to grasp.

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Posted By: mozzy
Date Posted: 04 Oct 17 at 10:48am
You need to work on your problem setting iGRF. There are so many things left out or wrong. 
Originally posted by iGRF

 
I have another scenario for you to consider, in your {doesn't make any difference which angle the boat is to the tide) assumptions.

Lets take the typical triangle with a cross tide, but the 1st leg has been set broad and the return leg of the triangle is tight.
Okay, so by triangle course you mean a course that start on a reach? 1st leg a broad reach, second leg a tight reach and third leg presumable a beat? 

Originally posted by iGRF

 
The advantageous cross tide assists on the 1st leg, but is disadvantageous on the 2nd.
Okay. I get this, that's fine. But really, the extent to which a boat has to crab across the tide to stay on the rhumb line is determined by:
  • the strength and direction of the tide 
  • the strength and direction of the wind
  • the boats speed through the water over various headings and wind speeds (speed polar) 
  • distance and heading between marks.
You provide none of these key bits on information which would allow a definitive answer to the problem. You are either leaving this information out because you don't realise it is required, or because you don't understand how to solve the problem and want it to be ambiguous so you can blag your way out of it later.

Originally posted by iGRF

 
It is still a reach though but the wind is failing
So, you're saying the wind is dropping, but not so much as they have to start tacking to get over the tide and up to the mark. I think I understand that. But again, you don't say what the wind started out at, or what it drops to.  

Originally posted by iGRF

 
Boat A gybes and sails for the mark with the tide still on the weather side of the foils.
Sails for the mark... so ambiguous. Do you mean points it bow at the mark? Most people in this situation would sail for the mark by taking a transit to show them where the rhumb line was and crab the tide. AS you can tell from my bullet point list above calculating the true heading needed is difficult, so just taking a transit and keeping the boat on it is a very practical way of solving the problem. 

Tide on the weather side of the foils? Ha. The tide is coming from the weather side of the boat, the foils, the rig and everything. It is a single factor, moving the whole boat in one direction as one. Meanwhile, the foils will experience flow in from the direction the boat is moving (through the water). In terms of flow on the foils where the tide is coming from makes no difference. The boat isn't moored to the bottom. 

Originally posted by iGRF

 
Boat B gybes higher and presents the bow directly into the tidal flow.
So, boat B sails higher, directly in to the tide. But, the tide is still slightly across the reach (although we don't know for sure as you haven't provided the information). If they only sail directly in to the tide, they will never reach the mark. (unless the mark is directly in to the tide as well... in which case boat B is doing the same as boat A, which is sailing directly to the mark as well).

There must be a second part to B's strategy. I.e. they sail directly in to the tide, then bear away later on the reach to cross tide and drop down to the mark. 

Sailing high then dropping low could be a good strategy in a failing breeze. You'll spend more time fighting the tide when you have breeze to do so, then can ferry glide down to the mark later on when the breeze dies... none of that is in your description of the strategy though. 

Originally posted by iGRF


Who gets to the lee mark first?
Well, unless boat B bears off, he'll never get to the mark, so I'd say boat A. 



Posted By: mozzy
Date Posted: 04 Oct 17 at 10:57am
Originally posted by iGRF

Tide can and does lift on both tacks.

It just doesn't lift in the same manner as a windshift lift which is what you and your maths fail to grasp.

Hilarious. Prove it. 

But, bear in mind whilst you're proving this, its also going against why you had that one decent beat and got the better of the 'squaddies'. For if it's true that tide lifts you on both tacks then both the starboard boats in you scenario and you on port would be lifted. So if this was the case, how did you get an advantage? 

Like I said iGRF, even a broken clock is right twice a day. It sounds more and more like you lucked out and experienced a tidal gain one day and beat some decent sailors. 

There are plenty of times in sailing when people adopt the best strategy for all the wrong reasons. It doesn't prove they know what they are talking about, or are good sailors. 


Posted By: iGRF
Date Posted: 04 Oct 17 at 2:46pm
Originally posted by mozzy



[QUOTE=iGRF]Tide can and does lift on both tacks.


Hilarious. Prove it. 


Easy.
Point 1, favorable tide say up river, wind blowing downriver, like say Columbia River Gorge Both Tacks lifted.


Point 2. Your gammy start line prediction.

All boats off the line, relative to the wind will stay at the same angle to the wind yet the course they collectively sail on is moving sideways creating the illusion of a lift, but not altering the attitude of the boats against one another as would be the case with a windshift favoured starboard end.

I take it I don't have to point out to you what happens when wind lifts a fleet as it departs a start line.

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Posted By: iGRF
Date Posted: 04 Oct 17 at 2:51pm
Originally posted by mozzy



]Like I said iGRF, even a broken clock is right twice a day. It sounds more and more like you lucked out and experienced a tidal gain one day and beat some decent sailors. 



Yep, I must have lucked out, then again and again and again and again year in year out, I guess I must be just lucky.

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Posted By: mozzy
Date Posted: 04 Oct 17 at 3:09pm
Oh dear, that is not proof. 

Stating something is the case in one location, but not describing why, isn't proof. Show me why both tacks are lifted on the river. 

Also: Jonathan Mckee says the left is favoured by a wind shift and the right by more current.  http://https://www.cgra.org/McKee-interview-long.pdf" rel="nofollow - http://https://www.cgra.org/McKee-interview-long.pdf

You still don't understand the start line situation. I showed the vectors. I can't make it any clearer. 

You seem to be arguing against yourself. One minute you say you beat 'squadies' because you got a lift from the tide, next you say it's an 'illusion'. 

The tide going across the course does collectively move everyone that way. In doing so it produces a very real shift in apparent wind. 





Posted By: mozzy
Date Posted: 09 Oct 17 at 12:21pm
Watch from 10 minutes in... 

[TUBE]6WzOrK3y9nI?t=10m2s[/TUBE]


Posted By: A2Z
Date Posted: 10 Oct 17 at 8:00am
Originally posted by mozzy


All boats are affected the same by tide, here is why:
A2Z; ask yourself this. When it's a flat calm on the lake, do the boats move through the water? The boats aren't rigidly attached to the water on a lake, but if they are becalmed they don't move. You don't see water flows past and hydrodynamic forces moving boat through the water on a lake do you? 

So if you accept all boats becalmed on a lake sit still,  then why would they move through water when becalmed in a current? If there is no force to propel the boat through the water they will sit still, apparently attached to the water and from the shore the boat will appear to move with the tide. 

Now, add in wind. All boats are still affected just the same by tide (this is where iGRF is wrong about boards being more or less effected by being lighter or having less hull in the water or whatever). The tidal vector is the same for all boats. 

Boats sit still on a becalmed lake because there are no aerodynamic or hydrodynamic forces to move them.  However, if the water is moving due to a current, there are hydrodynamic forces in play. Each boat will then accelerate until they achieve a steady velocity (in the direction of the current) and with a magnitude such that the drag of the boat moving through the water is equal to the force of the water (current) on the boat. This requires that the boat speed must be less than current speed, because if the boat moved at the speed of the current there would be no hydrodynamic force. The boat would have to slow down until such time as the hydrodynamic force equalled the drag again. This is true even ignoring aero drag.  So I maintain that boats drift downtide at a speed less than the current. This doesn't alter the basis of the argument that a shift in apparent wind occurs, but the size of that shift might not be as big as expected.  


Posted By: mozzy
Date Posted: 10 Oct 17 at 9:08am
Originally posted by A2Z

 
However, if the water is moving due to a current, there are hydrodynamic forces in play. 
What are these 'hydrodynamic forces' you talk of? if the boat is stationary in the water (albeit moving in relation to the land)?

Originally posted by A2Z

Each boat will then accelerate until they achieve a steady velocity (in the direction of the current) and with a magnitude such that the drag of the boat moving through the water is equal to the force of the water (current) on the boat. .  

If the boat achieves a steady velocity in the direction of the current there is no force from the water. 

Newton's first law. Once the boat is accelerated up to the speed of the current and reaches a steady speed it will remain in that uniform motion. I.e. it needs no additional water force to keep moving. 

Put it another way:
The water on your lake is moving at 1000 miles an hour and yet when you put a boat in it, and it is becalmed, it doesn't move through the water. 

Or, imagine a boat on an achor:
In 2 knots of tide a boat at anchor (fixed to the land) will experience 2 knots of flow past it's hull. On the boat it will appear as if your anchor line is pulling you through the water at 2 knots. Relative to the land you will be stationary. 

Release the boat from it's mooring and it accelerates up to 2 knots until the water is now longer flowing past the boat, on the boat you will be stationary in the water. From the land the boat will now be moving at 2 knots with the current. 


Posted By: Sam.Spoons
Date Posted: 10 Oct 17 at 9:21am
I agree with mozzy. The only thing causing the boat to move at less than the speed of the current is aerodynamic drag, in the hypothetical situation of the wind exactly matching the current in speed and direction the boat would quite soon exactly match the current (and wind) speed and would appear becalmed. If there was truly no wind (which is actually much more likely than the wind exactly matching the current) the boat would not appear becalmed but would experience an apparent wind with a vector exactly opposite to the current. 

The interesting question is "would that apparent wind be sufficient to allow a boat to make progress against the tide?" (I think no but I could be wrong) Wink


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Posted By: iGRF
Date Posted: 10 Oct 17 at 9:22am
I can't see that link, but it's pointless to argue with him, he's done his maths, he can't possibly be wrong.

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Posted By: iGRF
Date Posted: 10 Oct 17 at 10:12am
I think it's time for another anecdote.

Once upon a time they used to hold a race up the Thames, there were hundreds of boards. The race went ahead wind or no, because we sailed with the tide.

So with no wind whatsoever we criss crossed the river the water movement creating a 'true' wind and the movement we generated forward producing a 'created'wind so our apparent wind was exactly where it might be if the true wind were the couple of knots that the current generated.

But what was wierd, as you might normally do, watch up wind for wind obstacles, trees, bushes etc, it didn't matter, even under bridges, no change the only thing that was important was to judge where the tidal flow was greater and play the shorter routes the bends in the river allowed, from Putney up to Barnes as I recall.

But,here's a thing even as the river bent around the bend, no change in the appearance of a wind direction, both tack were equal, very little difference the only place changes that occurred were thanks to tidal flow points and being boards with a free sail rig, you could feel them.

Now finally that other point about boards, the reason the tide is very important is that the planing threshold for a board is around 8 knots wind, give or take board volumes widths etc, so if you have only 5 knots of wind, but find 3 knots of tide, you get to plane which then increases the created wind and you plane even better, you can even pump yourself onto the plane and stay there, it's not something you can easily describe to fixed rig sailors, but like ice yachting, it works.

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Posted By: mozzy
Date Posted: 10 Oct 17 at 10:55am
Originally posted by iGRF


Now finally that other point about boards, the reason the tide is very important is that the planing threshold for a board is around 8 knots wind, give or take board volumes widths etc, so if you have only 5 knots of wind, but find 3 knots of tide, you get to plane which then increases the created wind and you plane even better, you can even pump yourself onto the plane and stay there, it's not something you can easily describe to fixed rig sailors, but like ice yachting, it works.

Pretty similar planing speed to the 49er I used to sail. Not only would getting in tide take you to the mark 3 knots quicker, but you would be sailing in 8 knots breeze. 8 knots was enough for you to plane so you would see a significant increase in boat speed through the water. 29er was the same, but you needed more like 10-12 knots. 

Interestingly, for moths, if it is wind against tide, they are better sailing against more tide downwind. The increase in boat speed from the additional apparent more than outweighs the additional distance they have to sail through the water. Only works for boats which have VMG greater the wind though. 


Posted By: A2Z
Date Posted: 10 Oct 17 at 9:12pm
Originally posted by mozzy

Originally posted by A2Z

 
However, if the water is moving due to a current, there are hydrodynamic forces in play. 
What are these 'hydrodynamic forces' you talk of? if the boat is stationary in the water (albeit moving in relation to the land)?

Originally posted by A2Z

Each boat will then accelerate until they achieve a steady velocity (in the direction of the current) and with a magnitude such that the drag of the boat moving through the water is equal to the force of the water (current) on the boat. .  

If the boat achieves a steady velocity in the direction of the current there is no force from the water. 

Newton's first law. Once the boat is accelerated up to the speed of the current and reaches a steady speed it will remain in that uniform motion. I.e. it needs no additional water force to keep moving. 

Put it another way:
The water on your lake is moving at 1000 miles an hour and yet when you put a boat in it, and it is becalmed, it doesn't move through the water. 

Or, imagine a boat on an achor:
In 2 knots of tide a boat at anchor (fixed to the land) will experience 2 knots of flow past it's hull. On the boat it will appear as if your anchor line is pulling you through the water at 2 knots. Relative to the land you will be stationary. 

Release the boat from it's mooring and it accelerates up to 2 knots until the water is now longer flowing past the boat, on the boat you will be stationary in the water. From the land the boat will now be moving at 2 knots with the current. 
Your anchor scenario is exactly my point.  Here you have hydrodynamic forces acting on the hull and tension in the anchor cable is holding it still.  Cut the cable, the hydrodynamic force now accelerates the boat until the drag of the hull equals the hydrodynamic force. At this point the boat moves at constant velocity, but the magnitude of that velocity surely depends on how draggy the hull is.
 



Posted By: sargesail
Date Posted: 10 Oct 17 at 9:35pm
No it doesn't. 

The hydrodynamic drag will affect the acceleration....the change in velocity.  

Given 2 boats of different profile both end up accelerating......and as they approach the speed of the current the flow past the hull tends to zero....so that the hydrodynamic drag tends to zero so that the difference in hydrodynamic drag tends to be even smaller than that.....

A 'fascinating' race during that very high pressure week in June.....drifting up and down the Solent with the same piece of flotsam alongside us.


Posted By: mozzy
Date Posted: 10 Oct 17 at 9:35pm
How quickly the boat reaches equilibrium will depend on it's weight and how 'draggy' it is. 

The point is, shortly after coming off the anchor, that force (the anchor) is no longer acting on he boat and it slows through the water to zero. Once it's off the anchor the only forces pushing the boat through the water is the sheer between wind and water. 

The tide doesn't move a boat through the water, because the tide is the water. So how fast a boat moves through the water does not depend on tide, but on how much power the sails have and how draggy the hull is, exactly the same as a lake. 


Posted By: A2Z
Date Posted: 10 Oct 17 at 9:41pm
Originally posted by sargesail

No it doesn't. 

The hydrodynamic drag will affect the acceleration....the change in velocity.  

Given 2 boats of different profile both end up accelerating......and as they approach the speed of the current the flow past the hull tends to zero....so that the hydrodynamic drag tends to zero so that the difference in hydrodynamic drag tends to be even smaller than that.....

A 'fascinating' race during that very high pressure week in June.....drifting up and down the Solent with the same piece of flotsam alongside us.
LOL


Posted By: craiggo
Date Posted: 11 Oct 17 at 7:19pm
[QUOTE=mozzy]
How quickly the boat reaches equilibrium will depend on it's weight and how 'draggy' it is. 
The point is, shortly after coming off the anchor, that force (the anchor) is no longer acting on he boat and it slows through the water to zero. Once it's off the anchor the only forces pushing the boat through the water is the sheer between wind and water. 
The tide doesn't move a boat through the water, because the tide is the water. So how fast a boat moves through the water does not depend on tide, but on how much power the sails have and how draggy the hull is, exactly the same as a lake. 

[/QUITE]

Correct, which means a low drag canoe body hull with highly efficient rig, thats in a tidal flow with no wind, will experience an apparent wind initially equalling the speed of the tide, which in turn will allow the boat to start making some forward progress at an angle to flow of the tide. At some point the hydrodynamic drag will equal the forces generated by the apparent wind and the boat will have achieved its maximum speed. A draggy hull with inefficient rig will accelerate slower due to rig and will stop accelerating earlier due to the increased hull drag however it may be able to point higher!

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Posted By: mozzy
Date Posted: 11 Oct 17 at 7:44pm
Yes; but what you are describing is a fast boat and a slow boat. Not really anything to do with the tide.  

A low drag boat with efficient rig will go faster in a 3 knot tidally induced apparent wind than a high drag boat with inefficient rig. But the speed difference between the two boats will be same as in a 3 knot true wind with no current. 


Posted By: Sam.Spoons
Date Posted: 11 Oct 17 at 7:49pm
But will either of those boats be able to make ground against the tide (which means sailing 'downwind')?

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Posted By: iGRF
Date Posted: 11 Oct 17 at 9:08pm
Seen that video, at last, the bit from ten minutes where he talks about a cross tide half way up the course and draws a similar vector illustration to yours, which I have to think of a method to disprove it to you both so you get where I'm coming from.

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Posted By: mozzy
Date Posted: 11 Oct 17 at 10:29pm
But here's what I don't get GRF. The situation and he describes is exactly the reason you get a triple bonus in your original situation. Not only is the tide tacking you up to the lay line, but it's also increasing your breeze and giving you a lift. 

The starboard boats get swept toward the layline, and they get the increase in breeze, however, the shift in wind is a header for them, which is why you won out coming in from the left. 

So you start a thread describing a situation where you get a tidal lift, and now, ten pages later your going to think of a method to disprove it ever happened. 


Posted By: bustinben
Date Posted: 18 Oct 17 at 2:00pm
Your commitment to this argument is admirable mozzy LOL  many have tried...

On the plus side the clear explanations of tidal effects are probably very helpful to people who don't already know this stuff, so it's probably worth it from that point of view!


Posted By: iGRF
Date Posted: 18 Oct 17 at 2:06pm
The only bit we are in disagreement over is the difference between a tidal lift, and a windshift lift, on your position in a fleet, when you understand that, you'll be fine. I can't help anyone who doesn't get that.

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Posted By: mozzy
Date Posted: 18 Oct 17 at 4:53pm
It would give me hope if iGRF could understand. But, i can't think of any more ways to explain it. 

I showed a situation where the wind was veered for sailors on the water compared to the true wind felt by the RO.

The video I posted showed how as the boats went from no tide (feeling true wind) to the current, where the tide backed the true wind, favouring boats which had created leverage of the left (just like a regular wind shift). 

I can't think of another way to frame effect tide has on apparent wind. 

This sort of tidal knowledge is useful. You will know that not only when you hit a cross current will it sweep you sideways toward a side of the course (and layline), but it will also shift the wind lifting you up toward the layline as well. 

Hopefully the thread has been useful for people and some have learnt a few things. 


Posted By: 423zero
Date Posted: 18 Oct 17 at 5:07pm
Would a gybing board work with cross current?



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