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Sailing in tide..quiz.

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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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? 


Edited by mozzy - 05 Oct 17 at 4:06pm
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post 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?

Edited by iGRF - 04 Oct 17 at 10:00am
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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. 


Edited by mozzy - 04 Oct 17 at 10:08am
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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. 



Edited by mozzy - 05 Oct 17 at 9:24am
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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. 


Edited by mozzy - 04 Oct 17 at 1:09pm
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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

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. 





Edited by mozzy - 04 Oct 17 at 3:22pm
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