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Sailing in tide..quiz.

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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 17 at 9:28pm


So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

Edited by iGRF - 02 Oct 17 at 9:31pm
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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.
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Post Options Post Options   Quote KazRob Quote  Post ReplyReply Direct Link To This Post 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.
OK 2122 & 2148
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post 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!
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post 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?
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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. 




Edited by mozzy - 02 Oct 17 at 9:57pm
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post 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.

Edited by iGRF - 02 Oct 17 at 10:00pm
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post 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?
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post 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?
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post 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!).
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