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Lee-Bow..... Windsurfers...etc (Dons tin hat)

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Oatsandbeans View Drop Down
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 19 at 4:42pm
Mozzy I am with you on that👍
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Post Options Post Options   Quote Oinks Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 19 at 7:19pm
Since reading Dave Perry's book that covers this argument specifically, I've always been convinced that the lee-bow thing was a myth. now I find myself questioning this!

I appreciate that the flow of the tide acts like a conveyor belt and generally speaking affects all the boats in the same way on a particular tack. It is trying to move everybody backwards at the same rate.

In a flat calm, all boats will move backwards at the same rate irrespective of their angle to the breeze. There is no forward drive from the sails and everybody is freely drifting on the conveyor belt.

However, take the example say, of a stake, buried in the seafloor and protruding above the water level. It is being subjected to a knot of tide flowing past. There is 1 knots worth of pressure on that stake. Because the stake is immovable, there is flow and there is pressure – it’s not being subject to the conveyor belt dynamic.

Now take a boat that is managing to make a couple of knots over the ground. It is not now subject to the “conveyor belt” dynamic – it is not freely drifting. It is moving forwards against the tidal flow and, like the stake, will be receiving flow and pressure over its foils – equally on each side if sailing directly into the tidal flow.

Then take another boat, being sailed in exactly the same way, in the same tidal flow. Let’s say that the person then sets that boat up to go more for pointing (not necessarily pinching) - and manages to point a degree or two higher thereby increasing the pressure to some degree of the tidal flow over the leeward side of the foils (decreasing pressure on the windward side) – in terms of simple logic (!), that would surely result in this effect of “pushing the boat up to windward”.

I agree that we’re talking a degree of precision that may not be easily achievable, but not impossible by any means, but the simple logic remains.

I don’t know an awful lot about windsurfers, but looking at what the thread instigator was saying about his son…a windsurfer is a bit different to a dinghy. The board can pivot around the base of the mast. If by doing something with his feet that keeps the rig in the same attitude to the breeze, he managed to angle the board above the tidal flow and thereby the angle of attack of the foils, would it not experience the same effect of being pushed to windward?


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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 19 at 8:40pm
No, where you're getting mixed up is that the boat doesn't know anything about the tide per se. As GRF said its a hull in one moving fluid and the rig in another. Whether the water is moving relative to the mud however many fathoms down is completely irrelevant. As far as the boat is concerned the tide just changes the perceived wind direction/speed.

So the boat is always subject to the conveyor belt dynamic. Its moving against the conveyor belt, not the sea floor.

Edited by JimC - 18 Sep 19 at 11:08am
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Post Options Post Options   Quote CT249 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 4:05am
Originally posted by Oatsandbeans

A windsurfer allows you to have a amazing control over the angle of attack of both the sail ( in 3 axis) and foil (in 2 axis) .

In the pic the board to windward has the rig oversheeted past the centreline. If you think what the foil is doing it will be at a high angle of attack to the waterflow, generating a lot of lift ( and drag). This sounds as if it would give a high pointing low leeway mode. So I would not be surprised if this gave separation to the boards below that are in a more fast and free mode.

Yes, it does look to be in a high mode and that may well have made the difference; those old windsurfer sails really like being trimmed hard.

But you can't make them go forward if they are actually oversheeted past the centreline. The thing is that because the mast leans to windward, the luff is even further to windward than the leach is, even if the leach is past the centreline. So what looks like a sail oversheeted is actually a factor of the extreme windward position of the luff.


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Post Options Post Options   Quote PeterG Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 8:05am
Then take another boat, being sailed in exactly the same way, in the same tidal flow. Let’s say that the person then sets that boat up to go more for pointing (not necessarily pinching) - and manages to point a degree or two higher thereby increasing the pressure to some degree of the tidal flow over the leeward side of the foils (decreasing pressure on the windward side) – in terms of simple logic (!), that would surely result in this effect of “pushing the boat up to windward”.

But difference of forces on foils will occur with different degrees of pointing regardless of how the body of water is moving. Some people will make a boat point higher effectively, others may sail better a bit off the wind, others just don't sail as well. 

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Post Options Post Options   Quote CT249 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 11:00am
Originally posted by Oinks

Now take a boat that is managing to make a couple of knots over the ground. It is not now subject to the “conveyor belt” dynamic – it is not freely drifting. It is moving forwards against the tidal flow and, like the stake, will be receiving flow and pressure over its foils – equally on each side if sailing directly into the tidal flow.

Then take another boat, being sailed in exactly the same way, in the same tidal flow. Let’s say that the person then sets that boat up to go more for pointing (not necessarily pinching) - and manages to point a degree or two higher thereby increasing the pressure to some degree of the tidal flow over the leeward side of the foils (decreasing pressure on the windward side) – in terms of simple logic (!), that would surely result in this effect of “pushing the boat up to windward”.

I agree that we’re talking a degree of precision that may not be easily achievable, but not impossible by any means, but the simple logic remains.

I don’t know an awful lot about windsurfers, but looking at what the thread instigator was saying about his son…a windsurfer is a bit different to a dinghy. The board can pivot around the base of the mast. If by doing something with his feet that keeps the rig in the same attitude to the breeze, he managed to angle the board above the tidal flow and thereby the angle of attack of the foils, would it not experience the same effect of being pushed to windward?



Go back to the conveyor belt analogy; if you change your angle while walking on the conveyor belt you don't change the pressure on your shoes in a way that lifts you to the side. 

You don't change the effect of the pressure of the tidal flow by pinching any more than you change the effect of the pressure of still water by pinching.


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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 11:52am
I think the 'ferry glide' analogy is the best explanation, it seems to explain the 'being pushed to windward' bit very neatly.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 12:25pm
I don't.

A ferry glide is when in a channel with flow, you face the current and small alterations in heading, whilst maintaining a speed equal to the current will see you move transversely across the river. It creates the optical illusion of the boat moving sideways through the water. Of course it is only moving sideways relative to the land and the boat is still having to propel itself through the water to achieve this. Through the water the boat is still moving in the exact direction it is pointing and that's forward. I think it's fine to think in this land based reference when docking a boat because you're approaching something fixed to the land. It's the same reason why you always approach a mooring from down current. 

But, it's not helpful term for racing. This is the exact phenomenon that H2 describes, and where many people get tripped up. Boats appear to move sideways at a greater rate than heir heading suggests they should, making them appear to 'slip'. When combined with shifts when sailing against the current they seem exaggerated when framed against the land. 

But, this is of no help at all when trying to get a boat around a course quicker. And it still gives no reason, what so ever, why pinching up high might help you get to the next mark quicker or increase apparent wind, or alter flow on the foils, or anything that mythical lee bow purports to do. 


Edited by mozzy - 18 Sep 19 at 12:40pm
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Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 12:48pm
Yes I am describing Ferry Gliding - exactly - and in the example I gave a small lift allowed some of the fleet to ferry glide to windward whilst the rest of the fleet did not see any windward lift due to the few degrees of wind shift not being uniform across the course and the result was those that benefited did so to a greater degree than the windshift would have warranted.

I do not subscribe to views on apparent wind or anything else mythical but to me it is just really obvious that boats sailing to windward with the same wind strength so they are moving through the water at the same speed all other things being equal then those that get a "ferry glide" lift off their foils will get pushed to windward and that this will result in a better VMG to the windward mark. I do agree that it happens rarely because it needs all these factors to stack up and it is not something that I go out and try and make happen but every so often conditions are such that it can and does happen. That is my experience and I have seen it happen and everyone in the fleet knew it had happened as we all talked about it for weeks afterwards!


Edited by H2 - 18 Sep 19 at 12:48pm
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 18 Sep 19 at 12:55pm
I wonder if the roots of the myth, as set out in the seminal early works of dinghy tactics, can be found in tidal shifts in the apparent wind.

Imagine 3 boats sailing on starboard tack into current which is directly against their average close hauled course. They all achieve the same VMG through the water despite sailing different angles.

Footing Fred has the tide on his weather now. Average Andy sails straight into the current and pointing Pete sails high.

Understanding/accepting tide as a conveyor means we know that there is no difference in their VMG over the ground if their VMG through the water is equal.

But when they encounter more tide (from the same direction) they all also experience s lift. The same lift. But pointing Pete looks like he gains most and Footing Fred loses most. Actually, if the wind doesn’t shift again and the tide doesn’t change then those are real gains and loses.

Of course the opposite is true if the tide lessens. But we tend to focus on the lessons of success not failure. I can thus see how sailors might perceive empirical evidence for the Lee bow myth.

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