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Debunking Dodgy Sailing Theories
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G.R.F. View Drop Down
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    Posted: 20 Oct 11 at 10:27pm
Originally posted by Oatsandbeans

No I haven't - fill me in please!

Well without getting over complicated both work in the same way, the flow over the surface needs to be exhausted, so it is the same as the propellor blade in water, the sail in air, both fluid flow over a foil generating a force propelling a boat forward.

Sails generally were slow acting foils, as speeds have increased, apparent wind increases, foils have twisted more and become higher in aspect ratio.

For a while windsurfing drove that increase in twist, but it has passed on to high aspect cat foils and even fixed wings have to have twist in order to function.

Old school dinghy sails had some twist but they were pin heads, so getting 'twist' used to be a complicated affair. With the advent of higher aspect (fatter headed) sails the twist is easier to dial in.

But twist is there for more than just the perceived variation of wind speed at different heights, that's another old wives tale. The primary reason is to increase the speed of the flow by exhausting without turbulence at the leech or tip vortices (like you try not to get with glider wings).


Edited by G.R.F. - 20 Oct 11 at 10:28pm
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Xpletive View Drop Down
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Posts: 320 		Post Options Post Options   Quote Xpletive Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:50pm
Coincidentally, I am one of those who have an appalling understanding of what aspect ration is.... and, for that matter, what effect it really has.......I am humbly aware of how evident this may be in my posts.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:44pm
Aspect ratio, what it is and what effect it really has strikes me as generally poorly understood.
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Xpletive View Drop Down
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Posts: 320 		Post Options Post Options   Quote Xpletive Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:37pm
Originally posted by Menace

..... a venturi effect accelerating flow through the slot gap
 
Reminds me of a little known theory regarding the flight of birds actually being related to the discharge of wind from the anus. The wing flapping bit is to get as far away from it as possible.
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Posts: 296 		Post Options Post Options   Quote Menace Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:09pm
Another load of crap, even pedalled by the RYA guys from time to time is that the jib causes a venturi effect accelerating flow through the slot gap. It's the other way round, creates an area of high pressure, hence the reason you get backwinding in the main when the slot is wrong and the wind is up. What effectively happens is the slot increases the stall angle of the main, allowing the main to operate at higher lift coefficient.
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Posts: 3011 		Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 9:07pm
Originally posted by G.R.F.

Never studied the action of power boat propellors then OaB?


Power boat propellors have twist in the blades because for a constant pitch distance (the distance the prop would move forward with no slip in one turn) the pitch angle varies with the radius along the blade. The tip moves forward say 1ft in 1 metre of circumference, halfway alon the blae, it moves forward the same distance but runs through half the circumference, hence it must be at a steeper angle.
But it's made more complicated by other effects I'm sure.
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Posts: 1151 		Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 12:43pm
He's one ... the V Turd will be faster round the cans than a Laser  Wacko
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 11:47am
Originally posted by Oatsandbeans

that the benefit of having twist in sails is so that in instable wind conditions at least one section of the sail is set correctly.  Well it is surprising how many sailors hold this view. 

Well here goes- Rubbish-


Why do you think one precludes the other?

Twist can do a lot of things for you.

Yes, twist to deal with wind sheer is the most important thing most of the time, but that's not all...

An overtwisted rig is more tolerant in highly unstable winds when its impractical to have the sail sheeted correctly and correct twist set all the time.

Twist is also a tool in the armoury for depowering the rig, especially if you have a fully battened rig.

For example I sail on a raised reservoir, so the wind is very unstable on the final approach to the windward mark. I find it faster to release some kicker and overtwist the sail for the last few yards of the beat.
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 11:27am
No I haven't - fill me in please!
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G.R.F. View Drop Down
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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 20 Oct 11 at 10:48am
Never studied the action of power boat propellors then OaB?
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