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Debunking Dodgy Sailing Theories

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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post 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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RS400atC View Drop Down
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 8:19am
Originally posted by G.R.F.

[...
....
But twist is there for more than just the perceived variation of wind speed at different heights, that's another old wives tale. ....


Only in your world!
In the real world, we can measure the different windspeed (and direction) as a function of height. Its significance can vary, but sometimes you want a lot of twist for this reason.

Or were you taking the view that old wives sometimes knew a thing or two?
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2547 View Drop Down
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Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 9:22am
Originally posted by G.R.F.

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.


yeah ... after no doubt millions spend researching the matter look how heavily twisted they are running ...


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rogue View Drop Down
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Post Options Post Options   Quote rogue Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 9:47am
Seamonkey- it's a cat, it doesn't count, cats are not tactical, they are in GRF's words, "gehey"

of course, if you close the bow up and form the hull from one piece of plastic, then you get a tunnel-hulled scow... these of course are really cool.

Edited by rogue - 21 Oct 11 at 9:48am
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Post Options Post Options   Quote JohnW Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 10:39am
Originally posted by seamonkey

Originally posted by G.R.F.

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.


yeah ... after no doubt millions spend researching the matter look how heavily twisted they are running ...



I think if you do the vector maths, the twist required would be less when you have a large apparent wind component which is probably why cats such as the ACs dont appear to need much twist in their wings.

(EDIT: Im not a cat sailor, nor have I sailed with a wing sail - just considering the mathematical point of view which as we all know is often not as simple as we think).


Edited by JohnW - 21 Oct 11 at 10:42am
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Post Options Post Options   Quote fudheid Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 10:41am
Originally posted by Menace

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.

this is an all time favourite of old dudes on 4ksbs
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Post Options Post Options   Quote JohnW Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 10:51am
How about this old chestnut:

"the fastest way to round the leeward mark is 'wide in tight out' "


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Post Options Post Options   Quote fudheid Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 11:03am
Originally posted by JohnW

How about this old chestnut:

"the fastest way to round the leeward mark is 'wide in tight out' "


not the fastest but the best way, inside track upwind.....
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2547 View Drop Down
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Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 11:08am
Originally posted by rogue

Seamonkey- it's a cat, it doesn't count, cats are not tactical, they are in GRF's words, "gehey"

of course, if you close the bow up and form the hull from one piece of plastic, then you get a tunnel-hulled scow... these of course are really cool.


Ah ... that would be a great idea ... then it would slap up & down on the chop to slow you down whilst the cats go creaming past ...
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I luv Wight View Drop Down
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Post Options Post Options   Quote I luv Wight Quote  Post ReplyReply Direct Link To This Post Posted: 21 Oct 11 at 11:23am
What is planing? and how do you define it?

http://www.bloodaxeboats.co.uk
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