Select to expand quotePacey said..yoyo said..Ben1973 said
I think there's a fair bit we could do to reduce rider drag. A wetsuit is the perfect suit to fit some fairings under.
Yet surprisingly despite all the effort and cost spent on sails, boards and fins no one seems to have made much effort on this front. One guy at Luderitz had a slightly streamlined vest (weight under arms instead of on back).
But in case you are thinking of sticking bits of shaped foam under a baggy wetsuit be warned. It is a good way to drown. I know 
When your legs are buoyant it is almost impossible to keep your head/mouth above water.
If Barney is correct that the highest speeds are achieved when the apparent wind angle is 90 degrees to the direction of travel, then drag on the rider is just going to increase side force, rather than adding significantly to the total drag of the system. So skin suits and body fairings are not going to have much effect.
It's when true wind is at 90 degrees to apparent wind. The apparent wind will still be an angle beta off the nose, and contributing to drag.
en.wikipedia.org/wiki/High-performance_sailing
Ian, is that diagram correct for his planing speeds in strong winds as a speed to true wind ratio of 1.2:1?
The course theorem makes the assumption that the lift to drag ratio of the aerodynamics (includes sail plus all windage on hull and rider) remains constant for all wind strengths. At different angles in a constant true wind though the apparent wind will be of different strength. Your sail with your weight may have best lift to drag at say 20 knots of apparent wind but it drops off in efficiency at 25 knots apparent wind . If the "90 degrees" coincides with the 25 knots apparent wind you may not get your best speed there. Same assumption for the hydrodynamic lift drag of the hull/fin combination.
So maybe your best bet, if you have the luxury of sailing fast in any direction on a flat course (Primbee, L George), is to see if you're sailing broad or tight to the theoretical 90 degrees when you've determined your best angle by feel or beeping GPS. If you're going broad of 90 degrees try and tune up with a smaller sail and run tighter. If you're tight of the 90 degree optimum tune up the next larger sail and run broader. You'll need pencil, paper, calculator all the anemometers etc.
But there's no assumption in the theorem on overall efficiency, works in no particular order for ice boats, AC72s, kitefoilers , moths, windfoilers and slappers .
Thanks Ian.
There is something about this that does not ring true to me. I cant get my head around it but it is a feeling that as efficiency increases and speed to true wind increases, one should than be able to sail a broader course to true wind as apparent wind will comes around further towards your course. (more from ahead of the boat).
But MI was doing 50 knots in barely more than 25 knots of wind, and at what appeared to me to be a lesser angle off the true wind than what us windsurfers did. I seem to remember them throwing around the 118-120 degrees number.
Clearly, they were able to sail much more efficiently at a close angle to the wind.
We windsurfers need significantly stronger wind (30kts) to go 40 knots, and we seem to be at a significantly broader angle to true wind (130-135 degrees)
In my mind, that means the Beta angle will be larger. If I understand it correctly (A big IF! 
) that means the apparent wind would be closer to 90 degrees from our actual course than 45 degrees on the nose.???
And doesnt the article linked above say as much??
Select to expand quotesailquik said..
In my mind, that means the Beta angle will be larger. If I understand it correctly (A big IF! ) that means the apparent wind would be closer to 90 degrees from our actual course than 45 degrees on the nose.???
And doesnt the article linked above say as much??
Yes the beta is larger if your best speed is further off the wind. There's only 180 degrees in a right angle triangle. If one of the spare angles goes up the other must go down. More efficient craft will go fastest at tighter angles.
But as the wikipeadia article says a lot of the most efficient craft are designed with VMGs in mind rather than absolute speed. They would probably choose not to sail efficiently in that direction for fear that the craft might explode. They'd keep that small but constant beta in the corner of something other than a right angled triangle!
( en.wikipedia.org/wiki/High-performance_sailing
must be a recent inclusion on Wikipedia, I'm sure it didn't come up the last time I googled this subject. It's pretty good. )
Select to expand quotesailquik said..
But MI was doing 50 knots in barely more than 25 knots of wind, and at what appeared to me to be a lesser angle off the true wind than what us windsurfers did. I seem to remember them throwing around the 118-120 degrees number.
Yes if they get best speed at 120 degrees then beta must be best guessed at 30 degrees. Then Vt/Vy = Sine 30 degrees = 0.5 . So yes they'd expect to be going twice wind speed.
Edit. I am impressed that back in the day a yachtie doodling around with all this was aware of this quirk of geometry and the circle. The circle has nothing directly to do with sailing. With any line BC the loci of points of all the triangles with that angle beta is a circle!
Like putting a loop of string around a pencil and a stick in the ground. It draws a circle.
In this case you'd get a piece of 2.4 by 1.2 ply from Bunnings and cut it in half along a diagonal. Put two sticks in the ground say 600 mm apart. But any distance less that 1.2m will do. Attach a pencil to the sharpest point on one of the plywood triangles you have and swivel it around while poking it hard between the two sticks. Bingo you'll draw a circle! (Well most of a circle) In this case beta is atan 1.2/2.4 = 27 degrees.
The apparent wind angle (beta) is typically 18 degrees off the nose for the standup boards with a windsurfer rig and between 11 and 14 degrees off the nose for my quad miniyacht with a cantilevered mast. The sensitivity of the impellers have a cosine response (relative unit response along the axis of rotation and zero at right angles to the rotating shaft). The rotating shafts are orientated +/- 45 degrees relative to the board or yacht centreline.
I forgot to mention that the board is positioned such that the wind is blowing parallel to the starboard impeller shaft in the above photo. The impeller is rotating at its max (rpm/knot) sensitivity. The port impeller is not rotating.
The board is not moving so that apparent wind is equal to the true wind.
Select to expand quoteIan K said..sailquik said..
But MI was doing 50 knots in barely more than 25 knots of wind, and at what appeared to me to be a lesser angle off the true wind than what us windsurfers did. I seem to remember them throwing around the 118-120 degrees number.
Yes if they get best speed at 120 degrees then beta must be best guessed at 30 degrees. Then Vt/Vy = Sine 30 degrees = 0.5 . So yes they'd expect to be going twice wind speed.
Edit. I am impressed that back in the day a yachtie doodling around with all this was aware of this quirk of geometry and the circle. The circle has nothing directly to do with sailing. With any line BC the loci of points of all the triangles with that angle beta is a circle!
Like putting a loop of string around a pencil and a stick in the ground. It draws a circle.
In this case you'd get a piece of 2.4 by 1.2 ply from Bunnings and cut it in half along a diagonal. Put two sticks in the ground say 600 mm apart. But any distance less that 1.2m will do. Attach a pencil to the sharpest point on one of the plywood triangles you have and swivel it around while poking it hard between the two sticks. Bingo you'll draw a circle! (Well most of a circle) In this case beta is atan 1.2/2.4 = 27 degrees.
Beta is the performance metric for sailing - the faster you go for a given wind speed - the smaller is Beta. This is exactly the reason why I suggested that speed sailors use a bit of string to measure Beta.
The locus of point A is a circle, if and only if, Beta is a constant. Beta is approximately constant in light winds when your speed is limited by drag. Most of the time, however, your speed is limited by stability. Read my paper at AYRS.org
Beta remains the sailing performance metric for all points of sailing, however, even when it is not constant for different headings.
Just plot the true wind, apparent wind and board speed triangle to see what happens when you increase the board speed while keeping the true wind constant.
Select to expand quotebarney831 said..
The apparent wind angle (beta) is typically 18 degrees off the nose for the standup boards with a windsurfer rig and between 11 and 14 degrees off the nose for my quad miniyacht with a cantilevered mast. The sensitivity of the impellers have a cosine response (relative unit response along the axis of rotation and zero at right angles to the rotating shaft). The rotating shafts are orientated +/- 45 degrees relative to the board or yacht centreline.
The bottom line is 'you can't beat a boat' and the stability speed limit is reached well before the drag speed limit.
Or you hit a drag speed limit because of body form drag on the stand up, of which you have far less in the presumably 'body position reclined' land yacht.
But I see your point, that if we want to measure efficiency, measuring beta angle would be a way to do it. Unfortunately, so far I see no practical way to do that, on a windsurfing speed steup, in the conditions we need to.
Sailquik said:
"Or you hit a drag speed limit because of body form drag on the stand up, of which you have far less in the presumably 'body position reclined' land yacht."
It has nothing to do with whether you are standing up or not although the drag limit is more severe on my standup board.
The drag limit for my quad mini is shown above by the locus of the plus signs. The solid blue lines are the stability limits imposed by capsize. You cannot go faster than the blue lines on any heading without turning over - and must ease the sheet to prevent it.
Select to expand quotebarney831 said..
Sailquik said:
"Or you hit a drag speed limit because of body form drag on the stand up, of which you have far less in the presumably 'body position reclined' land yacht."
It has nothing to do with whether you are standing up or not although the drag limit is more severe on my standup board.
The drag limit for my quad mini is shown above by the locus of the plus signs. The solid blue lines are the stability limits imposed by capsize. You cannot go faster than the blue lines on any heading without turning over - and must ease the sheet to prevent it.
Go broader?
How did you come to those 'drag limits?
Pretty well all fixed mast sailing craft have a stability limit, except of course SailRocket. Longshot was pretty amazing as well with active roll control, but of course, it too would eventually reach a limit.
'Turning over'. do you mean the 4 wheel skateboard type land sailer rails up and rolls over?
That not an issue we have with windsurfers.
I don't inderstand your proposition that it is 'stability limit' that windsurfers reach??? Un;ess of course, by that you mean just the physical limit if how much power a windsurfer can balance against his body weight??
I have some experience with this 'windsurfing 'Land Sailer my friend Mick and I produced back in the '80's The speed limits of this craft was definitely sailor bravery. It was a matter of how fast do you want to hit the hard sand when it goes bad. After some nasty wipeouts, we came to the situation where we only sailed it in very light winds where we had a fair chance of stepping off and running if it went bad. Some other young brave loonies went very fast though! 
I would not do it now without full body protective gear and a good helmet! 
What does my head in are the creases, I get that twist works, even if (and this thread backs it up) no one is EXACTLY sure why, but from my dingy sailing days creases are air flow killers, and even the fastest guys at Luderitz have sails with massive creases. Im no expert but in a game that is now getting down to hundredths of a second surely this is an issue? How could it be improved? Does it need to be? Am I just a neat freak?
Select to expand quotesailquik said..barney831 said..
Sailquik said:
"Or you hit a drag speed limit because of body form drag on the stand up, of which you have far less in the presumably 'body position reclined' land yacht."
It has nothing to do with whether you are standing up or not although the drag limit is more severe on my standup board.
The drag limit for my quad mini is shown above by the locus of the plus signs. The solid blue lines are the stability limits imposed by capsize. You cannot go faster than the blue lines on any heading without turning over - and must ease the sheet to prevent it.
Go broader?
How did you come to those 'drag limits?
Pretty well all fixed mast sailing craft have a stability limit, except of course SailRocket. Longshot was pretty amazing as well with active roll control, but of course, it too would eventually reach a limit.
'Turning over'. do you mean the 4 wheel skateboard type land sailer rails up and rolls over?
That not an issue we have with windsurfers.
I don't inderstand your proposition that it is 'stability limit' that windsurfers reach??? Un;ess of course, by that you mean just the physical limit if how much power a windsurfer can balance against his body weight??
Go broader?
Look at my plot!
The max measured speed (red circle) was obtained on the heading for max predicted speed (blue line).
Broader is slower!
drag limits?
I measured the drag using coast-down tests.
stability limits for Sailrocket?
I guess that you have never seen the photo of Sailrocket a hundred feet in the air - inverted!
I suggest that you study 'Airplane Performance, Stability and Control' by Perkins and Hage, but be forewarned - it is heavy on math.
'Turning over' refers to my quad snow-ice-land mini-yacht. I guess I should have said 'capsize'.
Windsurfer stability limit?
The sail is held up by the weight of the sailor hung out off the boom as far as possible - thus the advantage nature gives to big, heavy sailors. This restoring moment is constant so the sailor must sheet out in gusts - or the sail must twist off in gusts to lower the centre of effort - or catapult! Drag reduction due to twist, if any, is secondary.
Twisty sails are not a windsurfer invention. They were copied from dingy racers who didn't like to swim or to bail.
Select to expand quotebarney831 said..
stability limits for Sailrocket?
I guess that you have never seen the photo of Sailrocket a hundred feet in the air - inverted!
That was Sailrocket 1, with the pilot at the stern. Sailrocket 2 had the pilot near front and was a very different beast in terms of aerodynamic stability.
Barney said:
stability limits for Sailrocket?
I guess that you have never seen the photo of Sailrocket a hundred feet in the air - inverted!
That was the original Sailrocket which had several design flaws one of which was to link the angled wing to the forward planning hull thereby increasing the AoA of the wing (hence the lift) when the forward hull got lifted by the wing. Ergo, backflip at speed.
Needless to say, they they went back to the drawing board, built a new boat called Vestas Sailrocket, eliminating those flaws and linked the wing to the rear hull so any lifting depowered the wing. This is the boat that smashed the record and generally is regarded as "the" sailrocket by the sailing community and is the one Andrew refers to not the one you noted was good for backflips and other problems.
Exactly as pacey and YOYO said.
And I was referring to heeling stability. Sailrocket2 is the only succesful craft I know of which has no heeling stability issue.
Barney. Have another look at your plot. If you go broader, you could have gone faster and had more stability. By going broader you decrease your apparent wind slightly, and therefore your heeling moment, and increase the forward vector thrust. Methinks you are too blinded by your sums.
Stability limits for windsurfers are not as you imagine it. It is extremely rare to see a windsurfer on a speed course simply get catapulted by being overpowered. I dont think I have ever seen it. It may be because they sheet out a little, but that most likely increases thrust in a vector closer their course, increasing speed. On a free course, we simply go broader, and faster. The only times I can remember running out of stability were on the tight run up where I have had to feather the sail to stay on the water until I can get broad enough to fully sheet in. One memorable occasion was around 2007-2008 when I was physically lifted off the water at over 30 knots on the square run in by a huge gust, and flew for 15meters under the sail like a hanglider. It fair knocked the wind out of me when I finally landed on my back. . That just isn't going to happen on the best speed angle, or at least it is very hard for me to imagine it, and I have never experienced or seen anything close to it.
On a fixed angle course where the wind is too tight, there are issues with being lifted off the water, but speeds will be lower anyhow..
Nobody here has claimed that windsurfers 'invented' sail twist, that would be ridiculous. But I dont think any windsurfers consiously 'copied' dingy sailors either. It just evolved in windsurfing to counter particular issues related to that platform. And of course there is always crossover of ideas between all the forms of sailing.
Select to expand quotesailquik said..barney831 said..
Sailquik said:
"Or you hit a drag speed limit because of body form drag on the stand up, of which you have far less in the presumably 'body position reclined' land yacht."
It has nothing to do with whether you are standing up or not although the drag limit is more severe on my standup board.
The drag limit for my quad mini is shown above by the locus of the plus signs. The solid blue lines are the stability limits imposed by capsize. You cannot go faster than the blue lines on any heading without turning over - and must ease the sheet to prevent it.
Go broader?
How did you come to those 'drag limits?
Pretty well all fixed mast sailing craft have a stability limit, except of course SailRocket. Longshot was pretty amazing as well with active roll control, but of course, it too would eventually reach a limit.
'Turning over'. do you mean the 4 wheel skateboard type land sailer rails up and rolls over?
That not an issue we have with windsurfers.
I don't inderstand your proposition that it is 'stability limit' that windsurfers reach??? Un;ess of course, by that you mean just the physical limit if how much power a windsurfer can balance against his body weight??
I have some experience with this 'windsurfing 'Land Sailer my friend Mick and I produced back in the '80's The speed limits of this craft was definitely sailor bravery. It was a matter of how fast do you want to hit the hard sand when it goes bad. After some nasty wipeouts, we came to the situation where we only sailed it in very light winds where we had a fair chance of stepping off and running if it went bad. Some other young brave loonies went very fast though! I would not do it now without full body protective gear and a good helmet!
I have some questions about this landsurfer. Because my questions have nothing to do with sail twist I has started another thread on the Landsailing forum.
That looks like a constant entry angle all the way up the mast. I'm thinking now this is a big factor.
Select to expand quotedecrepit said..
That looks like a constant entry angle all the way up the mast. I'm thinking now this is a big factor.
I agree Mike.
I am also noting less twist in the winning PWA foiling sails, as well as higher aspect ratio than most slalom sails, which makes sense from what has been discussed here.
Select to expand quotesailquik said..decrepit said..
That looks like a constant entry angle all the way up the mast. I'm thinking now this is a big factor.
I agree Mike.
I am also noting less twist in the winning PWA foiling sails, as well as higher aspect ratio than most slalom sails, which makes sense from what has been discussed here.
Andrew, I would be interested in your quantitative estimate for wind speed and the speed of Charles' board shown in the photo above? You may have more experience at estimating wind speed from surface conditions than anyone I know. I have written down my guess for wind speed and board speed - but I'd like to see how close they are to your estimates. Thanks
Select to expand quotebarney831 said..sailquik said..decrepit said..
That looks like a constant entry angle all the way up the mast. I'm thinking now this is a big factor.
I agree Mike.
I am also noting less twist in the winning PWA foiling sails, as well as higher aspect ratio than most slalom sails, which makes sense from what has been discussed here.
Andrew, I would be interested in your quantitative estimate for wind speed and the speed of Charles' board shown in the photo above? You may have more experience at estimating wind speed from surface conditions than anyone I know. I have written down my guess for wind speed and board speed - but I'd like to see how close they are to your estimates. Thanks
My two cents......Sail size about 8.5 to 9.0 Wind speed 12-15kn board speed 20-23kn??????
Select to expand quotechoco said..
Haha! People need more sense of humour like you choco!
Select to expand quotebarney831 said..
Andrew, I would be interested in your quantitative estimate for wind speed and the speed of Charles' board shown in the photo above? You may have more experience at estimating wind speed from surface conditions than anyone I know. I have written down my guess for wind speed and board speed - but I'd like to see how close they are to your estimates. Thanks
I would not be confident to judge. I dont sail much in those conditions or with larger sails like that and dont know how big the sailor is. Photos, in my experience are notoriously difficult to judge wind srength from. Wild guess on wind would be around 15 knots or slightly less. No idea of board speed or sail size.
Select to expand quotesailquik said.. >> I would not be confident to judge. I dont sail much in those conditions or with larger sails like that and dont know how big the sailor is. Photos, in my experience are notoriously difficult to judge wind srength from. Wild guess on wind would be around 15 knots or slightly less. No idea of board speed or sail size.
Exactly what I was thinking. But at very wild guess, I'd put board speed in the low 20s
Select to expand quotesailquik said..barney831 said..
Andrew, I would be interested in your quantitative estimate for wind speed and the speed of Charles' board shown in the photo above? You may have more experience at estimating wind speed from surface conditions than anyone I know. I have written down my guess for wind speed and board speed - but I'd like to see how close they are to your estimates. Thanks
I would not be confident to judge. I dont sail much in those conditions or with larger sails like that and dont know how big the sailor is. Photos, in my experience are notoriously difficult to judge wind srength from. Wild guess on wind would be around 15 knots or slightly less. No idea of board speed or sail size.
Here is my take on that photo. There appears to be a wind line about a hundred metres in front of the board. Beyond the wind line my guess at wind speed is 15 knots. Where the board is, I estimate that the wind speed is about 12 knots. My guess at board speed is about 10 - 12 knots. It looks like the sailor has a gps on his arm so he should know the board speed but I very much doubt that he is exceeding the wind speed. My guess is +/- at least 2 knots.
Thanks to all who made a speed estimate.
Here is another photo (very old, predates gps) that I would like some estimates of wind speed and board speed. The sailor was 5' 10" and weighed 163 lbs. Sail was a 6.3 m sq Gastra.
At a guess Charles pic, wind 12kts board 15-18kts. Check how it is planing nose up n in straps.
2nd pic id think board is doing 20-23kts. Wind 15-18kts.
Is there a purpose or a point to this guessing? 
It's very hard to estimate wind strength from photos as I have said.
Estimating wind strength when standing in it and watching the signs live is very different. 
But to humor you : Sails from that era and that size had a very small wind range. I would guess they were out of their controlable range at around 15 knots of wind, so probably less than that. The appearence of the water is not much help as it could be behind a sand bar. There are no signs of cresting wavelets. If not behind a sand bar, it is very light wind. The board is on a broad reach from the look of the wavelets, and there is a bit of spray off the rails, so my guess is 'maybe' up to 20 knots of speed, but most likely high teens.
Here is a good example of what I mean:
Check how flat the water is! Not many clues there. But looks quite fast eh? When you have had a guess at the sail size, wind strength and board speed, I am sure Dezza can fill us in on the exact numbers.
