Ian, is that diagram correct for his planing speeds in strong winds as a speed to true wind ratio of 1.2:1?

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??

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.

Ian 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.

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.

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?

barney831 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.

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.

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.

barney831 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??????

choco said..

Haha! People need more sense of humour like you choco!

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.

sailquik 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.

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.