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Chris 249 said..

mathew said..
Sail-trim is everything - be it big boats or little ones...It is not just "reducing power in the head" -> it is to dynamically-react-to-the-current-gust in a way which allows the sailor to maintain efficient control of the rig.In the case of windsurfing -generally speaking, you want to shed some of that power -> if you can keep yourself locked-in, then you can will go faster.
....

Sure, I agree that using leach twist for gust response works well, and I never said it didn't. I've gone to sailmakers for custom sails myself, and specified a bigger roach for better gust response.

What I'm objecting to is that TBT is claiming that twist has no other purpose. We often want it to allow for wind shear.

I'm actually fairly sure that TBT and Wing 11 are probably just the sock puppets of our aggressive wingsail maker, the one who insults and abuses all the other sailmakers.

I have read your posts - from my understanding of what you have written, it is not clear that you hinted that twist had anything to do with gust response. If you did explain that, I missed it completely. As is described in this thread - it is not clear whether wind sheer has much effect in windsurf craft... we suspect there is some, but we are not sure to what extent - indeed the MI boat with its tall mast, didn't need much. So any discussion of wind sheer, is largely just pontification.

One important note - AFAICT no one has clarified at what board-speed we are having this discussion. As you stated, wind sheer is less important as speed increases. There was mention of raceboards which means boat speed is topping out at say 20kn, vs say speedsailing at 45kn - the dynamic-twist will need to be different for the same-size-sail due to the bodyweight of the sailor (for the given windspeed) because the strength of the gusts/lulls will be significant.

I will state it more pragmatically - twist [in windsurfing] is for automatic power control - to reduce power during gusts. Twist does also enhance some other aspects - reduced drag at the mast-tip, wind sheer, etc - but dominant reason for it is for automatic response to wing-loading.

TBT, I apologise if I was incorrect about you being a sock puppet, but you throw around many of the same insults as our wingsail designer friend did and your style of posting is similar.

Oh dear !
Popcorn anyone ?

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Chris 249 said..
TBT, I apologise if I was incorrect about you being a sock puppet, but you throw around many of the same insults as our wingsail designer friend did and your style of posting is similar.

I was thinking the same ...

Its all fascinating stuff and probably mostly maths (or magic) but it would be awesome if we could see the vortices caused by sails like you can sometimes see those created by aeroplanes. I might just stare in to my morning coffee a little while longer.

^^^
Maybe sailing in thick fog ?

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gorgesailor said..

Chris 249 said..
TBT, I apologise if I was incorrect about you being a sock puppet, but you throw around many of the same insults as our wingsail designer friend did and your style of posting is similar.

I was thinking the same ...

^^^ Ditto, since the first week of TBT n Wing11 posts. Gone from beginner to sail design technician in a week or so. Think im reading an MW inspired write up.

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mathew said..



I have read your posts - from my understanding of what you have written, it is not clear that you hinted that twist had anything to do with gust response. If you did explain that, I missed it completely. As is described in this thread - it is not clear whether wind sheer has much effect in windsurf craft... we suspect there is some, but we are not sure to what extent - indeed the MI boat with its tall mast, didn't need much. So any discussion of wind sheer, is largely just pontification.

One important note - AFAICT no one has clarified at what board-speed we are having this discussion. As you stated, wind sheer is less important as speed increases. There was mention of raceboards which means boat speed is topping out at say 20kn, vs say speedsailing at 45kn - the dynamic-twist will need to be different for the same-size-sail due to the bodyweight of the sailor (for the given windspeed) because the strength of the gusts/lulls will be significant.

I will state it more pragmatically - twist [in windsurfing] is for automatic power control - to reduce power during gusts. Twist does also enhance some other aspects - reduced drag at the mast-tip, wind sheer, etc - but dominant reason for it is for automatic response to wing-loading.

Sorry, I didn't mention that twist plays a part in gust response because I thought we all accepted that.

As you say, wind shear varies with boardspeed and windspeed. The vectors indicate that the fact that M1 didn't have much twist would be related to the fact that she was a speed record machine and not something that sailed in conditions where wind shear was larger. Part of my issue with that TBT was saying is that he only seems to be considering high speed sailing when most windsurfers spend a lot of time when they are not overpowered and not going flat out.

From experience with other rigs about the same size as a windsurfer rig, and from sailing with quite a few world championship and America's Cup winning sailmakers, I feel it can be quite important in some situations. Perhaps the issue is that I'm thinking about racing in all conditions, where getting out of a light-wind patch to gain one metre in one race can separate the champion from the runner-up. When things get that close, paying a lot of attention to wind shear and twist are probably vital. For the sailor who goes BAFfing in medium and fresh winds, it's probably no big deal.

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Al Planet said..
Its all fascinating stuff and probably mostly maths (or magic) but it would be awesome if we could see the vortices caused by sails like you can sometimes see those created by aeroplanes. I might just stare in to my morning coffee a little while longer.

Watch this video from Primbee, at the 4:57 mark you can see a mast-top telltale effected by the tip vortex.

Nice..... Mr Curly goes windsurfing....had to watch it a few times . it looks like a trick.

Wow. Good observation to spot that jetlag.

It really is quite and interesting video to study the behaviour of different sorts of sails at speed. Some reasonably powered up.

For this sort of sailing there seems to be definite advantages in having the COE lower in the sail, despite probable losses to induced drag. Of course, one cannot take that to rediculous extremes, but the trial and error approach with high speed salom and speed sails design has definitly led us in that direction.

Likewise, twist in the upper part of the foil seems to help lower COE to enable the sailor to generate more power, and probably reduce induced drag from the tip.

I think the hard part is getting maximum, or just the right amount of dynamic twist for gust control. I still think many speed and high wind slalom sails struggle with optimising this. It is not difficult to speculate as to why. The variables are sailor weight, and to some extent boom height and mast differences. All these things can have subtle, but quite meaningful effects on dynamic twist. Every now and then I have fluked it pretty good, and I have definitely felt the difference, but even a different day and slightly different wind and water conditions can stuff up what you thought was the finding of the holey grail on your rig settings.

If you looks closely at the rigs in the video, you can see quite a range of dynamic response in the different sails and conditions.

Martin van Meurs ,nice explain 8:00 - 9:45

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Chris 249 said..

. Bjorn also sails very differently from the usual sailor - I know, I was his training partner at a world championships when he was a kid. Even around that time he used lots of twist, but that's because he sails in a very different way to most windsurfers.

Can you explain what is his style and what makes him diffrent then others,technically?

Do you maybe know how many years Bjorn and AA sail for NP together,did Bjorn beat AA at that time,which years was that?

I only noticed he allways keep harness lines far apart,approx 25cm,even on wave,salolm,speed etc,99% today keep them close together.

That was '85-'86, when Bjorn was a little kid (in '85 he seemed small for his age - he must have had a huge growth spurt) but already recognised as the upcoming star. Almost all of us had harness lines set that far apart at the time, I think. KA43 and Doug may remember the details.

I can't remember particular details of his style, partly because when the racing started he zoomed ahead of me, but the thing is that arguably he and others are so damned good that their sail trim should be different from the average sailor. Bjorn's strength and skill allowed him to carry a bigger sail than the average sailor. His skill means his apparent wind is higher and he's not going to slow down or fall off in gybes, so he can trim the sail more towards top speed than towards acceleration.

So Bjorn's settings for a 7m2 sail*, for example, could be aimed at carrying it in 22-27 knots of wind at a boardspeed of 25 to 32 knots and an apparent windspeed of 32-40 knots. However, when Joe Average uses the same sail, he may use it when the wind is puffy and 22 knots maximum. Joe's boardspeed will be be zero knots when he falls off every fourth gybe, three knots when he falls off the plane every second gybe, 12 knots in the lulls, and 20 knots top. Joe will also struggle in gusts that Bjorn will handle easily, and he won't trim the sail as efficiently. The apparent wind he sails in will be 12 to 27 knots. Joe's settings should be different from those of Bjorn - he's effectively sailing in different winds at different speeds.

So it used to be strange to see someone rigging a 5m Pryde to sail on a puffy day of 15-22 knots, when they were going to spend lots of time going slow and trying to accelerate, and then have another very average sailor come up and say "you've got to rig it like Bjorn rigs his 5m!" Bjorn wouldn't use his 5 in those winds, and if he was there that day he'd be sailing at different speeds with a different sail.

You can see the difference in one design classes. In Windsurfer One Designs, for example, the average sailor (who isn't a newbie, but someone who has been racing each summer week for most of their life) may want to carry two to three inches more downhaul some days than the top sailors. In Laser dinghies, sometimes there can be 30-45cm difference between the twist the top sailors carry and the twist a sailor in the middle of a club fleet should carry. It's a bit like the fact that I can't ride up the Alpe d'Huez using the same gears and cadence as Chris Froome does, and I shouldn't try.

* I'm just pulling those figures out of thin air to use them as an example; they aren't realistic.

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Chris 249 said..
That was '85-'86, when Bjorn was a little kid (in '85 he seemed small for his age - he must have had a huge growth spurt) but already recognised as the upcoming star. Almost all of us had harness lines set that far apart at the time, I think. KA43 and Doug may remember the details.

I can't remember particular details of his style, partly because when the racing started he zoomed ahead of me, but the thing is that arguably he and others are so damned good that their sail trim should be different from the average sailor. Bjorn's strength and skill allowed him to carry a bigger sail than the average sailor. His skill means his apparent wind is higher and he's not going to slow down or fall off in gybes, so he can trim the sail more towards top speed than towards acceleration.

So Bjorn's settings for a 7m2 sail*, for example, could be aimed at carrying it in 22-27 knots of wind at a boardspeed of 25 to 32 knots and an apparent windspeed of 32-40 knots. However, when Joe Average uses the same sail, he may use it when the wind is puffy and 22 knots maximum. Joe's boardspeed will be be zero knots when he falls off every fourth gybe, three knots when he falls off the plane every second gybe, 12 knots in the lulls, and 20 knots top. Joe will also struggle in gusts that Bjorn will handle easily, and he won't trim the sail as efficiently. The apparent wind he sails in will be 12 to 27 knots. Joe's settings should be different from those of Bjorn - he's effectively sailing in different winds at different speeds.

So it used to be strange to see someone rigging a 5m Pryde to sail on a puffy day of 15-22 knots, when they were going to spend lots of time going slow and trying to accelerate, and then have another very average sailor come up and say "you've got to rig it like Bjorn rigs his 5m!" Bjorn wouldn't use his 5 in those winds, and if he was there that day he'd be sailing at different speeds with a different sail.

You can see the difference in one design classes. In Windsurfer One Designs, for example, the average sailor (who isn't a newbie, but someone who has been racing each summer week for most of their life) may want to carry two to three inches more downhaul some days than the top sailors. In Laser dinghies, sometimes there can be 30-45cm difference between the twist the top sailors carry and the twist a sailor in the middle of a club fleet should carry. It's a bit like the fact that I can't ride up the Alpe d'Huez using the same gears and cadence as Chris Froome does, and I shouldn't try.

* I'm just pulling those figures out of thin air to use them as an example; they aren't realistic.

Odd didn't Barry say in Rig it Right Bjorn often doesnt use enough downhaul?

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mkseven said..

Chris 249 said..
That was '85-'86, when Bjorn was a little kid (in '85 he seemed small for his age - he must have had a huge growth spurt) but already recognised as the upcoming star. Almost all of us had harness lines set that far apart at the time, I think. KA43 and Doug may remember the details.

I can't remember particular details of his style, partly because when the racing started he zoomed ahead of me, but the thing is that arguably he and others are so damned good that their sail trim should be different from the average sailor. Bjorn's strength and skill allowed him to carry a bigger sail than the average sailor. His skill means his apparent wind is higher and he's not going to slow down or fall off in gybes, so he can trim the sail more towards top speed than towards acceleration.

So Bjorn's settings for a 7m2 sail*, for example, could be aimed at carrying it in 22-27 knots of wind at a boardspeed of 25 to 32 knots and an apparent windspeed of 32-40 knots. However, when Joe Average uses the same sail, he may use it when the wind is puffy and 22 knots maximum. Joe's boardspeed will be be zero knots when he falls off every fourth gybe, three knots when he falls off the plane every second gybe, 12 knots in the lulls, and 20 knots top. Joe will also struggle in gusts that Bjorn will handle easily, and he won't trim the sail as efficiently. The apparent wind he sails in will be 12 to 27 knots. Joe's settings should be different from those of Bjorn - he's effectively sailing in different winds at different speeds.

So it used to be strange to see someone rigging a 5m Pryde to sail on a puffy day of 15-22 knots, when they were going to spend lots of time going slow and trying to accelerate, and then have another very average sailor come up and say "you've got to rig it like Bjorn rigs his 5m!" Bjorn wouldn't use his 5 in those winds, and if he was there that day he'd be sailing at different speeds with a different sail.

You can see the difference in one design classes. In Windsurfer One Designs, for example, the average sailor (who isn't a newbie, but someone who has been racing each summer week for most of their life) may want to carry two to three inches more downhaul some days than the top sailors. In Laser dinghies, sometimes there can be 30-45cm difference between the twist the top sailors carry and the twist a sailor in the middle of a club fleet should carry. It's a bit like the fact that I can't ride up the Alpe d'Huez using the same gears and cadence as Chris Froome does, and I shouldn't try.

* I'm just pulling those figures out of thin air to use them as an example; they aren't realistic.

Odd didn't Barry say in Rig it Right Bjorn often doesnt use enough downhaul?

I have no idea. I really respect Barry Spanier, but champion sailors and their sailmakers can have different opinions. I know of "team" boats for sailmaking companies where the sail trimmmer (a sailmaker who won the America's Cup on Australia II) and the skipper (two time world champion and another sailmaker) set the sails differently to the way their company's chief said, and they won the nationals ahead of Olympians, world champs and America's Cup skippers. The top two guys in Australia's biggest sailmaking company used to set up and adjust the rig of the most popular yacht in Australia in different ways. One has been a top guy in about five America's Cup wins so he's not exactly a dill. In Lasers and Windsurfer One Designs, to name two classes where everyone uses the same gear, the top sailors can use different amounts of twist and downhaul to match their own preferences and sailing styles.

Maybe Bjorn was so good and so strong that he didn't mind if his back hand loaded up? Maybe he preferred having a deeper sail with more grunt (ie lift)?

How windsurfing twist was "invented"...
text from joewindsurf blog;

"I believe it was in the late 80's when Bruce Peterson of Sailworks now (Gaastra back then) discovered by accident the whole concept of the sail leech twisting off and opening up. This twisting allows sails to have much more range in that they do not become overpowered. The extra wind will "spill" out of the leech. This seems to allow a gradual powering up of the sail. Apparently, the leech also lowers the CoE or centre of effort of the sail. I call this the point of power or PowerPoint. When the CoE is lower, it is more stable, like a person with a lower centre of gravity is more stable for gymnastics or martial arts. It seems the CoE needs to align with the CLR/ centre of lateral resistance that runs off the fin. In order to do this with a lower CoE, the mast track had to move back on the board.

Older sails had more power, but less range and control. To cover this, "newer" sails became larger. Earlier the biggest sails were in the 8.x range with many cams. Now we are up to 12.x. I imagine that over 12.x it just becomes unmanageable in terms of human capabilities i.e. imagine uphauling such a sail? With the larger sails come the larger fins/foils. Larger foils require wider boards. Wider boards plane up earlier and do not need as much low end power. What I am trying to say here is: this twisting off of the leech thing had a major impact on the whole windsurf model. "