Basher said..
I think I've already broken down this issue in my earlier itemised post, but there are a lot of comments here about 'wind shear' which seem to be a bit confused.
Let's try and clarify this, because there are two definitions of that term, one which is relevant, and the other which is not. The goal issue here is still why we want twist in our sails.
1) There is a theory where the wind at ground and sea level is streaming at a different angle to the wind flow way up high, like at cloud height. And we can often see that difference when we feel wind on our face, and then the clouds above seem to be flowing in a different direction. This is true 'wind shear', where the wind at higher atmospheres is often travelling faster and in a very different direction than at sea level. There are a number of reasons for that differential, one of which is surface friction acting on the wind at ground and sea level, slowing it down. The wind directions are also affected at different heights in the stratosphere by changes in atmospheric pressure, with wind in low pressure systems heading towards the centre of the low, etc. The atmosphere above us is in fact a complex mix of different winds, sometimes with uneven wind flows - like a badly mixed cake has lumpy bits and sweeter bits.
2) But the other wind shear people talk about is where a simple differential occurs in 'apparent wind', with a different wind angle felt across the deck of the board (or boat) compared to the wind angle seen at the mast tip.
Apparent wind is what the sail sees and that's the wind angle when sailing along. Apparent wind is made up of the 'true' wind of the day and the 'created' wind which is down to the speed of the board. The true wind across the deck of a non-moving board is lighter in strength compared to the true wind higher up, say at mast tip level. You can test that difference in strength by measuring the wind at ground level, and then climbing a ladder to take another reading. But note that those two wind readings may well have the breeze flowing in the same or a similar direction - because the height difference is not that great.
The created wind however is purely down to board speed and that is the same at deck level and at the mast tip, and that wind flow is fixed in direction, parallel to the centre line of the board or flowing opposite to the direction of travel..
It's only when we combine these two, created wind and the true wind, that we get the wind strength and wind direction we call apparent wind. The ratio of created wind to true wind changes at the mast tip purely because the true wind is measured stronger up there. And that is why there is a different wind angle for apparent wind at the mast tip compared to below the boom.
So that, in turn, is why we set the sail with more twist at the head.
Sometimes it takes a while to get your head around this, although it's actually not that complicated.
It's no different for windsurfers and the various dinghies used, except where the board or boat speed varies, and where the craft are used in lighter or stronger true winds.
We won't learn much more about our windsurf rigs by looking at racing dinghies or yachts, as each craft has a different need for full or flat sails and for more, or less, sail twist. Each craft just develops the rig where, over time, competition shows what works best.
Note that the angle the craft sails to the true wind also makes a difference, because the true wind may flow at right angles to the board on a beam reach whereas the created wind still flows with the direction of travel. When we sail upwind the true wind is a bit more in line with the created wind flow, but on planing days we may still want sail twist where that allows us to dump excess power.
When not planing or when sailing in relative light wind, a windsurf or other craft may prefer a tighter leach to maximised power, so in those circumstances sail twist is less of a thing.
The key in all conditions is to be maximising drive and lift from the rig while minimising drag. And sail twist becomes a very useful factor in controlling that relationship.
Note that foiling dinghies (like Moths) that sail fast in light wind will of course use a lot less twist in their sails - simply because created wind plays a bigger part in the apparent wind make up. They can sail faster than the true wind of the day.
And I should know, given my competition past.
Yes, but while there is "wind shear" due to apparent wind, that would affect all rigs of the same height in the same way (assuming speed and angle were similar) - and yet rigs of the same height but on different craft have very different optimum twists even when sailing at the same sort of speed and angle. Therefore, wind shear seems to be only a minor reason for the greater twist in windsurfer rigs.
Yes, each craft has a different need for full or flat sails or leach twist. My point is that we CAN learn from that, because we can see how the different needs and ratios of those craft affect the optimum twist for the different craft.
Moths still use a lot less twist than boards even when at very high speeds, which indicates that the twist in windsurfer rigs is only about wind shear to a limited extent. I know your competition past and I also know and sail against guys like Landy (who won the same worlds, but in a boat he built and with a sail he built, and who then went on to win more worlds and an Olympic medal) and Steve S, Mark T, and Emmett L who all won multiple worlds in the same class, and they all use much less twist than most windsurfer rigs when required, which shows that it isn't all (or mostly) about wind shear. Landy's world champ A Class sails may often sail at the same sort of speed and angles as a board and therefore would experience similar wind shear to a board, but he uses far less twist than a board much of the time and it's a quite different sort of twist.
I'm away from home so don't want to do the calcs, but it would be interesting for you to run the numbers to show how much the apparent twists over the 5m or so of a windsurfer rig.
I agree about most of your reasoning behind the twist we use; I just don't think that wind shear is responsible for the far greater degree of twist we use compared to other sailing craft because they experience similar wind shear and yet find that much less twist works better. Therefore different factors are at play in many respects, and we can learn from them.
Myself, Pascal Maka and Jeff Magnan
