P.S. Rightly or wrongly, my understanding is that the loose leech exhausts the air flow backwards and upwards from the main body of the sail, to allow that part to develope more thrust. If it didn,t do so the sail would 'lock up', a feeling only too well known to those of us who were ever caught out by a sudden blow in old sail days, and had to fight our way back in.

In other words, a tight leech sail is akin to shoving half a potato up the exhaust pipe of my Bonneville. It wouldn,t break any speed limits then!

Every dog has his day etc
So KA steps up to the plate and has a go.. good.

Barry Spanier discovered the RAF sail serendipitously, now a bit of science might help.
But he says "Marketing is what debases it all" in his interesting historical speedsailing stuff:
http://www.mauisails.com/barry_corner.php
Fins as thin as paper, lines a meter long, slotted sails like the America Cup carbon, all been done before..

And you mate!

wildblue said...

A ws sail's closest aeronautical equivalent is a propeller blade, they have major twist. The twisted sections supposedly still make useful lift, but don't overpower the rider and keeps the COE low. Used to sail a 1997 NP V8. Lots of low end range, but it was a crazy high COE catapult machine downhauled and overpowered.

Propeller blades are not a good comparison to sails. The large change in blade angle on a prop is due to the fact that it goes around in circles, thus the blade velocity at the prop tip is typically 4 to 5 times faster than at the prop hub.
Thus there is the absolute necessity to reduce the blade angle all the way along the blade to keep the angle of attack constant all the way along it.

Coming back to sails then, the effect of having the top of the sail "spill wind" in the gusts is not just to reduce the force on the sail but it also acts to keep the centre of pressure (or centre of effort in the case of a sail) in the same place.
There is another thread going on this subject. www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=63881
A sail design which spills wind is tending more along the lines of a reflex aerofoil and thus by having the trailing edge spill air in gusts, it has the effect of counteracting the movement of the centre of lift which a wind gust would normally produce.
If they get things just right and if it is rigged exactly right, the two should negate each other resulting in the centre of effort remaining in the same place in a wide range of wind conditions. Thus the sail feels very "soft" or comfortable because you don't suddenly have huge forces on your back hand or front hand.

A sail is just a wing mounted vertically. All the same principles apply to sails as low speed aeroplane wings.
There is heaps of info on the web on plane wings and the various types of aerofoils if you want to look it up.
You could start with a quick look at wikipedia on airfoils en.wikipedia.org/wiki/Airfoil
The old sails with the cams and battens are closely related to the "cambered" aerofoils.
The new sails with the loose trailing edge which spills the gusts are tending more towards the "reflex" aerofoils which are used in the flying wing design aircraft.
Have a look at reflex aerofoils en.wikipedia.org/wiki/Camber_(aerodynamics)
No point in reproducing it all here. If you're interested, have look. ( Not now! This is WORK time. Get back to work! )

I thought part of the reason for the twist off of the sail, (when under constant load) is to allow it to maintain a constant angle of attack. The wind 4 metre from the ground will be travelling faster than the wind 1 metre from ground. Thus the apparent wind vector will be different. This is certainly true for big sail boats, perhaps not so much for wave sails. I thought the other reason and most important was so that when a gust hits and the masts bends the loose leech allows the main panels lower down to keep there existing shape, so the COE stays were it is. If it was tight the bend would distort the main panels of the sail, and the COE would move abrumptly making life fun...

Just a thought.