3rd factor is the top bending off to lee taking the leech with it and thus opening up the upper part of the sail and leech thus spilling air.

You want these 3 factor working in unison.

This is the reason why prebend wing masts have such a large range of (de) power levels.

Because you derotate a wing mast to loose power 3 things happen that reinforce another :

-1- You reduce the angle of the leading egde effectively reducing draft of the sail.

-2- You flatten the sail by use of prebend.

-3- You present to weak axis of the mast to the wind coming from the side thus pushing the (unsupported) top away to lee twisting off the top of the sail and flattening the top part even more.

Actually point 3 on skiff masts causes the top battens to invert (bending to windward) and this totally depowers the top.

End result is a rig that is totally depowered spilling air all over the place but with very little drag. In summary these rig both reduce lift (drive) AND drag by derotating the mast. These rigs are seen on A-cats, M20/M18, F18HT, Tornado's (I think) and Formula 16's. Ohh and the capricorn F18

Rigs that can not be set up with prebend (no-diamond wires) or that need the mast to bend over its weak axis to flatten the mainsail must rotate there mast further to depower. Making the top of the mast stand up. These rig reduce lift and drive but don't reduce drag. These rig are unsupported masts like Hobie 16's and Nacra 5.0's but also boats like Nacra 5.2 that have diamond wires but can't effectively produce prebend.

A 3rd way is the current F18 masts setup (excluding the capricorn F18) These are prebend rigs and they derotate first to depower but when it really starts honking they fully rotate to make use of the same principle as that described for Hobie 16's. So inititial these rig first depower by reducing lift and drag simulataniously but at a certain level they can only reduce power more when accepting to increase drag as well. It is my opinion that these rig are now seen with larger and larger square top sails in order to delay this switch over point. Hobie Tiger design is in my opinion as good example of this 3rd way.



A note on the side; I really took to wingmasts simply because the leech tension and mast rotation act like a pretensioned spring setup. By these two controls you can effectively set the point at which the top falls away WHILE sailing. It is a spring system that want two by at one of two extremes, roughly said. That is standing up when the sail force it to low to push it away OR falling away completely when the sail pressure is enough. Its reaction can be controlled very well. If the top falls away to quickly you just increase mast rotation a little bit and the leech tension will support the weak axis a little more thus setting the system to fall away at more sail pressure.

It is quite a neat system. If you really want to know invite yourself on a baot with a wingmast owned by a sailor who knows how to trim it. You'll be amazed how much control you have over such a rig.


I hope this helps

Wouter