Actually, it takes energy to create a vortex. The gain in efficiency by
reducing the vortex is a decrease in apparent drag. Sometimes the methods
used to quench the vortex use more energy than the vortex itself.


"Wally" wrote in message
...
Thom Stewart wrote:

I'll give it a try. Wally, the shelf is made of a very stretchy
material made to fill the opening at the bottom of the main when the
sail is trimmed to have a deeper draft. The theory being that the
separation between the foot and the boom will be a cause for the air
to form a vortex and lower the efficiency of the main. It is supposed
to act like the fins on the tips of aircraft wings.

I see. Is it the vortex itself that reduces efficiency? I'm wondering if
it
has something to do with a loss of pressure near the foot of the sail, due
to it spilling out of the bottom.


Now, since the foot of the main is of flexible material and the free
footed main can carry the trim to the very bottom of the sail ( Sails
with there foot attached to the boom actually lose their trim at the
bottom of the sail) The possibility of the Vortex was eliminated by
putting a very light material that wouldn't have enough force to
effect the curve of the foot of the sail but would send the vortex
off the back of the sail rather than under it.

Does the shelf 'inflate', so to speak?


The shelf had to be made a material that would pull even with the foot
of the main when the main was flat. If it didn't that would have cause
its' own eddies.

I hope I haven't screwed you up but maybe at least you will be able to
see that there isn't any force on the center of the boom that could be
considered support.

I didn't think it was there for support (the photo of Neal's shelf foot
shows that it isn't set up to do that), just wondered what it was for.


Wally, I'm very surprised at how many in this group have no knowledge
of what the main shelf is.

I read somewhere that it also helps in light wind.


--
Wally
www.makearatherlonglinkthattakesyounowhere.com
Things are always clearer in the cold, post-upload light.