Love a Sheep ) writes:

However, if the wind is aft of the beam on say the starboard side then
surely we have a choice where to set the sails ie they can be on the
starboard side (ie the boom is pointing to the starboard side) where
they act as an airofoil or on the port side where they act as a
'parachute' - is this right or am I missing something. If so which is
best?

When a boom gets much forward of 90 degrees to the centreline it's hard
to pull back in from behind, reducing your ability to control the sail.

I have small boomless sails on my tiny boats which I can let forward all
the way and rotate them right around the mast. I often let the sail go
forward of the mast on a run in the manner you suggest so it's partially
acting as an airfoil. The sail is much more stable in that position. Sails
are least stable when running downwind. Spinnaker sails work much the same
as my rotating sails when let out forward.

Even in the standard upwind airfoil position only 2/3 of the power of a
sail is the airfoil vacuum on the front of the sail. There is still 1/3 of
the power from the "parachute" pressure on the back of the sail. It's
never one or the other but always a combination of both. Even on a dead
run air is pushing around the edges of the sail making a vacuum on the
front side. (Actually I imagine a true parachute has a vacuum on the upper
surface caused by the air from below passing over the curved surface.
Modern parachutists use an airfoil parachute which glides.)

There are some sails like the crab claw sail that aren't an airfoil. The
crab claw works like a kite. The air passing the edges of the sail makes
powerful vortecies which create the vacuum on the front side of the sail.
Crab claw sails produce the most power per squate foot of any type of sail
I'm aware of. Unfortunately they're not as easy to handle as a sail with
its front edge attached to a vertical mast.

On my website you can see photos of my rotating sails and my of crab
claw sail which lifts over the top of a short mast for easier handling.

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