Most RO systems work by pressurizing water against a membrane that
only passes fresh water. Pressurizing and the pumping is done either
electrically or mechanically from the engine. This involves running
the engine. I imagine you could do it by having the prop turn your
alternator as you sail or use a towed generator behind the boat but
the efficiency of turning motion into electricity and then into motion
to produce pressure may be low. I propose to use a variation on the
old hydraulic ram pumps to directly produce pressure from the boats
motion.
HYdraulic ram pumps use the momentum of falling water to force a small
part of the falling water to a higher level than the water falls. In
other words, if your water falls 20', you can pump a very small
portion of it to say 50' higher without any electricity or engine.
Used on a sailboat, the momentum is not produced by falling, but
simply by the boats motion produced by the wind. This directly
produces pressure in a small portion of the water against the RO
membrane. Of course this produces drag but with proper design the
drag can be varied thus enabling the user to produce more fresh water
in a lot of wind.
Havent done th ecalculations yet but it may be reasonable.