I converted a standard futaba servo to winch type, it's not particularly powerful though, so i'd only really recommend using it if you're using a relatively small sail area or have light winds. it works find when i use it for my ls-4 with wing mast only. i do use it when i have the mast & sail, but it struggles when the wind picks up.

I'll write a short explanation of what i did.

when you open up the servo, there's a few cogs at the top, the motor & a potentiometer in the middle, and usually a circuit board at the bottom. now... from memory, where the cogs are there was a small black plastic cap type thing, it basically turned the potentiometer when the servo went round. i had to drill and cut a small tag from the centre of this black cap, this stops the potentiometer from being turned, and the motor is free to continue one way or the other.

there, real easy i know it's not much of a description, but if you open up the servo it may seem a bit clearer.