Is far as I remember, it think it can be modified by removing/adjusting the potentiometer inside the servo BUT! I'm pretty sure that this servo does not have the strength to pull the sails on a 40" yacht. You're talking about a standard, 13-in-a-dozen cheap servo that comes standard in the box when you buy a new transmitter. There good for 1/10 scale cars, and small planes (trainers etc) and eventually as rudder servo in smaller boats. Anything else is way more than they can handle....

It might work at first sight, until you're actually sailing and there's a gust of wind, it could actually rip all the gears inside (remember, the standard 148 futaba servo is all plastic gears!)