Hi WTP, Bit of large subject is servos. In general a standard servo converts the angle movement on the transmitter stick to a push / pull action to use for say a rudder. But I will just refer to sailing boats.
The power of the servo can be assessed by how much torque or power there is for the push pull action. It is measured in kgrms / cm. The further away from the centre turning point of the servo arm the less power is available.
For a 5kgrm / cm. servo you have 5Kgrms pull or push available at 1 cm. from the servo arm centre. If you extend the arm to 5cms from the centre you have a force of 1 Kgrm available. 2.5 cms is 2 kgrm and so on.
There are also very small servos for doing small jobs on small models.
So for a dedicated sail arm servo it needs to have a bigger or more powerful motor to give a reasonable pull at the end of the arm. This usually involves more substantial or metal gears inside. So the box is a little bigger.
This might be quoted as 13.8Kgrms at 1 cm (Hitec 725B) or by arithmetic 10 cms (4 inches) 1.38 kgrms.
The other option is a drum winch, the difference here is that the pull is fixed by the drum radius. I think the standard drum supplied (Hitec) is 1.9 cms radius. The output power from the motor is a little lower giving 11.5kgrm / cm. so you have a pull of 6.05 kgrms pull (11.5 / 1.9).
The arm winch will have a transition of about 0.8 seconds through about 90 degrees movement so it is quick.
There were sail servos from Sanwa with 180 degrees of movement.
The drum winch will have several rotations from one end to the other so to speak (Hitec 3.5 revs). This is at a speed of 1 rev in 1.3 seconds, about 4.5 seconds. This is enough power to work the sails of most non-competitive sailing boats.
So you sacrifice some speed for power.
If you look at some of the cheap high power standard size servos form Howes, I am sure there are others, you will find a 10 kgrm servo which could well be made into a sail winch for a small yacht.
Take into account that if there is a lot of work to be done, then you really need a rugged servo to do it.
I would suggest that a 36 inch loa yacht needs a proper sail servo BUT there is an alternative.
You can use 2 servos in parallel from one socket on the receiver by putting in a Y connector. The Y connector single end goes in the receiver and this leave 2 ends for servos. Then you can have one servo for the jib and one for the mainsail.
They do not have to be the same size / power servos.
It all comes down to what you might have in the spares box or whether its cheaper to go one way or the other.
I use a sail arm servo in one of my yachts and decided to slow it down by using an Action kit servo slow down module and this works very well, in general you do not want a very fast action from a sail servo. The slow down module is placed between the receiver and the servo.
Hope this helps, Roy