That servo would normally be used as an arm winch, programmed for 180 degrees rotation. It can be modified for continuous rotation and a drum added, but why do that extra work?
The drum dia will depend upon the length of sheet line that you need to move, and the number of turns you want, More turns allows a smaller drum and more torque, but increases travel time.
Note that the servo uses the large 'D' size spline.
You can calculate the length of main sheet to be moved by the winch. Assume that the boom going from close hauled to full out to the shrouds, creates a 90 degree angle, then the length of the sheet line is the hypotenuse, and the base and side are the distance from the goose neck to the main boom attachment point  call it L.
the diameter of the drum is square root of (L squared*2) divided by Pi divided by number of revolutions desired.
For example, Gooseneck to main sheet attach point L = 200mm, Three revolutions required
L squared (200*200) = 40000 times 2 = 80000
Length of main sheet movement is Sq root of 80000 = 282.8mm
Divide by 3.14 =90
Divide by 3 revolutions = 30mm
So the diameter of the drum is 30mm
John
