Next stupid question - with a servo arm set-up where is elastic used? Please I'm a visual person and a diagram would be great.
With an arm, I wouldn't use any elastic. With a drum, and a continuous loop, the elastic would live in the return just to make up the changes that you can't otherwise account for when winding. (The line NEVER lies perfectly on the drum, therefore the effective diameter changes)
With a single drum, the only time you need elastic is when you let the sails out in light wind, and the line falls slack. If you try to design a machine that can tie a knot, you will have a hard time. A single drum in light wind will do the job every time. So the elastic needs to be placed so that it can pull any slack away from the drum.