I suspect the answer may lay in welding. When two actors are welded they can be pulled apart without breaking. In in the attached image, the translate channels have been unhidden in Posette's shin. The shin has been moved down and to the side, but it still remains connected to the thigh because of the welding. I think you may be able to exploit welding to stretch out your mainsheet as the boom swings. "Point At" may also be useful to you. The way I'm thinking (this is only theory) is that a block on the deck could Point at another block on the boom, which in turn points back at the other block, the sheet would either be a child of one of the blocks, or an actual part of one block, and would be welded to the other block. vertices in the sheet would need to line up exactly with vertices in the block for the welding to work. Perhaps if the sheet is a closed cylinder, you could spawn a cylinder cap and make it part of the block, to give the sheet something to weld to.

This is all just brain-storming on my part, never done anything like it, not sure if it would work, or what problems you might face with it. Good luck!

P.S. The boat looks great!