For halyards, staset-X or similar lines
work very well in that they have very low stretch and keep the sail up and tensioned.
- - However, for
sheets, you want a line that has "give" or stretch to absorb shock loads when the sail empties and quickly refills. And the line need to be easy on your hands when you
work with it. For sheets then I use
Regatta Braid which is both soft and very easy on your hands.
- - Size of the line had to do specifically with the
boat and what loads the
sails will be exerting. But even more so, the
rope clutches and brakes and winches pretty much limit the size to one or two choices.
- - Length is a matter of choice but typically I and others make the lines too short. Of course you need enough line to reach from the boom up to the
mast head and then back down and then to the
cockpit for boats equipped that way. Then you need to add enough extra to allow the line to be taken back to and around a primary
winch so that if you get a "over-wrap" on the line you and use another
winch to pull the line free. Plus some more so you can "end for end" swap the line after sheaves, etc. have worn one end too much.
- - There is no set formula since each boat's standing
rigging is different. With sheets it is about the same process except that you need to allow enough extra line to wrap around the furled headsail.