If you are going with New England Rope
, I'd go with T900. Its a low stretch synthetic core
(dyneema??) with a dacron cover so will have virtually no stretch and, hopefully, decent suppleness. StaSet X is an all dacron line with a braided cover and a straight fiber core
. It's low stretch claims are largely because their are more fibers in the core because it's not braided. StaSet X is okay for a halyard but it is a very stiff line that is not pleasant to use. In my case, it wanted to go straight instead of around the self tailing
jaws of my coach roof mounted main halyard winch
which made the winch
non self-tailing. Since it's so stiff, it doesn't coil easily or like to be stuffed in the halyard bag. The dynex cored line I replaced the StaSet X with doesn't have these issues and is way way less stretchy. The stretch wasn't a big deal with the sail for the StaSet X but was with my ATN Mast Climber.
Since the spinnaker
halyard needs to go up quickly and can suddenly load with tension, using a double braid line of larger diameter has it's merits.
I've used synthetic line halyards on two boats that previously had wire halyards. One boat has done more than 30,000 miles for us and subsequent owners, the other a TransPac without a problem. If the sheaves don't have burrs and are wire/rope grooved, they should be fine. Still not a bad idea to replace the sheaves if it's an older mast.