i just recently replaced my main halyard on p-30 hull # 346. i used a dyneema low-stretch from west marine. the line is 8mm. the reason i choose that diameter is because i put an 11mm line on the jib halyard last year and it was too large in diameter. when i went to take the jib down it was stuck in the sheave at the top of the mast and i had to send someone up the mast to unjam the spliced part of the line to get the sail down. so this year i replaced the jib and the main halyards with 8mm line. i had them made 110 ft long which is just a little too long but better than the alternative. my halyards lead from the bottom of the mast to stoppers and cleats on the cabin top just in front of the cockpit.
now on to the sheave. the new main halyard that i put on is getting chafed about 12 inches from the shackle so i figure that the sheave at the top of the mast is probably the original and made for a wire/line halyard. if you find out the sheave that is the correct replacement i would be very interested in knowing which one will fit. probably a pain to replace is what i'm thinking if you plan on doing it in a bosun's chair. the chafe is so bad on my brand new main halyard that i'm either gonna have to shell out lots of cash for replacement line or get to the bottom of the problem and put in the proper type of sheave up top.
let me know if this info helps. i will be more than happy to let you know exactly what line and captive-pin shackle i bought. it fits perfectly and is plenty strong enough. just need to solve the chafe issue.
satellite beach, fl/abaco, bahamas