I took a little bit of everyone's posts...........trying to find the cross section of the sheave from Westcoaster, I took Hagar's cue and looked up the replacement or refit components for an Isomat boom for an IP27 it calls for sheaves with an inside diameter of 3/8" and 10mm line, 3/8" line is about 9.5 something mm's, so thanks to you all who suggested 3/8" double braid. Seldon suggests the short pieces of line be = to the E distance, long pieces should be 4X the E distance for ref #1 and 6X the E distance for reef #2 where E is the distance from the boom to the eye in the sail. I'm thinking 150' total of 3/8" should do the trick.