My old lines are at least 1/2" three-strand, but I left them in the boat and can't get to them now. The eye splices just fit through the cleats, and I loop them over the horns. Could be 5/8" I suppose. Most guides suggest 1/8" per 9 ft of boat, which puts them at 3/8". Looks kinda puny, even though the (new, unweathered) breaking strength is 4400 lb, probably enough to rip the cleat off. I've even seen power boats larger than my Mac in the marina using 3/8", though it is fairly protected in there. And I do use a spring line to relieve the one stern line, which is closer to vertical than horizontal, of the job of keeping it from moving forward. And I use a rubber snubber on it, since it's so short.
I've also noticed that it's cheaper to buy 50 ft 'premium' dock lines and cut them in half or thirds and eye splice them than it is to buy bulk line of the same spec, by the foot. Go figure.
Anyone out there using 3/8" three-strand on a Mac or T power sailor? I know it sounds like a rookie question, but I've always wondered and never asked.
p.s. I don't care for braided, as I like the stretch you get with three-strand. And it's easier to splice.
