Went to the local RTM shop here chatting too a yachtie got talking boat mooring lines and fittings, he told me braided line as little a 200 k breaking strain would hold my girl in a storm, so with that I thought 1600 kg breaking strain will do me so picked up 30 meters of braided line, my 10 mm swivel looked cheesy to me so a 16 mm swivel which is almost as heavy as the boat will do, 2 ton rated shackles.
im still stumped on couple things, the chain that attaches to the block itself he recommended 10 mm chain , that sound ok? And how do you attach the line to the swivel, he said tie it, I thought me and knots and actually pulling of a knot that would secure it, which knot would I use? I still like the idea of doubling the rope,