Bosun's chair
I'ts been a while, but I used to sail 'deep sea' on a freighter & all we ever used was a bosun's chair, a line and a single sheave block.The line you use has to be large enough diameter to enable your hands to get a grip on it, so 1/2" is probably the smallest I'd use, but it isn't particularly hard physically to haul yourself up with a single part line as you're only effectively only lifting half your weight, plus a small amt for the friction loss in the sheave.A loop from the fall of the line is pulled thru the vee made by the ropes of the chair; this loop is passed over your body and snugged up, making a slip knot at the point where the working end of the line is attached to the rope of the chair. It's dead simple and it works. To raise of lower yourself from wherever you're working, just grab both parts of the line above the 'slip knot' to cinch it, and work a little slack from the free end of the line thru the knot. Any book on knots, rigging & ropework should show how its done. Using a multi-part tackle for a bosun's chair is a waste of time; you'd have to haul feet of line just to move a few inches. Take forever.The line you use has to be fat enuf to grip and soft enuf to be easy on the hand and to provide some friction in the slip knot though: I'd be careful with a slippery dacron or some of the high-tech cordage, but a soft nylon braid should work just fine.