A proper gantline is essential for both long-distance racing and offshore cruising unless you always sail with a large, strong crew. If you have a varnished or painted spar it is even more important. A gantline is simply a four-part block and tackle arrangement that lets you haul yourself aloft with ease. We use two neoprene shelled double blocks (one with a becket) a 250-foot long 3/8-inch line (the same one we use as a stern anchor line) plus a galvanized hook for this purpose. This makes a gantline sufficient for work on any mast up to 47-feet tall. If we put a tail on the warping line we could stretch the gantline to work on spars up to 52-feet tall.
When it is time to use the gantline we secure the double becket block directly to our halyard using a bowline with a long tail. For security we tuck the tail through a lay in the line. The other double block is shackled to the chair. Then we feed the line through the blocks as we haul the gantline aloft until it is snug against the masthead and we secure its end to our anchor windlass. (This keeps the secured line away from the spar while we varnish. The four 1/2-inch bolts holding the windlass in place will definitely make anyone in the chair feel well supported.) It’s amazing to see how versatile this rig is. Because it increases your pulling power by four, you can pull yourself up the mast. (Even I can do it alone.) If you add a galvanized hook (or a small cleat) to the shackle on your chair, you can be in complete control of your assent or descent. Simply take three turns around the hook with the tail line of the gantline and the weight and friction of the line will hold you right in place where you want to work. Ease the line, you’ll slide slowly down the mast, haul on it, up you’ll go. This gantline leaves your deck-bound assistant free to get you the parts you need. (This is excerpted from Capable Cruiser, chapter 16 – Safety Aloft.)
Diagram of gantline