Going aloft with climbing gear
Chris has the right idea, although I'd recommend using three prussiks, two for your feet and one for your harness. The line used for the prussiks needs to be sized to the line your climbing (in this case, the main halyard), generally, the prussik line will be one half to two thirds the diameter of the line to be climbed. It does take a bit of practice to get good at climbing this way, if there is a rock climbing gym in your area you may want to go over there and practice for a few hours with a spotter to top rope belay you in case you have any problems. Also, I'd make sure to have a proper harness, with both a seat and chest harness to prevent 'going inverted' in case of a fall. Check out the harness systems from 'Black Diamond' they are good quality and my personal choice. It's also a good idea to wear a helmet. If you do have a rock gym in the area, they should also have a pretty good selection of gear and enough 'rockheads' hanging around to help you get what you'll need to do the job safely. Ask them to show you a pair of 'ascenders' which is basically a form of jam cleat that replaces the prussiks. You may also want to look at a variety of self-belay devices. Whatever you choose, get some professional instruction and practice before you start, a fall from even 20' can have devastating effects!BTW, for what it's worth, I'm a certified Cliff Rescue Technician for El Dorado Search and Rescue and have been a climber for over 20 yearsJeff