From this article (first google hit)
1. Do it while the boat is afloat. Most boatyards ban the practice of climbing the mast of a boat on the hard, and for good reason. A keel boat propped up ashore is defying gravity and your weight jerking around aloft can cause it to lose balance.
Thing is, if you somehow overbalance the boat afloat it will rotate around it's center of bouyancy and lean over... if you do it on the hard the boat rotates around the bottom of the keel.. if a stand slips, or your actions on the end of a 30+ foot lever otherwise compromise the delicate balance of a boat perched on the ground, you're going for a short (but nasty) ride to the ground, along with the boat.. and most likely trying to go through one or two neighbouring rigs too.
One average guy up the mast of a Formosa 41 in the hard?? maybe.. on a lightweight 30 footer? maybe too, but.....On a 25 footer I'd say no way...