Just finished that job on a 21' Cal, with a 400 lb swing keel. The total weight of the boat is 1100lbs. The toughest work is removing the old paint, unless there are no compatibility problems. Jacking it off the trailer was fairly straightforward, once I decided upon an approach.
The underbody of my hull aft is fairly flat, approaching horizontal, and this dictated the type of boat jacks that I purchased. The ones for my aft end are actually planing hull boat stands. The two for the bow are more angled as you'd commonly see on a sailboat in a yard. The main issue with them is, of course, getting the trailer out from under. I accomplished this with a 3x5" timber, 12' long, and cement blocks. First I laid down a couple of 3/4" sheets of plywood, athwartships, under bow and stern. My driveway is unpaved and I planned on having the boat on the stands all winter. I wasn't looking for a midnight grounding in my own driveway come spring and so the stands sat on the plywood.
First I placed the planing boat stands under the counter (stern) and jacked them up slightly to take some load. Then I used a 20 ton bottle jack under the bow, aligned on the keel, with a 2x4" that I had cut a 'V' notch into. I bought the bottle jack at a discount tool warehouse for around $30. I obviously did not need the 20 ton capacity for this job, the size of the jack promised more stability. And, if I decide to relevel Casa Squalor, I've got just the tool. I blocked up the bottle jack under the bow with scraps of 4x4" and slowly jacked the bow up off the trailer. The rear stands, under the counter, provide athwartships stability. When I had it as high as it would go, I placed the timber athwartships under the bow, supported outboard by the cement blocks. The 3x5" timber was ideal for this as I could rotate it for either a 3" height or a 5" height as needed. With the timber under the bow, safely far enough aft that the rake of the bow was not a factor, I released the strain on the bottle jack and lowered the weight of the boat onto the timber. I then reblocked the bottle jack and jacked it higher yet. Cement block, 8", by cement block I brought the bow up. I jacked the stern up enough to clear the trailer bunks. I ended up with the hull about three feet off the ground. I could go higher but that was the limit of my boat stands. Even before I got that high, I was able to rest the bow on the timber and pull the approx. 8' wide trailer out from under the boat.
Once i had the trailer out and i had the boat to the limit of the aft boat stands, I was able to level it up at the bow, and bring in the forward boat stands to support it. Make sure you chain the boat stands together athwartships. Also, as you are raising, make sure the stands do not start to tilt or ****. The bottom is now painted, with the exception of the portion where the boat stands are holding the hull. I plan to slide the timber under the stern, remove the stands, sand and paint, replace the stands, do the same procedure at the bow without replacing the stands, and slide the trailer back under her. And you can call me a moron if i forget to rewire the trailer with the boat off it again this year!
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.