I just finished modifying a Karavan trailer for my Hunter 22. The trailer picture is at
http://www.karavantrailers.com/KCBunkTrailers.cfm Found the trailer on Craigslist last month.
I will try to get more pix of my trailer when I go over to the lake next week.
But I wanted to share the way I set my trailer up, understanding that my centerboard would not support the weight of the boat ( or any part of that weight IMO ) so I had to make sure I had clearances.
I used 1/2 threaded rod as extensions for the u-bolts holding on the bunks. I used the original ubolts, some threaded couplings and the threaded rod to extend my u-bolt length for my spacers. The spacers were made from 1 foot long pieces of 2X4 ( shims ) 4X4 and 4X6 treated lumber from my scrap pile. I used a skill saw to cut notches for the bolts, as my trailer has 3" wide frame members, and the block material was 3.5" wide.
I used the drawings on line to create a hull outline for my boat, scaling them like in engineering school 40 years ago. Now it can be done with software but I didn't have what I needed at the time. That with an outline drawing of the trailer from Karavan gave me a good starting point.
I ended up with a single 4X6 on the front bunks, raising them 5.5" total. I put a 4X6 and two 4X4's on the rear bunks. The line drawings said my ballast would be centered slightly behind the front bunks. 'Sea trials' indicated my rear blocks needed to be only one 4X4 and right now a 2X4, and the boat slides on the trailer flawlessly. I have about two inches of clearance from the centerboard to the front axle.
I post this because it was a simple task to take a pile of wood scraps, build up a tinkertoy system that allowed me to adjust the lift with only an open end wrench and a socket (deep well is nice). I spent a total of $30 and about 4 hours on the modification, and now I know what size 'blocks' to make from thick wall steel tubing to make a permanent solution.
Oh, and I spent about two weeks worrying over the drawings ( which were garbage by an engineer's standards (c: ) hoping I didn't destroy the centerboard because I missed a measurment.
When we put the boat on the trailer first time, I slid it on to where I thought it should go, listening carefully for a collision between the CB and the trailer axle/frame. Then hopped on board and let down the CB. It lowered a bit and went 'thunk'. Ahh I'm high enough. Then we pulled the boat out a bit, and tested again. And again repeat till boat out of water.
Found I was high on the back end, the boat sat nose down by about 2 inches, thus the removal of one 4x4 block and the insertion of a 2X4 block, giving me a 1.5 inch boost.
The 1/2 inch threaded rod extensions are plenty tough for this application, and I am sure that someday I'll replace the wood blocks with custom galvanized steel tubing, all welded together to fit. Sure I will.