Try This
You need to know the draft of your boat, the beam and the length overall. Al lthe other measurements can be made while the boat is in the water.
Calculate the center of gravity of your boat it is in the center of the beam and the center of the water line. The center of gravity was to be positioned such that about 10% of your total boat and trailer weight is on the tongue of your trailer.
Once you know where the CG of the boat is going to be on the trailer, measure to the end of the trailer. Compare that distance to the distance from teh CG of the boat to the end of the stern. In Texas the boat can extend past the end of the trailer but it is limited to only 5.5 feet I think.
You may be able to adjust your trailer axle or may have to extend the trailer lights aft to get into compliance and meet the toung weight requirements.
Now measure forward from the boat CG point halfway to the bow. Go down to the water lne and measure (inside the boat) the distance across the hull. add to that 2x the thinckness of the hull., That is where your forward pads will be.
Do the same at the middle of the boat and wherever the rear of the trailer is going to be. (You measure the length from the trailer back to the ppint where the CG of the boat is going to be on the trailer to get this distance)
Your pads are going to be located about 6 inches below the water line of the boat. so you need a pad height above the keel rest point that will be whatever draft of the boat is minus 6 inches. Then you can allow for adjustment beyond that. I used Acme threaded rod in a 2 inch square pipe to adjust my pads and I left a foot of downward adjustment room for a two foot rod. I got the pads on the rods from Borwnell boat stands. There are other ways to do this but the acme rod allows quick adjustments even in the water. That allows you to use the trailer for many similar size adn weight boats
I cut my posts to be about 18 in less than the height of the keel to the water line.
You angle your posts in so that they intersect the bow at the width of the hull right at the water line you which you measured in an earlier step.
This sounds trick but iut is easy if you make a model out of cardboard or PVC pipe so you can visualize it.
After you get your pad posts located, you need to brace them in two directions at approximately right angles. Add some keel guides to help locate the keel as you sail on to the trailer or crane the boat in to it. Pad everthing well. You can use marine grde carpet that is available cheaply from Overton's.
With the 6 posts in you can get he boat on the trailer, tie it down securely and pull out of the water and up the ramp or crane the boat into place and make the final measurements for the bow stop.
If you are going to submerse the trailer for launching and retreiving, you can add a 1000 lb weld on bulldog jack and rotating caster wheel on the front and get a suitable rope or tow strap to allow the trailer to be pulled out of the water with your tow vehicle.
