The lower the hull on the trailer the easier to launch and especially retrieve back on to the trailer. As stated, putting the boat back on the trailer is the hard part! So, when looking at sailboats I recommend measuring from the ground up to the water line on the hull, some boat just sit very high because of the trailer configuration. The Catalina 22’s have a stripe along the lower part of the hull and the water line in the water would be about 1 inch below this line. My 22 is 41 inches to this line at the trailer axle to the ground, the hull is as low as it can sit on this trailer and I have to back into the water till the rear bumper is wet to put the boat back onto the trailer. Different ramps have different slopes so, my vehicle would have to be placed at different depths on other ramps.
At an average ramp slope, I would figure almost 1 foot into the water for each inch of height on the trailer. My trailer axle is about 31 feet from the water edge at a man made lake, the ramp would be steeper than at a natural lake from my experience.
As these fellows have stated, the biggest problem is the length of the ramp. When power boaters put their hulls on the trailer, they gun the engine to help slide the hull up the slopping trailer, over many times and many seasons this prop wash pushes the lake bottom away from the very end of the paved ramp and creates a hole for you to back or drop the trailer wheels into. With the weight of the boat on the trailer even a 4 wheel drive vehicle cannot pull it out on a wet smooth ramp.
All keels have to be maintained, I did my keel 23 years ago for the first time, it sits at a dock for 6 months a season on a Pennsylvania lake. I bottom paint my hull every 2 to 3 years with a little touch up on the keel and I re-finished this keel 3 years ago.