In the past I've had to get my car fairly wet to launch on ramps but this year I am determined to keep my rear brake rotors away from the saltwater.
I made 3 modifications to my trailer: First, I replaced the keel rollers with a single long center bunk. Second, I added an 8 foot extension that pulls-out of the existing frame. And third, (and I wish I could claim credit for this, but I can't) a simple and ingenious device to push the boat off the trailer instead of having to float it off!
The original keel rollers worked adequately for launching and retrieving from water, but they didn't work at all well for launching or retrieving in my back yard (for working on the centerboard, etc.)! I usually tie the boat to a tree and then drive the trailer out from under the boat, but, the bow gets scarred by the roller bracket, and it's hard to get the bow to go past each roller when retrieving, not to mention the black rollers always made black marks on the hull. So, I wanted to install a center bunk.
I bolted 2 2x4's next to each other to the frame and screwed indoor/outdoor carpet over it. Then made 2 wedges that started a couple feet in from the back and sloped to 1.5" near the front (3" was too much and didn't allow the forefoot to rest on the flat in the middle). I covered the wedges with carpet and also decided to keep the rear roller. The new bunk worked just as I'd hoped and made easy work of retrieving the boat.
The trailer frame uses 3.5" x 3.5" x 1/8" square tube and the center portion has only 1 through hole at the end, the hinge, and is about 7 ft long from receiver to hinge. I ordered 2.5 x 2.5 x 1/8" x 8 ft tube for $48 at some on line metals place. (If you buy longer than 8ft then you'll pay common carrier rates.) The receiver was transferred from the frame to the extension and I drilled 2 half-inch holes through the frame and extension for pins. Later I found I could not find any 1/2" hitch pins for the 3.5" through holes and had to re-drill for 5/8" pins. Then I extended the extension and drilled 2 more holes just through extension to match the frame's hole pattern when extended.
Since I nearly always sail alone I had to find a way to extend the extension without pulling it out of the frame, while I'm driving the car. I made a wooden block to hold a 1/4" pin. The pin is under tension from a small bungy and the block is held in place over the front hitch pin hole and when the extension hole lines up with the frame hole and the pin the pin drops into the extension hole.
To push the boat off the trailer I replaced the webbing on my winch with rope and rove the rope through a single block (with snap shackle) between the bow eye and the winch reel. I added an eye to the trailer near the axle. Release the reel winch and enough line to allow the snap shackle to be belayed to the eye by the axle. Now, start pulling in the rope on the winch reel and the boat backs off the trailer. Of course wire rope will work fine too.