My trailer has two 12 ft long 2 by 4's mounted on edge on the trailer. Each board is about 2.5 ft apart. My boat is an aluminum flat bottom john boat type bass boat. I have carpet covering the wooden 2x4's. I am able to back the boat down the ramp without the boat slipping off the trailer. I unhook the boat from the tow rope and unhook the safety chains as well. Then I hook a 70 ft long rope to the bow of the boat and the trailer. Then when I back the boat into the water and it starts to float off the trailer it will go back into the water until the rope catches the boat. I can even drive the truck and trailer up the ramp until the back of the boat trailer is free of the water's edge. I fish an electric motor only pit that has a very shallow concrete boat ramp. The pitch of the concrete ramp is shallow. If I launch at a lake with steeper ramps I normally will keep the safety chains hooked up to the boat so that it can't slip off the trailer until the boat is floating off the trailer in the water.

I have at times wanted to redo my carpet on the 2 x 4 rails as I call them to help the boat slide of the trailer a little easier. If I used rollers it would come off while backing down the ramp and that's not a good thing.

With the long rope method and quick snaps on each end of that long rope I can easily launch the boat by myself.
Regards,

Moose1am