What kind of angle is the ramp?
I suspect you have a steep ramp and thus why your only getting about half-way up the bunks when the trailer is shallow, too much bunk out of the water. Sounds like when you go in deeper (trailer further in the water) so you can load all the way your floating off and damaging things. Do you have side guides?
If this is a deep ramp, is the nose of your boat hitting the trailer before the bunks? If so, you may need another roller where the nose hits the trailer.
Maybe I misunderstood your dilemma.
The object of bunks is to float on and off the trailer, means the trailer must be deeper in the water.
The object of roller bunks is to drive on/off the trailer and makes for shallower trailer.
On this trailer I have roller bunks and put the last roller just under water. The rest of the trailer is out of the water and I power load all the way up to the bow eye. Unloading is just as simple. Although I can't unhook until I am ready to launch, I climb on the bow and unhook, then push myself off the trailer.
when I had a smaller boat and trailer with bunks, I had to float on and off. No way my 10hp motor was pushing me to the bow eye. Also meant my feet got wet or I walked the tongue and climbed on my SUV's rear bumper to keep my feet dry. I could crank the boat on the bunks, however that was a pain in the butt.
Having said that, sounds like you have a hybrid of each system, couple rollers and bunks. Sounds like something need adjusted, you need another roller, of the ramp is too steep for your set-up. Maybe side guides will help too?
A trick you can try is to submerge the bunks to get them wet before loading and unloading. It will help make the boat slide easier. Some also install poly material, which is slick as snot. Be careful with that stuff, don't drop the boat on the ramp.
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