I'll try to address your questions since I have a similar setup. I have a 2 speed, 2500 lb winch. My deck is 36" above ground and my winch axle is 30 inches above the deck. I have it mounted on top of a 3"x 2" piece of steel square tubing welded to my trailer's tongue and mounted at an angle. This post does double duty as a mount for my winch and a spare tire holder. I thought about making it removable, but I haven't because I was worried about the winch getting stolen. I want to put a motorcycle carrier on he tongue. That will be pretty crowded. If I have to I might end up cutting it off above the tire, welding in a post to allow me to remove it and pin it in place.
The extra height above the deck doesn't shine when floating a boat on. It shines when loading a boat on from the ground. The bow of the boat goes a few feet above the deck until the equilibrium point of the raft clears the roller bar and then the bow comes down onto the deck. Then winching gets a lot easier.
I almost always winch in high gear unless the boat is loaded heavy for camping or my wife or kids is doing the cranking. The 2 speed is nice because you have high gear for speed but the low for when you are heavy, or trying to get the bow onto the deck. I have only used the brake when the raft caught and we had to back it up. That winch handle spins so fast it can hurt your arm.
I use a rope instead of a cable or webbing. No real reason except I already had the rope. I spliced a hook on. The rope has to be watched so it doesn't get caught in the teeth of the gears. If I could be sure that webbing would go 8-10' past the back of my trailer, I would probably switch. That hook and rope make nervous if the rope broke while cranking.
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