.......There's this inertia thing that keeps them where they are. Lightweight boats are too light to roll off alone and heavy boats are too heavy to roll off--trailer rollers aren't like ball bearings, and many hardly roll at all!
Winches hold boats in place--40 years of trailering and I've never seen a boat rolled back against the safety chain. Doesn't mean it doesn't happen, that's why you use the chain; it is just highly unusual.
40 years of launching and I have never had a boat start rolling after I unhooked it unless I push it. In fact, I always loosen the winch and push the boat backwards about 8" so I have enough slack to unhook the winch snap. They don't keep going. Then I have to push right hard to get it going. Once it gets going, though, it's gone. That bi-polar inertia again.
The self-launching boat is 100% a function of operator error, being the combination of premature disconnect and a back/stop motion. This frequently happens to people in a hurry.......
