re: boat trailer problem

Some have a shock shock absorber that helps lessen the thud, I had to change mine a few years ago. It helped a lot.

Another thing to check is if there is air in the lines bc that will compress and allow the coupler to move more like a soft brake petal.

One other thing is to make sure the brakes are adjusted properly, it’s another thing that will let the coupler move more than needed.

A sticking wheel cylinder or caliper are famous for the jerk when taking off.

Master cylinder may need to be replaced.

If you have a sloppy receiver it will amplify it a lot, the best thing I did was buy one of those u bolt and plate thing that stop the rattle when not towing. It actually helped when towing.

You can also try stopping less aggressively and letting off the brakes and rolling a little before laying on the gas to let the trailer brakes release and the slack be take up on the coupler.

There will always be some clunk, it’s just the nature of surge brakes.