-- Last Updated: Oct-10-12 6:53 PM EST --
A potentially huge problem is that your trailer is not providing any shock absorption as does the suspension of a car. With that combination of wheel, spring, and axle, you need 500+ lbs on it just to smooth out the ride. I tried this (including taking the extra spring leaves out) and it still beat the hell out of the boats.
Speaking from experience, I would not haul a composite boat on a trailer like this, unless it was one that I didn't care for very much.