I agree, the speed limits were bought in to limit the wash, and in reality its the wash that the rangers keep an eye open for, on a martham boat with no way of gauging your speed other than the wash, i have never yet been pulled up by a ranger for excessive speed, but then those boats do have very little wash, even when flat out. at 3mph (checked by gps) there is barely even a ripple.
hull speed is inversely proportional to the length of the boat, so a longer boat has a greater displacenment speed than a short boat, once you reach your hull speed, then your wash will increase as you are physically shoving the water out of your way (and using more fuel).