How much is too much?

I've got a 1980 Diamarco 21ft daycruiser. Has a 7.4L Chevy, Borg Warner Trans, with a Mercruiser TRS outdrive. The boat currently runs a consistant 58mph at WOT with family inside (3 people and ice chest). However, want to go faster. The boat is not a full stringer boat. Stringers for motor mounts are run only from transom to just in front of the motor (behind the rear seat). Should I be concerned about how much horsepower the boat can handle and tearing a stringer from the floor of the boat? I'm not talking crazy horsepower increases, but maybe talking a total of 375 - 400hp total.

Any ideas?

 

There are many issues here. The engine you have is heavy and puts out lots of Torque. Your engine probably puts out about 330 to 350hp. Increasing HP 50hp should not make such a big increase in speed. You may have trouble with your boat structure in general at this speed or higher. I dont know how well built your boat is but I cracked the fiberglass in hull of a boat in half at 65mph hitting a rogue wake and being launched 10 feet in air. The increasing pounding at higher speed could be the problem.