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?

 

Hard to answer without knowing more details of the hull construction. You're talking about a 20 -25% power increase, I think, as the 7.4 L was usually rated in the range of 310 - 330 HP. I can't find any info on Diamarco; there was a DiMarco that used to build high speed boats in the 70's and 80's, the type with the engine exposed, flat or shallow V bottom, often called drag boats.

That much increase in power might have implications beyond the stringers. You should have the HIN; with it you can go to the USCG web site and find information on the hull manufacturer. That might lead you to more info.

Of course, the easy way to gain speed is to leave the family and the ice chest at home; the boat will go faster without them, and you'll save lots of money.

 

It would be nice to know more details of how it's built, as Charlie says.
This boat is now 27 years old. It is very possible that there could be fatigue cracks in the structure; a thorough hull survey would probably be worthwhile before you consider putting a significant amount of money into upgrades.
The possibility of a stringer failure if the boat is made faster and more powerful is not a trivial matter. I recall hearing about an offshore speedboat that sank near here (Great Lakes) a few years ago. Fairly new 30 or 40 foot hull, twin Merc HP500s or something like that. Not a cheap rig. As the story goes, the boat caught some air coming over a wave and one of the engines went right through the hull when it landed. Crew had less than a minute to get out a "Mayday" and abandon ship (they were all rescued IIRC) before the boat was completely gone. There have been similar stories on here of speedboats whose chines cracked open or whose hullsides disintegrated in spinouts because of inadequate structure for their speed. So yes, caution is definitely in order.

 

I agree with Marshmat, "caution is definitely in order". Speed is a funtion of the Sq. root of HP. A 50HP increase will only get you about 4 MPH. Hardly worth the Expense and Risk IMO.

Jango