Originally Posted by Mark P
I am sure there must be a government enforced manufacturer limit on HP on boats - am I right? So, Mercury can make any size motor they want, but the boat maker is restricted in terms of a HP limit? I can't wait to see what happens when some guys straps one of these 600 HP Verados to the back of a 16 foot bass boat - I think the evening news report will be spectacular.
On small boats there are formulas, etc. for determining the max allowed hp. That's why you see dinghies and such rated for an outboard of up to X hp, etc. Once you get above the size range where capacity plates are required, I think it's pretty much up to the manufacturer to decide what they can build safely.
Locally, there are 2 guys with Nor-Tech 39 center consoles. One has 3x 400s on the back, the other has 4. Both are scary fast, but there's nothing other than the manufacturer saying they can't swap them out for something bigger. I did a little searching and found a magazine test for one of those with the quad 400s. With all 4 running, it's more efficient on plane than going slow, surprisingly enough (shutting 2 engines down would likely change that). Best efficiency came in at 1.34 nmpg at just over 36 kts. WOT was just shy of 81 kts burning a blistering 155 gal/hr
EDIT: Just found another test of the same Nor-Tech (a couple years newer) with triple 400s. With those engines it "only" does 70.8 kts, but oddly, it burns more fuel (per mile) than the quads (but that could be down to the testing).