[Fr. Frank]

The formula for multiple engines is different. I don't have the current (2005) publication.
My 1987 USCG home boat-builder's publication says the max rating is dependent on the height of your transom and length of your boat. For twin-OB vessels less than 20'0" in length with a transom of 20" or 15", the formula is the same as for a single motor. For twin-OB vessels less than 20'0" in length with a transom of 25" or greater, the multiplication factor is 2.5. So a 19'8" SeaCraft with a 25" transom and twin engines has a maximum hp formula that is (20X8X2.5)-90=310.

[b]For vessels over 20'1", there is no legal maximum horsepower, only "manufacturer's recommended maximum horsepower".

The USCG recommended using a test called the "quick-turn test" to determine the maximum safe horsepower limitation for home boat builders, where your maximum safe horsepower was whatever you could make a full-throttle turn with. The turn was accomplished by turning the steering wheel 180* at full speed with whatever prop you had installed. If you could do that without losing control, you were good. That's very subjective.

If I was using twin V6 Merc EFI's from 1997, with a maximum engine angle of 37.5* from center, and a SeaStar hydraulic steering system with 4.2 turns lock to lock, a 180* turn of the steering wheel will only turn the motor angle to 8 degrees from centered. While I wouldn't try turning like that with an Allison XTB at 105 mph, I would have no problem doing that with a 23' SeaCraft with twin 300 hp motors at full throttle and max trim.

So legal max OB horsepower for a 23 is pretty much whatever you want it to be. I would probably just stick the to old 500 hp max rating on the Potter built hulls.