Here is the formula taken form the US Coast Guard Backyard Boat Builders Regulations (Just do a google search)...
Kinda technical...but if you exceed Max. HP Limit, your insurance will not pay for any claims if that boat was in an accident and someone got hurt.. not to mention you would not have a leg to stand on in civil court,,,,
Sec. 183.53—Horsepower capacity
The maximum horsepower capacity marked on a boat must not exceed the horsepower capacity
determined by the computation method discussed in paragraph (a) of this section, or for certain
qualifying boats, the performance test method discussed in paragraph (b) of this section.
(a) The maximum horsepower capacity must be computed as follows:
USCG 2003 BOATBUILDER’S HANDBOOK | PART 1
58
(1) Compute a factor by multiplying the boat length in feet by the maximum transom
width in feet excluding handles and other similar fittings, attachments, and extensions. If
the boat does not have a full transom, the transom width is the broadest beam in the
aftermost quarter length of the boat.
(2) Locate horsepower capacity corresponding to the factor in Table 183.53.
(3) For a boat with a factor over 52.5, if the horsepower capacity calculated in Table
183.53 is not an exact multiple of 5, it may be raised to the next exact multiple of 5.
(4) For flat bottom hard chine boats with a factor of 52 or less, the horsepower
capacity must be reduced by one horsepower capacity increment in Table 183.53.
Table 183.53—Outboard Boat Horsepower Capacity
[Compute: Factor = Boat Length X Transom Width]
If factor (nearest integer) is 0-35 36-39 40-42 43-45 46-52
Horsepower Capacity is 3 5 7.5 10 15
[Note: For flat bottom hard chine boats, with factor of 52 or less, reduce one capacity limit (e.g. 5 to 3)]
No remote steering, or less than 20[sec] transom
If factor is over 52.5 and height
the boat has
Remote steering and at
least 20[sec] transom For flat bottom hard chine
boats
For other boats
Horsepower capacity is
(raise to nearest multiple
of 5).
(2 X Factor)-90 (0.5 X Factor)-15 (0.8 X Factor)-2
Outdoorsman