Nolkin, I found the following on the web. Hope it's helpful"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. (a) The maximum horsepower capacity must be computed as follows: (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 after most 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