Best Answer:?the USCG determines the max HP rating for a boat as follows:?
1. Multiply length x width at the transom.?
2 If Length x Width is.........then max hp is?
.........0 - 35.......................3?
.......36-39........................5?
.......40-42........................7....?
.......43-45........................10?
......46-52.........................15?
3. Except flat-bottomed hard-chined boats use the next lower rating.?
4. If length x width is greater than 52, use this formula:?
.......Length x Width x 2 minus 90, rounded up to the next multiple of 5.?
............Example: my boat is 15.5 feet by 5.5 feet. 15.5 x 5.5 = 82.25 x 2 = 170.5 - 90 = 80.5?
...................the rating plate reads "85 hp".?
5. Except if the transom is less than 20" high *or* the motor is tiller steered, in which case:?
..........Length x Width x 0.8 minus 25, rounded up to the next multiple of 5.?
A boat builder doesn't need USCG approval to rate a boat at or below the USCG formula.?
A boat builder can use a higher rating only if the proposed boat/motor combination passes handling tests designed by the USCG.
