New Boatbuilders Home Page - Safe Horsepower

HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS WITH ENGINES GREATER THAN 2HP

Taken from Title 33 Code of Federal Regulations Subpart 183.53

Outboard Horsepower Capacity
	Compute the Factor = Boat Length x transom width
If Factor is (to the nearest integer)			0-35	36-39	40-42	43-45	46-52
Horsepower is			3	5	7.5	10	15
Note: For flat bottom hard chine boats, with a factor of 52 or less reduce HP one capacity increment
If the Factor is over 52.5 and the boat has :
	Remote Steering and at least 20" Transom Height				HP =	(2 x Factor) -90
	No remote steering or less than 20" transom Height
		For flat bottom hard chine boats			HP =	(0.5 x Factor) -15
		For other boats			HP =	(0.8 x Factor) -25

Example 1: Bow Rider - Length = 19' 6" Transom width = 84" remote steering and 20" transom height.

(Length x Transom Width X 2)-90 = HP = ( 19.5 X 7.0 X 2) - 90 = 273 - 90 = 183

Horsepower = 185

Example 2: Length 12' Transom width = 46" Tiller steering and 15" transom height.

( Length x transom Width) = 45.96 Horsepower from table = 10

Horsepower is rounded to the next multiple of five.

Transom width: is to be measured at the widest point of the transom including permanently attached parts of the boat such as rub rails.

Boat length: Length does not necessarily include such things as swim platforms. However if the swim platform is molded into the hull and contributes to the buoyancy of the boat, it is included. If you need some more information on this call the Coast Guard at 202-267-0984.

Special Rules: For boats that meet the following qualification there is an optional test course method for determining horsepower:

13 feet or less in length.
Outboard Powered.
2 or less passengers.
Remote steering and 20 inch transom height.

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