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Reason for Safe Horsepower ratings

Start-in-gear protection requirements for outboards.

The Rules are in 33 CFR Subpart D 183.51 - 183.53

Outboard horsepower ratings are based on a variety of factors. These include;

Centerline length.

Maximum Transom width.

Transom Height.

Type of steering: remote or tiller.

Hard Chine Flat Bottom or Other.

There is a formula for computing Safe Horsepower based on various combinations of the above factors. See the table below.  This only applies to a monohull boat less than 20 feet in length (6.096 meters)

Canada uses similar formulas for computing outboard power in kilowats. (1 HP = 0.745 KW)

The below formulas do not apply to: sailboats, canoes, kayaks, and inflatable boats, that are designed or intended to use one or more outboard motors for propulsion. It does not apply to boats that are true multihulls.  A multihull makes two or more separate footprints in the water. A pontoon boat is a multihull.

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)






Horsepower is






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 (136 KW)

Horsepower = 185 (138 KW)

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

( Length x transom Width) = 45.96 Horsepower from table = 10 (7.5 KW)

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. Transom Height Boat Length

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  202-372-1077.

Transom Height Boat LengthClick on the image for the full size
Flat Bottom Hard Chine means: The boat is flat bottom if you can lay a straight edge across it, and there is no vee or curvature. A hard chine has no curvature. Flat Bottom Hard chine

Beware of steering changes:  If your boat is rated for remote steering, and the buyer wants you to reconfigure the boat from remote steering to tiller steering, the boat can no longer be rated for the higher horsepower allowed with remote steering.  You will have to change the labels to the lower horsepower rating.

Conversely: if the boat is rated for tiller steering and the buyer wants it to have remote, you will have to put on a  label reflecting the higher horsepower rating.

See Labels 

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.


Copyright 2010 All rights reserved.  Revised 11/21/2014