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HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS WITH ENGINES GREATER THAN 2HP
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;
Maximum Transom width.
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 kilowatts. (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
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.
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.
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