HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS WITH ENGINES GREATER THAN 2HP
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 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
Outboard Horsepower Capacity |
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Compute the Factor =
Boat Length x
transom width |
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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
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Remote Steering and at least 20" Transom Height
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HP =
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(2 x Factor) -90
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No remote steering or less than 20" transom Height
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For flat bottom hard chine boats
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HP =
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(0.5 x Factor) -15
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For other boats
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HP =
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(0.8 x Factor) -25
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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.
Click
on the image for the full size |
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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.
Click
on the image for the full size
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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.
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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
https://newboatbuilders.com/pages/labels.html
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.
See
https://newboatbuilders.com/pages/hp2_print.html
Copyright newboatbuilders.com All Rights
reserved. Revised 09/08/2021
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