On a small outboard boat, the US Coast Guard capacity plate gives the maximum persons, weight and horsepower. The calculator works out the horsepower the rules allow, so you can check the plate against the boat and the motor fitted against both.
Maximum horsepower
33 CFR 183.53 multiplies the length by the transom width, both in feet, to get a factor. Length is measured over the deck from the foremost to the aftermost point, leaving out pulpits, brackets and the motor (183.3). If the transom is less than half the beam, use the broadest beam in the aftermost quarter of the boat.
| Factor | Horsepower |
|---|---|
| 0 to 35 | 3 |
| 36 to 39 | 5 |
| 40 to 42 | 7.5 |
| 43 to 45 | 10 |
| 46 to 52 | 15 |
Flat bottom hard chine boats with a factor of 52 or less drop one step. Over 52.5 the rule uses a formula, and the result may be raised to the next multiple of 5:
- remote steering and a transom of at least 20 in: 2 × factor − 90
- other flat bottom hard chine boats: 0.5 × factor − 15
- all other boats: 0.8 × factor − 25
The rule covers monohulls under 20 ft that use outboards, except sailboats, canoes, kayaks and inflatables (183.51). Builders of qualifying boats of 13 ft or less can rate horsepower by a performance test, so a small boat's plate can read higher than the formula.
Persons
The plate gives persons in pounds and in whole persons. 183.41 converts the pounds by adding 32, dividing by 141 and rounding to the nearest whole number, and the calculator does the same.
For a boat without a plate, Boat-Ed gives the Coast Guard rule of thumb for boats under 20 ft: length × width ÷ 15, for people of about 150 lb.
On the survey
Find the plate at the helm or transom and photograph it. Compare the horsepower on the plate with the motor on the transom and with the calculator. Note any outboard over the plate rating.
Citing it in a report
"Capacity plate: 6 persons or 900 lb, 1,250 lb persons, motor and gear, 90 hp. Outboard fitted: 115 hp, over the plate rating. Recommend an outboard within the rated horsepower is fitted, or the builder's written approval obtained." Give what the plate says and what's fitted.