How is the maximum rated power determined?
In response to a mandate from Congress contained in the Federal Boating Safety Act of 1971 (FBSA), the United States Coast Guard issued regulations covering small outboard boats and the maximum rated horsepower for them. The regulations bind the manufacturer to provide a capacity plate that shows the maximum rated horsepower for each hull.
The applicable regulations can be found in 33CFR183.53 My interpretation of those regulations is that the manufacturer must state the horsepower as no more than that computed under the formula, except, if the manufacturer conducts the performance testing procedure, the results of that testing can be used (with some qualifiers). However, the manufacturer is apparently free to state a lower rating if desired, and sometimes does. Note that the regulations only require ratings for monohull boats under 20-feet in length, but Boston Whaler has historically provided a rating plate for its boats longer than 20-feet as well.
The formula provided in the regulations reduces to:
(2 X L X W) -90 = rated horsepower
Where:
L=boat length
W=transom width; if the boat does not have a full transom,
the transom width is the broadest beam in the
aftermost quarter length of the boat.
The rated horsepower may be rounded up to the nearest "5".
Here is an example of how the rating formula works in practice, applied to an older Boston Whaler boat, the V-20 model:
Boston Whaler V-20
Length = 19' 10"
Width = 7' 5"
Thus:
(19.83 x 7.42 x 2) - 90 = 204.15
Rounding up = 205 maximum rated horsepower
In the case of the V-20/Outrage 20, from 1978 through 1985 Whaler chose to give it a maximum rating of 180-HP instead of 205-HP. Affixing rating labels which are lower than the rating permitted under the formula is common.
Is powering above the rated maximum legal?
The United States Coast Guard has an opinion on this frequently asked question, and their answer from their website is reproduced below:
Can I use a bigger motor on my boat than what it's rated for?
It is not a violation of Coast Guard regulations to install or use an engine larger than specified on the capacity label, but there may be state regulations prohibiting it, and restrictions from your own insurance company regarding this.
There are no Coast Guard regulations against exceeding the safe loading capacity, however, there may be State regulations or restrictions from your insurance company which prohibit this. There is a Coast Guard regulation that gives Coast Guard Boarding Officers the power to terminate the use of a boat (send it back to shore) if, in the judgment of the Boarding Officer, the boat is overloaded. There is no fine for this, unless the operator refuses the Boarding Officer's order. We certainly hope that you will abide by the rating, as overloading may lead to capsizing or swamping of the boat.
NOTE: The Coast Guard Capacity Information label is required only on monohull boats less than 20' in length. The label is not required on multi-hull boats, pontoon boats (catamarans), or on any sailboats, canoes, kayaks, or inflatable boats, regardless of length.
As the Coast Guard mentions, local regulations may apply. For example, in the state of Ohio one should be guided by this regulation:
Capacity Plates
(ORC 1547.39 & ORC 1547-40)
No person shall operate or permit operation of a watercraft in excess of any of the stated limits on the capacity plate. When no capacity plate exists, no person shall operate or permit operation of a watercraft if a reasonably prudent person would believe the total load aboard or the total horsepower of any motor or engine presents a risk of physical harm to persons or property.
To help locate applicable law in your jurisdiction, you may find the website of the National Association of State Boating Law Agencies to be helpful. They provide a guide to state boating regulations.
It should be noted that in some cases there are pamphlets or guidelines issued by state regulatory agencies that contain recommended practices which may propose higher standards than those actually contained in the state law.
Will dealers or service shops install an engine that exceeds the maximum rated horsepower for a boat?
It is not unusual for a dealer to decline to install an outboard motor on a boat where the motor exceeds the horsepower shown on the rating plate. This seems to be fairly common practice, particularly if the dealer is also selling the boat. On the other hand, it does not seem to be particularly difficult to locate a service shop or facility that will install a motor which exceeds the rating plate horsepower. Most states do not appear to license the profession of installing outboard motors, so there does not seem to be any particular regulatory difficulty in performing this service. Many dealers who decline may cite risks of liability, but in most cases they already have insured themselves against such liabilities and claims. Separating the sale of the motor from the installation of the motor seems to make it easier to find a dealer or service shop willing to make such installations.
Can insurance coverage be purchased for boats powered above their rating?
Boats equipped with engines whose horsepower is above that shown on their rating plate can be insured, but often at greater cost than for the same boat with engines conforming to the horsepower limitation. It may be necessary to change insurance companies, as some will decline. At this writing it is known that the Traveller's Insurance Company will write policies covering boats with engines that exceed the rated horsepower, although they do charge more than for the same boat with engines conforming to the horsepower limitation. It is important to disclose the horsepower of your engine(s). Usually an insurance policy will contain the serial number of the specific engine being covered and its horsepower. Misrepresentation of the engine horsepower would be a fraud and could result in lack of coverage. Some insurance companies raise the cost of insurance in proportion to the boat's maximum speed.
Other Resources
For more information and previous discussion on this topic, please see the following archived articles from the forum:
Implications of HP in Excess of Rating
Exceeding Max HP Rating
Maximum Horsepower Capacity: Classics Versus New Classics
There is no law in Texas to prevent you from hanging whatever you want on whatever you've got.
