If you’re curious to know what the largest engine you can legally put on your boat is, try this calculator. It uses the federal regulations to compute the largest outboard engine you’re allowed to install.
The federal government regulates the horsepower of outboard motors that can be installed on boats. The maximum HP is determined by calculating the boat’s factor (the length multiplied by the width) and then looking up the allowable engine size in a table.
Horsepower Calculator
This step-by-step calculator will help you work out your maximum boat horsepower.
STEP ONE: Work Out Your Factor
First, you need to workout your FACTOR, this is calculated using boat length and transom width. This calculator will help work out your FACTOR:
STEP TWO: Convert FACTOR to Maximum Horsepower
Using your FACTOR worked out above, you can find your maximum horsepower.
If the FACTOR is 52 or below, use this table:
| Factor As Calculated Above | 0-35 | 36-39 | 40-42 | 43-45 | 46-52 |
| Horsepower Capacity (All Other Boats) | 3 | 5 | 7.5 | 10 | 15 |
| Horsepower Capacity (Flat Bottom/Hard Chine Boats) | n/a | 3 | 5 | 7.5 | 10 |
If the FACTOR is 53 or more, use these calculations (round up to nearest 5):
| If the boat has remote steering and at least 20″ transom height | (2 × Factor) −90 |
| If the boat has no remote steering, or less than 20″ transom height (Flat Bottom/Hard Chine Boats) | (0.5 × Factor)−15 |
| If the boat has no remote steering, or less than 20″ transom height (All Other Boats) | (0.8 × Factor)−25 |
Federal Regulations About Maximum Horsepower
I’ve been playing around with the idea of putting a larger outboard on my boat, and decided to read the rules and regulations to see what’s allowed. The federal government has regulations for just about everything, so it should come as no surprise that they created one for the maximum horsepower capacity of your boat.
The Federal Regulations governing outboard motor horsepower capacity are listed in 33 CFR § 183.53 – Horsepower capacity. Feel free to read these regulations!
I did, and I discovered they are written in lawyerly language with a lot of exceptions and footnotes. Every time I found an outboard engine that interested me, I had to perform a tedious calculation and refer to the rules to see if it was legal or not. After a dozen or so times, I became very frustrated and decided to developed this tool. It makes the evaluation process extremely simple and eliminates some of the mistakes that I kept making.
Feel free to use this handy calculator to determine the largest outboard engine that is legal for your boat.
Calculation Method
The federal government uses two methods for determining the largest engine allowable for your boat. One method is a calculation based on your boat’s dimensions and the other is a performance test. The calculator above uses the computational method. This approach calculates the “factor” for your boat that is spelled out in the rules. The factor is simply the length multiplied by the width of your boat. Based on this value, you lookup the maximum horsepower in a table. If you have a flat bottom boat, you’ll need to downsize the engine horsepower.
For large boats, there are a lot of special rules and things to consider. Each one of these factors results in an adjustment of the maximum allowed horsepower. Here are features your boat may have that will affect the allowable engine size:
- remote steering
- transom height
- boat bottom
Other Methods
Manufacturer Limits
Most boats have a capacity plate that gives you a lot of information about your boat’s design capacity, including the maximum engine horsepower. If your boat was built after 1972 then it almost certainly has a National Marine Manufacturers Association Certification Plate. This plate lists the maximum motor horsepower that the boat designers built the boat to handle.
The capacity plate also lists the maximum number of passengers as well as the maximum weight your boat can safely carry. You can find this plate mounted near the helm.
If your boat doesn’t have a capacity plate or you can’t find it, you can likely find the information in the owner’s manual. Most manuals clearly state the maximum horsepower that the manufacturer deems safe and appropriate for your boat.
Don’t have the manual? Just call your boat’s manufacturer and ask. I’ve found the manufacturers to be very helpful. Every time I have a question about my boat, I’ve been able to get answer.
The Final Word
If you’re thinking about installing a bigger engine on your boat, you need to know what the federal regulations say. This calculator is a great screening tool to play around with, but it’s not meant to be a substitute for common sense or an expert’s opinion. You need to talk with a competent professional before making the decision about whether you can safely increase your boat’s power and how much. Talk to your mechanic and then contact your boat’s manufacturer to see what they say. The last thing you want to do is spend a lot of money only to create a safety hazard for you and your family.
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