How Big an Outboard will Fit on my Boat?

outboard twin

Boats built in the U.S. after 1972 have U.S.C.G. mandated max horsepower capacity information supplied by the manufacturer.

In this world of bigger-is-better, I’ve often been asked, “Just how big an outboard can I put on my boat?”

The answer, of course, depends on the size of your boat.

For small powerboats less than forty years old, the answer is right there on the boat. Monohull powerboats less than 20 feet in length built in the U.S. after 1972 must have a builder’s capacity plate.  The max horsepower listed on the plate is arrived at by a formula that factors in the boat’s length and transom width. Also on the tag are max limits for the number of people and weight the boat is allowed to carry.

The capacity plate can usually be found inside the transom, or for boats with a steering console, near the wheel. Exceeding any capacities on the plate can result in safety fines by the Coast Guard. So unless you like to push your legal liability limits, keep the outboard size within those recommendations.

capacity plate

The National Marine Manufacturers Association (NMMA), Federal regulations (CFR 33) and American Boat and Yacht Council (ABYC) combine to govern max horsepower.

For powerboats bigger than 20 feet the answer to, “how big an outboard can my boat handle?” is not so straightforward.  Manufacturers of larger powerboats may make it easy for you by providing capacity plates, or (for boats over 26-feet) must provide the max hp in the owner’s manual, but how they arrived at those numbers is not a simple linear measurement. The naval architect who designed the boat and computed hull speed and drag formulations no doubt consulted a structural engineer to avoid ripping the stern off, and also spoke with the marketing department about how fast they wanted their boat to go. Then, unless the boat is a purpose-built raceboat, the suggested max hp rating also had to pass American Boat and Yacht Council (ABYC) handling standards.  The boat would have to be able to maneuver through courses at wide open throttle (WOT). So there is a practical limit based on safe boat handling.

If you came to me and wanted a recommendation for the biggest outboard you could put on your 20-foot plus powerboat and there was no manufacturer’s suggested max hp rating because the boat is homebuilt, older than 1972, or perhaps built oversees, I would ask a number of questions:

  • Have you tried different props to affect performance before spending big bucks on a new engine?
  • How does the boat handle now under the existing hp with WOT?
  • Does it cavitate? What are the handling characteristics?
  • How much does the boat weigh? Travel lifts or a roadside weigh station are good places to weigh a boat. Don’t forget to add weight for additional people, gear, and fuel you expect to carry.

The rule of thumb guidelines based on weight alone would be between 40 and 25 pounds of weight per 1 hp. In other words a 5,000 lb boat fully loaded could have a 125 to 200 hp engine. The wide range, based on weight alone, is due to the variety in design and handling characteristics of a given boat. Yes, you can ballpark the size outboard with either the NMMA linear standard or with the rule-of-thumb method, but then you’ll have to play professional driver and make a judgment call on ABYC boat handling standards.

Again, the first move would be to consult the capacity plate or owner’s manual. If that is not available I’d contact the company, or ask other owners of this model for their experience and recommendations. Worst case, I’d use the rule-of-thumb guideline. But remember, you’ll still have to be able to maneuver safely.

Comments

  1. charlie says:

    i have a 15ft thundercraft 1988 with 48 evenirudeoutboard- what would be the the most hp i could put on my boat – only doing 26 mph now -i want more power!! thankyou for your time

  2. Peter d'Anjou says:

    Hi Charlie,

    Their are three ways to find out how much hp your 15-footer can take. The first is the capacity plate, the second is the formula for small boats like yours. Unfortunately, this requires me to know the beam at the transom and the transom height. If you go to http://newboatbuilders.com/pages/hp.html and put in the boat’s measurements you can calculate the max hp yourself. Lastly use the rule of thumb rule in the story above, but again I don’t have the weight of your boat for this. Hope one of these methods gets you the answer you are looking for.

    Thanks,
    Peter

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