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 44 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 overseas, 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.

An earlier version of this article originally appeared on Boat Trader in November, 2012.




  1. Boat Trader says:

    QUESTIONS and SUGGESTIONS from the previous version of this post:

    charlie says:
    November 13, 2012 at 9:06 pm • Edit
    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

    Peter d’Anjou says:
    November 18, 2012 at 12:38 pm • Edit
    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 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.

    Bob Johnston says:
    June 17, 2014 at 7:46 am • Edit
    I purchased a 30 year old 16′ runabout and the outboard that is on the boat is a 77 evenrude 70 hp and I later found out that the boat is recommending only a 65 hp is this a problem

    Peter d’Anjou says:
    June 17, 2014 at 11:44 pm • Edit
    Hi Bob,
    I don’t think it is a problem. Just because you have a boat that can go fast doesn’t mean you have to run it wide open. Drive safely and within your comfort zone and have fun with your new boat. One other thing, check the transom to make sure there are no undue stress problems.
    Congrats on your new ride,

    Opps Mighthave bought to Much says:
    August 24, 2015 at 8:10 am • Edit
    20 ft CC boat. Plate says 200 Hp Maximum. If I add a 23inch bracket with swim platform to extend the motor and usable space of the boat back with some flotation. Could I then have a 225 Hp Maximum motor.
    Say maybe a 2 stroke Merc Optimax.

    Boat Trader says:
    August 24, 2015 at 8:36 am • Edit
    That would be hard to recommend. You’re talking about changing a major structural component of the boat and moving the prop way aft of where it was designed to be. This will change the running surfaces, the steering, and probably other things, too — all to get another 25 hp? Again. tough to recommend. Have you checked with the boatbuilder?

    Tim says:
    November 24, 2015 at 2:02 am • Edit
    I have 1976 fiberglass cuddy that was converted from i/o to outboard with a 34″ bracket. The boat dry is around 4000 lbs. It currently has an older 225hp 2 stroke. I am looking at a repower and was wondering if I could go up to a 250 hp 2 stroke (Mercury or Evinrude)?

    Boat Trader says:
    November 24, 2015 at 8:54 am • Edit
    Hi Tim. It’s hard to say from here, because the issue is all about engine weight and what it does to the balance of the boat, the transom height above the water, how it affects steering at low and high speed, and safety. Compare the weight of your current engine with the ones you’re researching, and you might find that a modern two-stroke with 25 more hp will be comparable in weight, or even lighter, than the old engine you’ve got. Good luck.

    Dwayne says:
    January 21, 2016 at 5:40 pm • Edit
    I have a 16ft Vangaurd bansee. its rated for a 90hp. Had a 4cyl 800 merc on it. I have to replace the motor. The only one I could find was a merc 150 v6. Obviously the intent isnt to go WOT. and the transom was previously repaired/beefed up. Cant find any specs on the engine weights. Is this gonna be nothing but a nightmare?

    Boat Trader says:
    January 22, 2016 at 11:21 am • Edit
    Hi Dwayne,
    A new Mercury 150-hp two-stroke engine weighs a minimum of 193 lbs. according to Mercury’s website. It’s harder to find a dry weight for the old Mercury 800, but there’s one anecdotal mention of 380 lbs. Even though a modern two-stroke might be both lighter and more powerful than your old 800, you’re talking about an 87% increase in horsepower over the boatbuilder’s rating on a boat that’s already fast. It doesn’t sound like a good idea. Why not hold out for a modern two-stroke that hits the max rating of 90-hp? You’ll be lighter, faster, safer, and can run the engine at its full rpm range up to WOT.

    stuart says:
    February 11, 2016 at 2:38 pm • Edit
    hi i have a 16.5ft grp mayland boat and i need to know if a 50hp honda engine would be ok on it any ideas thanks


    Hayden Redding says:
    November 7, 2016 at 5:47 am • Edit
    Hey Dwayne,
    I have a 23 foot single hull fibre glass long boat, centre console.I was looking to put a 130 HP outboard on the back would this create any problems in your opinion??

  2. Johnny Martens says:

    Hi I have a 1993 Sunbird neptune center console 18 ft. boat with a 115 1993 evinrude the motor.I want to up grade it to a 2004 Mercury 125hp. outboard motor motor115 115hp.EXLPTO .But the rating on the boat calls for a 120 hp. Is there going to be a problem with the size hp.

  3. Jason says:

    I have a 18 ft double eagle mud 70’s. What size of an outboard can I put on it. I’m just curious the minimum-max. Thanks

  4. Phil says:

    I have an older 14ft princecraft, hull tag rated for a 40, I want to put a newer 60 on it, I know for fact that the older 72 40 Evinrude will weigh more then the newer 60 Evinrude 1992, will this pose a problem with marine police?

  5. Lonnie D. Brewer says:

    I have a fairly new, (23 hours), Johnson 50-hp motor on an old 15′ Bayliner Capri that has been in storage for awhile. The boat needs some work. A friend has a 1988, 17′ Bayliner Bass Boat that needs a motor. My Capri weighs about 975 lbs. and his Bass Boat weighs about 1300 lbs. Would his bass boat be too much boat for my 50-hp motor?

  6. Matthew says:

    I have a 22 6 pro sport it has a 225 Honda on it now it’s a 99 hull and a 2002 motor the tag says 225 but can I safely put a 250-300 on it ?