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Your boat will tell you how much outboard it needs. Every boat has a maximum number of people it can carry, a maximum load of equipment it can carry and a maximum horsepower rating. The horsepower rating is based on the boat's "hull design speed." Even if your boating needs and circumstances may be different than those the designers used when doing their calculations, you can use their work as a beginning and, with a bit of math and a look at your boating habits, determine the right outboard motor size for your boat.
Items you will need
Locate the National Marine Manufacturers Association Certification Plate on the inside of the stern of the boat. The NMMA plate contains important information which you should know, including the number of people the boat can carry and the horsepower rating of largest motor you should use.
Ask yourself how many people are usually with you when you go boating. Use a calculator, if there are several passengers, to multiply the top horsepower rating by 1.10. If the plate says a 120 horsepower engine is the largest you should use, then: 120 x 1.1 = 132, or a 130 horse motor, since the horsepower of outboard motors is rated in multiples of 10. This adds ten percent to the horsepower and will make it easier for your boat to carry the load of several people more efficiently.
Work the equation the other way if you boat by yourself. Divide by 1.10: 120 / 1.1 = 109. Boating by yourself, you could get about the same performance from a 100 or 110 horsepower motor as you do from a 120 horsepower motor and save fuel.
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.