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 maths and a look at your boating habits, determine the right outboard motor size for your boat.
Locate the National Marine Manufacturers Association (NMMA) Certification Plate on the inside of the stern of the boat. This 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. If there are several, use a calculator 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 per cent 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. If you usually boat alone, 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.