What is Horsepower?

• Horsepower is a unit of force over time equal to 550 ft-lb/second. In other words, the force required to lift 550 pounds one foot high and hold it there for one second. Consequently, a 10-hp outboard puts out power equivalent to 5,500 pounds lifted one foot for one second and a 25-hp outboard is 13,750 pounds lifted foot for one second.
  
  

Boat Size Regulations

• A more powerful motor can propel and maintain speed for a larger boat. The Coast Guard regulations for maximum horsepower capacity can be found at 33 CFR 183.53. Generally, for a 10-hp motor, the ratio of length multiplied by the transom width of the boat cannot exceed 45. For a 25-hp motor, the length-to-width ratio ranges from 58 to 80, depending on the style of the boat.
  
  

Considerations

• Even though horsepower rating is linear -- the power output of a 25-hp outboard is 2 1/2 times that of a 10-hp outboard -- there are other considerations in choosing a motor. The 10-hp motor is limited to small boats. For example, to meet the federal requirements, a 10-foot boat can only be 4 1/2 feet wide at the transom. Conversely, with a 25-hp motor, that style boat can be 12-feet long and almost 5-feet wide. The larger motor is also appropriate for a wider variety of boats.