When you’re deciding how much power you want driving your boat, the obvious answer may be “as much as possible.” However, raw power doesn’t necessarily translate to capability. While a boat’s manual or capacity plate will state the maximum horsepower that can be safely harnessed to that vessel, jumping straight to that upper limit will yield diminishing returns.

When calculating boat horsepower needs, it’s best to consider your boat’s size, weight and intended use to inform your engine selection. A well-chosen motor setup will offer an optimal combination of handling, stability and performance.

Key Factors for Calculating Boat Horsepower

Once you’ve determined the maximum horsepower for your boat, you can work within that limit to choose an engine that suits your specific needs. When calculating boat horsepower, make sure to consider the following factors.

Speed and Fuel Efficiency

As speed is directly related to power, there is a simple equation that uses speed to determine a boat’s horsepower measurement: One unit of horsepower will move 40 pounds of boat weight at 20 mph. For a 2,000-pound boat to reach 20 mph, it needs 50 horsepower.

This relationship between speed and horsepower has a significant impact on fuel efficiency, as well, since diesel engines run best at three-quarters throttle and gas engines prefer the 3,000 to 3,500 rpm range.

Boat Use

Since a smaller engine running at its top range will be less efficient than a bigger one running comfortably, a primary consideration of how much horsepower you need for your boat is what you’ll be doing on the water.

If the same 2,000-pound boat is going to pull a skier at 36 mph, it will need 90 horsepower to reach that speed, and a 120 horsepower diesel engine would yield the best fuel efficiency. A pontoon boat built for pleasure cruises won’t have the same power requirements as a deck boat intended for water sports.

When calculating your boat’s horsepower needs, it’s also essential that you account for the weight of your passengers and gear, especially on larger vessels. Items like coolers, innertubes and fishing equipment can add up quickly, and you want to ensure the additional weight will not overtax your motor.

Multiple Engines

Put simply, horsepower is just a unit of energy, whether it is generated by one engine or three. Installing additional engines will increase your power requirements by adding to boat weight. However, this step can also improve efficiency by decreasing the amount of work performed by each motor.

The presence of multiple engines can also offer peace of mind, as in the unlikely event of a motor failing, you won’t be completely stranded.