How Much Horsepower Do I Need for My Boat?
Prospective motorboat buyers, whether in the market for a brand-new or used vessel, eventually have to decide how big of a motor they want. While powerboat companies and their respective print ads would have you believe that everyone needs a direct-injected, two-stroke, 250-horsepower engine, that’s not the case – but there is some truth to the saying “bigger is better” when discussing watercraft engines.
When deciding what size motor to put on your boat, there are three things to keep in mind:
1. Choose a motor that is in compliance with the rated-safety range of the boat.
2. Define and prioritize the main uses of the boat.
3. Look for the largest motor you can afford that will meet the required criteria.
“Ask the question: What am I going to use the boat for?” says Dan Conrath, president of Spokane Valley Marine, Inc., in Spokane Valley, Wash. “A general rule of thumb is to get as much power as the boat will take while remaining safe.”
Conrath recommends buyers take honest stock of the boat’s intended use, as well as who and what will be aboard. “If a guy buys a 17-foot boat for himself and a friend to fish with or ski out of, a 70-horsepower motor might be enough. But that same boat loaded with a family, food, skis and will be used to pull a tube all day, they might need a 90-horsepower,” he said. “Err on the side of a little more power. You can always back off if it’s too much, but if it’s not enough to get you to the speed you need to be, there’s not much you can do.”
To save money upfront in purchase price, as well as potentially at the gas pump later, go with the minimum horsepower engine required to get the job done correctly. Larger motors can be more expensive and typically burn more fuel – but not necessarily (as you’ll see below).
If you have a boat rated for a 50- to 90-horsepower motor, the 50 horsepower might get the job done, but it will work harder and require more gas, as well as be liable to less-than-desirable performance under some circumstances. At the other end of the spectrum, the 90-horsepower might be too much. So, a 70-horsepower is a good compromise between needed power, performance and cost. And that middle-of-the-road motor could run more efficiently than the smaller engine, which helps saves you gas money .
“If you always have the throttle wide open to get where you’re going, that 50-horsepower could use more gas than the 90-horsepower,” says Conrath. “Engines run their most efficiently at two-thirds throttle up to three-quarters throttle. It’s that last one-quarter of throttle that they burn more fuel.”
For those number-crunchers out there that just can’t stand estimated data when making choices, the Ocean Skiff Journal website has two interesting formulas that could be of help:
For a modified-V hull (assuming 16 degrees or less):
Total Weight / 30 = HP required to drive the boat at 25 mph, at a 75 percent throttle setting
For a deep-V hull:
Total Weight / 25 = HP required to drive the boat at 25 mph, at a 75 percent throttle setting
If you’re like me, however, and correct math answers are about as rare as a 25-pound large-mouth bass, the site also has a fill-in-the-blanks horsepower calculator that can help you accurately define your needs.