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 may not be the case – but there may be 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, according to Boating Magazine:
“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, and possibly at the gas pump later, go with the minimum horsepower engine required to get the job done correctly, says Boating Magazine. 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 vulnerable to less-than-desirable performance under some circumstances, according to DiscoverBoating.com. At the other end of the spectrum, the 90-horsepower might be too much. So, a 70-horsepower would typically be a good compromise between needed power, performance and cost. And that middle-of-the-road motor may 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, Boating Magazine provides some information that could be of help: “On average, an in-tune four-stroke gasoline engine will burn about 0.50 pounds of fuel per hour for each unit of horsepower. Likewise, a well-maintained diesel engine burns about 0.4 pounds of fuel per hour for each unit of horsepower it produces.”
Boating Magazine provides some simple formulas for calculating fuel consumption:
Specific Fuel Consumption (SFC):
Fuel Specific Weight (FSW):
So, for comparison, the simple formula for Gallons per Hour (GPH), which is GPH = (SFC x HP)/FSW, would look like this for a gasoline engine vs. a diesel engine:
300-hp diesel engine:
300-hp gasoline engine:
If you’re like me, however, and correct math answers are about as rare as catching a 25-pound large-mouth bass, the Ocean Skiff Journal has a detailed fill-in-the-blanks horsepower calculator that can help you accurately define your needs. Once you determine your needs, pick the proper motor and enjoy your time on the water, regardless of your speed.
Originally published August 2013.