How Much Fuel Capacity Do I Really Need?

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One of the worst feelings in boating is the rising tension that occurs when you realize you’ve miscalculated fuel consumption and your boat might not make it to your destination. The absolute worst feeling is when the engine begins to sputter and then goes silent. When buying a boat, one of the most important considerations is whether or not it will have the range to safely transport you to the places you want to go and back home again. Unlike cars, boats are subject to more variables, so just looking at boat-test results and factoring in the size of a gas tank won’t be enough information to make the right call. Being conservative and thinking like an airline pilot instead of a bus driver should be the mindset. 

Problem 1: Not All the Fuel in the Tank is Usable

You should only fill a tank to 90 to 95 percent of its capacity to allow for expansion when it gets hot and prevent gas from escaping from the vent when fuel sloshes in the tank. Nonspill vents are supposed to prevent this, but it can still happen if a tank is overfilled.

You’ll also lose some gallons because your fuel pickup doesn’t extend to the bottom of your tank. This prevents it from vacuuming up the nasty bits of crud that can accumulate over time, along with water from condensation that is heavier than gasoline. A safe bet is to shave 5 to 10 percent off your boat’s published capacity by assuming it can’t be used. Right from the get-go, and perhaps to your dismay, that generous 200-gallon tank might only provide 160 gallons of usable fuel. 

Solution: The only definitive way to learn how much gas in the tank you can use is to carry out some extra ­fuel and let your boat run out of gas. Take steps to be safe while doing this, of course, and make sure your propulsion package can run dry without complicating the restart. Add just enough gas to get back to the fuel dock, then fill up your tank until the pump clicks off. Note the total gallons. Once you know your usable ­gallons, use your engine’s fuel-­management system to calculate the amount of gas you’re burning. Don’t rely on a fuel gauge. Mechanical fuel gauges on boats are ­notoriously inaccurate. 

Problem 2: Rough Seas and Currents Can Sap Fuel Economy

Virtually all of the published reports you see from manufacturers and magazines are performed in calm conditions for the simple reason that it’s unsafe to run boats at top speed in rough water. Tests are often done with the minimum number of crew and light fuel loads. While this information is valuable for comparison’s sake, using it to establish your boat’s real-world range could lead you astray.    

A boat’s fuel economy is proportionally affected by increasing wave height. In calm seas, an engine can be trimmed up to reduce the hull’s wetted surface, which yields better mileage. In rougher seas, when a boat’s bow plows into a wave, it’s the equivalent of throwing out a sea anchor. The added friction on the hull can reduce fuel economy by 30 percent or more if a skipper has to ­zigzag to stay safe.

Solution: Because sea states can change rapidly, ­using the rule of thirds is prudent. Allocate one-third of your fuel for getting to your destination and fishing, one-third for getting home, and keep the last third in reserve in case of adverse conditions.

Problem 3: Many Other Factors Affect Fuel Economy

Other variables affect fuel economy, such as the number of crewmembers, amount of fishing gear and ice loaded on board, propeller choice, hull deadrise, number of engines, total horsepower and speed. Features like auto trim and autopilot can increase ­fuel economy.  

Solution: To estimate whether a boat you are looking at has the range to go where you want, start with a test performed by a party you trust, like a boat manufacturer, an engine brand, or a magazine like Salt Water Sportsman. Make sure to factor in the load the test boats are carrying. A company like Yamaha Outboards will sometimes show its Performance Bulletins with realistic loads, like the tests it did on a Pursuit 266 SC with twin F150s, then twin F200s. Those tests were based on full 139-gallon tanks, three people, and 20 gallons of water. Curiously, the twin F200-powered Pursuit got better ­fuel economy at 27.7 mph than the F150-powered ­model (2.45 mpg versus 2.17 mpg) because it could cruise at 3,500 rpm rather than 4,000 rpm.

Once you own your boat, the only way to accurately calculate how much fuel you’ll need is to keep a detailed log showing the different numbers you achieve under different loads and conditions. The next time you’re in 4-footers with a full load of fuel and gear, record both your miles per gallon and gallons per hour from the engine’s fuel-­management system. With fuel-economy numbers and your boat’s ­usable fuel capacity clearly defined, you’ll be able to base decisions on how far you can travel on the load you’re carrying and the conditions you expect to face. 

There are no filling stations at sea, so take the time before leaving port to calculate how much fuel you really need to safely deliver a happy crew and a load of fish back to the dock on time and with plenty of fuel to spare. 

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