Why Should a Boat Gas Tank Never Be Full?

The following is an easy-to-follow fueling schedule that should ensure that you, the boat, and the passengers are all secure while you are filling up.

A boat’s gas tank should never be full because a full tank is a risk of exploding. The gas kept in a boat’s tank tends to heat and expand. If the tank is full while this happens, then the gas pressurizes the tank. If the pressure rises too high, then the tank ruptures, and this can irreversibly damage the boat.

Most smaller boat owners fuel up their tanks when they are heading out to a boat launch, as this is the easiest way to do so. However, if you own a smaller boat or a PWC, and you have to fill up while you are out on the water, then you need to follow the same best practices listed above. If you have portable fuel tanks, you may be able to take them off the boat and fill them up at a marina if you prefer.

Larger boats have tanks that cannot be removed, and your boat will be filled just like a car, via an opening on the deck. PWCs, however, have an inline tank, and you need to use the same safety precautions you would use if you were on a larger boat with onboard engines. If you have a smaller boat with a hand-held tank, use it to blend stabilized fuel and put it through your motor.

Fuel Apportionment in Stern-Mounted Vessels

If a boat is stern-mounted, then the whole fuel goes in the back of its tank, and fuel gauges will appear great. Once you are on a plane, the boat levels off, all of the fuel moves to the front of its tank and the fuel gauge goes down. Now that the boat is fuelled, you will want to open up your boat again, if it has a cockpit, and dump out any residual fumes from the vessel. Once the fueling is complete, you want to open all of your doors, etc., so the air can circulate throughout the boat.

When filling up the tanks, put a Fuel Saver on top of the fuel intake vent (it has suction cups that keep it positioned, but you need to hold it down). The final safety precaution before loading your tank is to keep your fuel injector tightly positioned over the fill pipe outlet.

This will keep you 1/3 tank full on hand for times when the wind, waves, or drifts cause you to run more time than expected. For longer-term storage, top off your tanks, and in periods longer than three months, you may consider a fuel stabilizer. If you do, be sure to run your boat for a sufficient amount of time to achieve it. The trick here is to schedule late-season boating trips to consume a lot of your boat’s current fuel and then top up the tank with fresh fuel right before preparing for storage.

Wintry Tanks Are Empty Tanks

Some boaters prefer to keep the tanks of their boats largely empty during the winter, then refill them during the spring, hoping that they will be able to refresh the fuel to make up any lost octane. Some boaters, especially those who own a newer boat or motor, do not feel ill after using E-10 fuel.

While E10 fuel effects on your boat’s engines may be mitigated, the Marine Industry recommends using more than 10% ethanol in the fuel, which can harm the motor. The remaining 10 percent of ethanol will erode rubber, harm fiberglass, loosen fuel particles, and clog fuel lines.

The new fuel, called E-10 because it contains 10 percent ethanol, unfortunately, has a tendency to pull large amounts of water out, and Phase Separation, or forming two distinct solutions, into the tank, often for long periods. Once too much water has entered the tank and gas has begun to phase separate, no additive can bring the fuel back to the previous state. A nearly filled tank limits airflow in and out of the vents, reducing the chances that temperature swings add condensation (water) to the fuel, inviting phase separation.

This significantly reduces the amount of wet air that may be entering the tank through the fuel tank vent during the autumn and springtime temperature fluctuations. Marine motor manufacturers and technicians recommend keeping any boat almost filled to leave only some room for fuel to expand should temperatures rise. Fuel industry experts now advise keeping the tank at 90 percent capacity, a safe level of fill, to minimize the potential for condensation and to allow expansion.

Fuel Expansion Is a Real and Present Danger

Additional fuel expansion (and resulting spills out the tank’s vents) can occur as cooling fuel is pumped out of the subterranean tank to the boat during hot days, or as outside temperatures increase and decrease. When a rapid refueling or filling of a tank is done, a boat will expel air, which is frequently mixed in with the fuel.

Trailer-mounted boaters need to take extra precautions and fill the tank level, otherwise, bubbles of air may develop in the tank, which could cause fuel to bubble out or spray back. Caution is needed when fueling a boat, so that spills do not occur, nor do the fumes disperse or accumulate.

Fumes may accumulate in the bilges of larger vessels during the fueling process and must be given time to evaporate before starting an engine. It can take a lot of fuel, especially if you have a pump that runs slowly and your vessel is taking on more than 100 gallons.

You should still spend some time seeing what kind of fuel is best for your boat, specifically. Unless you have got a sailing vessel sitting in your garage, it is unlikely that you need to access all three types of fuel. Never plug the vents on a fuel tank–doing so can build up pressure, which can lead to dangerous fuel system leaks. The first time you inject fuel into the fuel tank, keep in mind that cold, unprocessed fuel vapors will be replacing air, and sometimes there will be an air-vapor pressure difference, creating an out-of-order pour.

If everybody at the front of the boat seems to have a low fuel level, but if everyone moves back, then you have a lot of gas.

Nicholas Finn

I've been the captain of a fishing boat for over 20 years, and I created Pirateering to share my knowledge of and interest in seafaring.

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