If you think gasoline is not an exciting topic, try throwing a match into it.
Yes, gasoline can be an interesting subject, especially if you are a boater. Do you know how much one gallon of gasoline weighs? A lot of boaters don't. It weighs 6.1 pounds per U.S. gallon. This means, when you add 40 gallons of gas to your boat, you are adding over 240 pounds. If you are planning to take a full complement of passengers, adding 240 pounds of fuel could put you over the legal weight rating for your boat. It could also affect the boat's handling characteristics.
Another fuel related question is: should I completely fill my boat gas tank? The accepted answer is "No." As air temperatures increase, gas in a fuel tank will expand. This can cause overflow out air vents, allowing dangerous gas vapors to escape. Also, a boat's filler hose is not designed to hold gas. As you fill your tank, listen to the sound of the flow and stop when the sound increases in pitch. This should leave your tank a little over 90% full.
You may be asking, "can a full tank of fuel really change my boat's handling?" This will depend on where the fuel tank is located relative to the boat’s center of balance (CB). The further the tank is mounted from a boat's CB, the more a full tank will result in a change in handling. And one handling concern that overweight will always create is longer stopping distances - albeit minimal.
You probably know that water weighs more than gasoline (8.4 lbs per US gallon). Your boat's bilge is prone to collecting small amounts of water and traces of gasoline. Because gas is lighter, it floats on top of the water and sits there evaporating into fumes. Any electrical wiring short or even a starter motor can create a spark that sets off an explosion. Hence the regulation to run your blower before starting the motor. While most boaters know to do this, remember that it is not only a smart safety requirement, it is also a legal one. Regular maintenance should include checking your bilge blower, fuel tank vent and filler tube. A bilge sock is a worthwhile investment, too.
What about ethanol gas in a boat? We have all heard that ethanol in gas can damage your boat engine. This is true. Since ethanol is an alcohol, it can dry out rubber engine components that it comes in contact with. So why do oil companies add it in the first place? The answer is simple -- they want to make more money, which is the main objective of every oil company. Ethanol is added to bring their fuel octane levels up to 87 or 89 with less refining. But there is more -- ethanol absorbs ten times the amount of water that gasoline absorbs. Fuel with absorbed water reduces the octane level of the gasoline, but as more water becomes absorbed a process called phase separation occurs, leaving two layers of liquid in your gas tank. A safe policy is to avoid ethanol fuel in your boat. Most marinas sell ethanol-free gasoline, which is one of the reasons marina gas costs more. For boaters who fill their tanks at road gas stations, remember to buy 'premium' gasoline, because high octane gas is ethanol-free in addition to being cheaper than marina gas.
Some weights to ponder:
Water - 8.4 lbs/US gal.
Regular Gasoline – 6.1 lbs/US gal. / 87 octane; contains 10% ethanol (E10)
Mid-grade Gasoline – 6.2 lbs/US gal. / 89 octane; contains 5% ethanol (E5)
Premium Gasoline – 6.3 lbs/US gal. / 91 octane; does not contain ethanol
Diesel Fuel - 7 to 7.7 lbs/US gal.
When you place anything into your boat, you should always know what weight you are adding. For most situations, the simplest way to remember fuel weight is that gasoline weighs about 6 pounds per gallon and is 25% lighter than water.