Seems everyone has a different opinion or answer for recommended fuel for marine use. The majority of watercraft we get into the shop have degraded fuel. How often do people check the fuel in there Waverunner or Sea-doo? Never. “fuel is used and fresh gas is put in every time” is the assumption. Unfortunately, no Watercraft can be completely drained of gas without removing the gas tank or physically removing the gas. All watercraft have a sending unit “gas pickup” that extends to the bottom of the tank. Most of the time this is near the bottom or an inch or 2 from the bottom. If the bottom layer of gas is contaminated, The new gas is mixed with the contaminated gas while filling up.
Phase separation is the separation of water and ethanol from fuel within a tank. When water is present in a tank, it bonds to the ethanol molecules in the fuel. Once the three-part mixture of water, ethanol, and fuel reach a certain point, the ethanol and water will drop to the bottom of the tank and separate from the fuel. Once this occurs, you are in danger of severely damaging your motor. Depending on where your fuel inlet is located, your motor might receive the fuel portion of what is in the tank or the ethanol/water mixture. Both will cause problems with your motor. While the water/ethanol mixture will cause damages, the fuel portion will be stripped of octane and leads to engine knock and further damage.
Seasonal storage with E10 fuel is another likely time for problems. During storage, fuel will tend to oxidize; it will become “sour”, and may absorb water from condensation. The water-holding capacity of E10 fuel is reduced with lower temperatures, so phase separation is more likely with winter temperatures. E10 can hold approximately 0.5% water at 60°F (.64 ounces in a gallon, or 12 ounces of water in a 20-gallon gas tank), but can only hold about 0.35% water at 20°F (.45 ounces in a gallon). If possible, store your boat for the winter with a full fuel tank. Add a fuel stabilizer to the fuel at the recommended dosage. Run the engine for 10 minutes to distribute stabilized fuel into the engine and fuel lines. Top off the tank to reduce the amount of exchange with the air that may bring in condensation. Note: Some storage facilities require that fuel tanks be empty for storage.
MYTH: Fuel additives can cure or prevent all issues from ethanol-blended fuel.TRUTH: There is no practical additive that can prevent phase separation from occurring. The only practical solution is to keep water from accumulating in the tank in the first place. In addition to using high-quality fuel additives, you should purchase quality fuel from trusted sources, check fuel filters and fuel tanks periodically for sediment and water, and keep up with manufacturer-recommended preventative maintenance schedules. MYTH: Fuel additives can make phase-separated fuel (that has separated into layers of water and low-octane gas) usable. TRUTH: “Bad” fuel should be completely removed and replaced with fresh fuel. Nothing can rejuvenate old fuel. To prevent fuel from going bad, most manufacturers recommend high-quality fuel additives to prevent sediment, gum and varnish buildup that forms when fuel goes bad, control moisture, help prevent phase separation and prevent fuel system corrosion. MYTH: Ethanol-blended fuels are bad and should be avoided. TRUTH: Ethanol blended fuels (E10) are common throughout much of the United States. After the transition period from non-ethanol fuel, E10 may be a superior marine fuel, as it tends to keep low levels of water moving through the fuel system, keeping the system “dry”. For over a decade, marine engines have been engineered to handle E10 gasoline. However, all types of fuels should be treated if they won’t be used in a few weeks.