While ethanol gasoline appears to be tolerated satisfactorily in automobiles and trucks, marine engines are an entirely different matter.
Auto vehicles have sealed fuel tanks as required by EPA regulations to reduce gasoline emissions to the atmosphere. Boat fuel tanks, however, are almost always vented to atmosphere.
Water is seldom found in vehicle fuel tanks unless it is pumped in from the gas station’s fuel tank. Boat tanks, however, always contain a water layer because it is vented to the atmosphere. The difference between daytime and nighttime temperatures causes the boat’s fuel tank to take in air at night to balance the pressure in the fuel tank. During the daytime, when temperatures are highest, the fuel tank vents to the atmosphere (the lighter gas components are vented). At nighttime, when the temperature drops, the fuel tank takes in air to balance the tank pressure. With the air comes moisture, and high relative humidity in South Louisiana means lots of moisture enters the fuel tank.
This moisture forms droplets on the fuel-tank walls, and eventually these droplets fall to the bottom of the tank and form a lower water layer (water has a higher density – it’s heavier – than gasoline, so the water appears as a lower layer in the fuel tank).
Water in the boat fuel tank presents a serious problem with ethanol gasoline. Ethanol is 100 percent soluble in water. It is “hygroscopic” and acts like a “water magnet.” So the ethanol in the gasoline will migrate into the water layer of the fuel tank. This creates several problems.
First, by losing ethanol in the gasoline, the octane of the gasoline will drop. Ethanol has a 113-octane rating. So, if there is enough water in the fuel tank, it is possible that all of the ethanol could migrate into the water layer, thus leaving the gas with a reduced octane rating of 83 to 84 instead or the original 87. Outboard engine manufacturers warn this lower-octane gasoline could damage the engine.
A second problem with ethanol gasoline is related to potential corrosion to aluminum fuel tanks. Since the ethanol/water layer is conductive, unlike gasoline, it can result in galvanic corrosion and could damage aluminum fuel tanks, eventually causing them to leak. Also, the ethanol/water layer is a great medium for algae and sludge to form. This could create plugging problem in fuel lines and engines.
A third problem is potentially the most serious: Ethanol is a great solvent. It will dissolve the gum/varnish film from gasoline which has accumulated on the walls of fuel tanks, lines, etc., over the years. It could also attack certain types of fuel-line hose materials. These products will be fed into the engine and cause carburetor plugging and in fuel-injected 2-stroke and 4-stroke engines plug the injectors. Serious engine damage could result. Certain older types of fiberglass fuel tanks also could be dissolved by ethanol.
If you must use ethanol gas because you cannot find ethanol free gasoline, see my article titled “Precautions to take if you use E-10 gas.”
