If engine rpm increases, but boat speed does not, this indicates that the crankshaft and the propeller may not be properly connected. There are a number of different systems between the crankshaft and the propeller on any boat, and a problem in any of these systems could cause similar symptoms. You could have a problem with the drive shaft, the shifting system or the gear case, or the propeller may be cavitating. However, the most common cause of these symptoms is a damaged propeller hub.
Some hubs are made of hard plastic and are readily serviceable. Others are made of hard rubber and are serviced by a propeller shop. In either case, the cause of the damage needs to be identified and corrected. Direct impact or loss of cooling could damage either type of propeller hub.
Ethanol in fuel is coming. The fuel is called E-10 and contains 90 percent gasoline and 10 percent ethyl alcohol (ethanol). Ethanol has about 30 percent less energy content than gasoline. This will cause a few percent drop in fuel mileage. The horsepower is a function of the octane rating of the fuel and should not change appreciably for the same octane fuel, so you should not see a major change in the performance of your boat.
Yamaha, Mercury, and Mercruiser engines are designed to handle up to 10 percent ethanol in fuel. Using fuel containing higher than 10 percent ethanol could void your warranty on these engines.
Ethanol will dissolve in water. As a result, if E-10 is added to a tank containing water or if water is introduced into a fuel tank containing E-10, the ethanol will combine with the water, separate from the fuel, and sink to the bottom of the tank. This is called phase separation. If this mixture reaches the pickup tube in the fuel tank and is pumped through the fuel system, filling the water separating filter and passing through, engine damage could result. Also, the remaining fuel now has less ethanol, so the octane rating has decreased. This will leave the engine more susceptible to detonation failures. If you determine that phase separation has occurred in your fuel tank, the tank must be emptied and dried and the source of water must be controlled.
Ethanol is a strong solvent and can dissolve corrosion and debris from fuel system surfaces. Hence, when E-10 fuel is introduced to your fuel system initially, you may need to change fuel filters more frequently. Also, ethanol is corrosive to some metals and can cause galvanic corrosion due to its ability to conduct electricity, especially when combined with water in a phase separated form.
The added amounts of particles associated with ethanol fuel may warrant upgrading filter systems. Many engineers are now recommending that boats using ethanol fuel need to use ten micron filters. Pre E-10 systems will likely be using twenty-eight micron filters. Manufacturers, such as Yamaha, have been quick to respond to ethanol fuel use by introducing replacement ten micron filters.
The corrosive nature of ethanol can affect fuel lines and other components, causing them to crack and fail. Many older boats will require replacement of all fuel hoses and, possibly, other system components. Especially affected are boats equipped with fiberglass tanks. Many older vessels must have the fiberglass tanks replaced prior to using E-10 fuels, or face certain engine failures. Please let us here at Muddy Bay Marine inspect your tank, and replace it for you if needed. We have tank resources available.
Muddy Bay Marine recommends and carries Stabil fuel treatment. Once tanks are cleaned of any old fuel and components are upgraded, fill your tank completely with fuel and Stabil fuel treatment.
A failure in a number of different systems or components could cause these symptoms. There are a few quick checks that any boat owner can perform that may solve this problem and allow him or her to get on the water shortly. Every marine engine has "start in gear protection". This prevents the engine from starting while it is in forward or reverse. A quick check to ensure the controls are fully in neutral may be all that is required in this case. wiggle or rotate the control to ensure the engine is in neutral and try again. If the symptoms persist, the battery cables should be looked at next. Liquid cell batteries use acid as the electrolyte. The battery is charged during engine operation. During the charging process, the temperature of the electrolyte increases as electrons are supplied to the battery. As a result, a minor amount of "gassing" may occur where a small amount of this electrolyte changes phase to a gas. Since acid is corrosive, this tends to corrode battery cables. Simply remove the battery cables and clean off the corrosion on both sides of the cables with sand paper or a pocket knife or a wire brush. If this still does not fix the problem, check the battery voltage. If the battery is weak, you may need to charge or replace the battery. If the battery is good, you will probably need to bring it to the dealer for further diagnosis.