Boats can accomplish their propulsion through the water using sails, propellers (e.g Evinrude outboard engines) or jets of water. Jet boats use the Newtonian concept that for every reaction, there is an equal opposite reaction. They work by pumping a jet of water out the back of the boat to move it through the water. A firefighter holding a hose under pressure is familiar with the force produced by a stream of water.
Boaters are seeing many changes in the marine fuels available for their use. Governmental regulation has slowly changed the composition of boat fuel to better help reduce harmful emissions. Boat operators need to be aware of the possible problems these fuels can pose to their craft. Currently there are three new fuel formulations showing up in the marketplace; they are ethanol blended gasoline, low and ultra-low sulfur diesel, and biodiesel. An understanding of these fuels will help boaters purchase the correct fuel, equip their boats properly, choose the right filters and improve marine propulsion performance.
Components - Jet boats use a gasoline or diesel fueled engine to turn a water pump. The turning pump takes in water channeled through screened water intakes mounted on the bottom of the boat hull, and then discharges it at high velocity through a nozzle centered in the transom. Turning the direction of the nozzle provides steering for the boat. Reversing the boat is accomplished by lowering a deflector or bucket over the nozzle and reversing the water stream thrust.
Marine water jet propulsion offers a number of benefits to boaters that include safety, maneuverability, economy, and environmental impact.
Ethanol-blended gasoline - Ethanol is a blend of gasoline and ethyl alcohol. Ethyl alcohol is an excellent solvent and is hydroscopic, that is it adsorbs water. As a solvent, ethanol will dissolve sealants used in fuel tanks. It has a cleaning effect on tanks and fuel system components that can release fine metallic particles and dissolved sealants into the fuel delivery system. This contamination can plug fuel injectors, filters and carburetors. The greatest problem associated with ethanol fuel is the adsorption of water out of the atmosphere. Fuel stored onboard a boat can degrade in less than a month with a combination of water and ethyl alcohol separating from the gasoline. This is known as phase separation and it can harm marine power plants, produce excessive engine deposits, decrease fuel economy and reduce power generated by an engine.
There are many things boaters can do to prevent problems with use of ethanol-blended gasoline in their vessels. Once it is apparent phase separation has occurred, the only good course of action is to pump out the fuel tank, clean it and replace to the fuel with fresh product. Inspect the condition of onboard fuel and fuel tanks regularly. Control the water that can enter the fuel system. Check the condition of fuel lines, filler hoses and gas caps. Use a quality water separating fuel filter and change the filter element often.
Safety - Jet boats eliminate the injuries caused by spinning propellers. Prop strikes cause severe injury and death each year to skiers, divers and swimmers in the water. Jet boats suffer less damage from striking floating debris or a prop to foul stray lines in the water. The risk of running aground in tidally influenced areas is greatly reduced.
Low and ultra-low sulfur diesel - The Environmental Protection (EPA) is requiring the use of low sulfur diesel by boaters and the marinas that distribute diesel fuel. Two forms of the new low sulfur diesel are available to marinas. They are low sulfur diesel (LSD) that does not exceed 500 parts per million (ppm) of sulfur and ultra-low sulfur diesel (ULSD) that can not exceed 15 ppm of sulfur. Marine fuel previously available at most marinas can run up to 5,000 ppm of sulfur.
Sulfur in diesel fuel acts as a lubricant that extends the life of seals and fuel injectors. Use of these new forms of diesel fuel can reduce protection provided to the engine and reduce performance. Low sulfur diesel is less stable, contains more moisture, is more prone to bacterial growth and causes fuel filters to plug up sooner. Diesel engines designed to ASTM standard D975 can safely use ULSD fuel. Diesel engine operators should consult with their mechanic and engine manufacturer on strategies to better cope with the new fuel. New seals, injectors or other components may be required for good longterm performance.
Maneuverability and Performance - Jet drives offer instant responsiveness and acceleration with no hesitation. Compared to a prop, jets deliver more horsepower from the engine to the water while at a stop or slow speed. Propeller capitation or churning is absent. At planning speeds, there is no torque feedback to the helm as seen in propeller driven craft. Jet boats are especially popular with water skiers. There are no transmission components used on jet boats, no gears or clutches to break. Moving a deflector or bucket over the jet stream changes the boat's direction and it can be done while under full throttle. Your passengers might appreciate a little warning before you try it. Panic stops can often be made within the length of the boat. Jets are maneuverable; their thrust can be vectored or sent in different directions to move the boat. This trait is especially true at low speeds. Control is infinite fore and aft in all conditions.
Biodiesel - This is a class of fuel that is derived from vegetable oils and animal fats. It is relatively clean burning, represents itself well as a renewable resource and is more environmentally friendly than use of diesel fuel. Government research, subsidies and emission regulations are driving the increased use of biodiesel.
Biodiesel displays good lubrication properties, but suffers many of the problems associated with use of low sulfur diesel. It has low storage stability, is prone to supporting bacterial growth and provides less energy for combustion. Standards are not uniform for biodiesel production and quality varies between producers. Most marine manufacturers will only approve the use diesel blend of fuel that contains less than five percent biodiesel. Conversion of the engine is required to use unblended biodiesel fuel. Major diesel engine manufacturers like Detroit Diesel, Cummins and Navistar have tested biodiesel blends of up to thirty percent that have not degraded fuel system performance.
Economy - A jet drive is only a water pump powered by automotive engine modified for marine use. Jets are easy on engines and there isn't the classical form of a transmission to break. Jet systems matched to vessel weight and hull shape can allow engines to operate within their optimum performance power bands. In boat operations that involve frequent starts and stops, the only moving part used to change direction is the deflector plate. The engine and pump are set to run at their most favorable settings. Basic maintenance involves monitoring fluid levels and the owners can service most components. Many repairs can be made without hauling the boat. Pumps running in clean water enjoy exception life spans.
Environment - Compared to propeller driven vessels, jet boats don't produce the same levels of noise or vibration transmitted into the water. Prop boats are linked to damaging sensitive reefs and tidal flats. In Florida, manatees suffer greatly from prop strikes. Striking a manatee with a jet boat isn't going to do them any favors, but they are going to suffer fewer injuries because of missing prop.
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