Boats can accomplish their propulsion through the water using sails, propellers 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.
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.
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.
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.
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.
