Jet boat engines propel the craft by ejecting a jet of water out the rear of the craft. Traditional boat engines use a propeller to essentially screw the craft through the water using the principle of Archimedes' inclined plane (screws are spiral inclined planes).
Jetting Above the Water Line
Jet boat engines do not expel the jet of water below the waterline. Though this seems counter-intuitive, the amount of thrust is not dependent on pushing against the water. In fact, jetting equal amounts of water at equal speeds below or above the waterline produces the exact same amount of propulsion.
Water Depth and Maneuverability
Jetting water out above the waterline allows jet boats to operate in very shallow water. This is because ejecting the water above the water line eliminates the need for props or vents that protrude below the hull. It also makes jet boats exceptionally maneuverable, as the hull skims across the surface without drags that would otherwise impede directional changes.
While jet boat engines do not have a physical limit to the size of the craft they can drive, the efficiency of the jet water engine depends on the application. Propeller drives are more efficient at speeds below 20 knots per hour. Above this speed, apparatus below the hull line (struts, shafts, rudders, etc.) begin to generate drag co-efficients high enough to significantly reduce efficiency. However, jet boat engines produce little torque. For heavy loads or against heavy resistance, as in tug boats pushing larger boats or freighters starting from a dead stand, jet motors would have to be too large to be practical. In these instances, a large prop blade turning at a low speed is required.
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