Conceived by sportsmen in the shallow streams of New Zealand in the 1950s, the jet boat has some distinct advantages in certain applications over boats using traditional propellers. Water jet propulsion systems also have their drawbacks, however, and the venerable propeller continues to be the propulsion system of choice for most boat manufacturers.
How Propellers Work
The propellers of ships are referred to as "screws," because the propeller acts essentially as a screw, pushing the boat along as it rotates and moves water behind it. In terms of physics, the angle of the propeller blades, or their pitch, creates a pressure difference in the water in front of and behind the blades, causing a mass of water to accelerate behind the propeller, pushing it (and the boat) in the opposite direction. Reversing the direction of propeller rotation will reverse the follow of water and thus the direction of travel.
How Jet Boats Work
Jet boats use a high-velocity stream of water directed out of the back of the boat to create forward thrust and propel the boat forward. Water is taken in through the bottom of the boat, passed through a series of impellers that increase its velocity, and then expelled from the back of the boat, just above the water line. Steering is accomplished by directing the stream from side to side, thereby changing the vector of the thrust and the direction of travel.
Advantages of Jet Propulsion
One of the most significant advantages of jet boat propulsion is that, unlike a traditional propeller, the jet mechanism is contained within the boat, with no mechanical parts extending below the hull. This means that jet boats can move at high speeds in shallow water without the danger of the propulsion system snagging or running aground. This advantage has made jet boats popular in sporting, tourism and military applications.
Disadvantages of Jet Propulsion
Because jet boat steering depends on the thrust of the jet and not a rudder as in a propeller propulsion system, simple jet boats cannot be steered when they are not under power. Traveling in reverse requires a system to redirect the jet's thrust, a solution that is relatively more complicated than simply reversing the rotation of a propeller. The efficiency of jet propulsion is sensitive to the drag of the hull in the water, meaning that jet boats operate best at high speeds, when the hull is planning along the surface of the water.
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