Jet boats
A jet boat is a boat propelled by a jet
of water ejected from the back of the craft. Unlike a powerboat or motorboat
that uses a propeller in the water behind the boat, a jet boat draws
the water from under the boat into a pump-jet inside the boat, then
expels it through a nozzle at the stern.
Jet boats are highly maneuverable, and many can, from full speed, be
reversed and brought to a stop within their own length, in a maneuver
known as a Hamilton turn.
There is no engineering
limit to the size of jet boats, though the classic prop-drive is more
economical than the jet-drive. Thus, the biggest jet-driven vessels
are found in military use. South Africa's Valour class frigates (approximately
120m long) are the biggest jet-propelled vessels so far. Even these
German-built vessels are capable of performing the Hamilton turn.
A conventional screw impeller
accelerates a large volume of water by a small amount, similar to the
way an aero plane's propeller accelerates a large volume of air by a
small amount. An aircraft's jet engine, by contrast, accelerates a small
volume of air by a large amount. Both methods yield thrust due to Newton's
third law — every force gives rise to an equal and opposite force.
In a jet boat, pumping a
small volume of water and accelerating it by a large amount delivers
the thrust. The acceleration of the water is achieved by using multiple
impeller stages. Steering is accomplished by small vanes that direct
the water jet.
The jet of water is expelled
above the water line, contrary to many people's intuition. Hamilton
discovered early on that this greatly improved performance, compared
to expelling below the waterline. From a physical point of view, this
is understandable because it is the momentum of the expelled jet which
provides the equal and opposite thrust. By expelling below the waterline,
the momentum is rapidly absorbed by the surrounding stationary water;
the jet does not "push against" the surrounding water.
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