HOW DOES THE JET
PROPULSION SYSTEM WORK?
A water jet generates propulsive thrust from the reaction
created when water is forced in a rearward direction. It works
in relation to Newton's Third Law of Motion - "every action has
an equal and opposite reaction". A good example of this is the
recoil felt on the shoulder when firing a rifle, or the thrust
felt when holding a powerful fire hose.
Put simply, the
discharge of a high velocity jet stream generates a reaction
force in the opposite direction, which is transferred through
the body of the jet unit to the craft's hull, propelling it
forward (see diagram to the left).
In a boat hull the jet unit is mounted inboard in the aft
section. Water enters the jet unit intake on the bottom of the
boat, at boat speed, and is accelerated through the jet unit and
discharged through the transom at a high velocity.
The picture to the left shows where water enters the jet unit via the
Intake (A). The pumping unit, which includes the Impeller (B)
and Stator (C), increases the pressure, or "head", of the flow.
This high pressure flow is discharged at the nozzle (D) as a
high velocity jet stream. The driveshaft attaches at the
coupling (F) to turn the impeller.
Steering is achieved by changing the direction of the stream of
water as it leaves the jet unit. Pointing the jet stream one way
forces the stern of the boat in the opposite direction which
puts the vessel into a turn.
Reverse is achieved by lowering an astern deflector (E) into the
jet stream after it leaves the nozzle. This reverses the
direction of the force generated by the jet stream, forward and
down, to keep the boat stationary or propel it in the astern
direction.
Information and images used under permission from Hamilton Jet.
Further information regarding jet propulsion systems :
http://www.hamiltonjet.co.nz/ |
