2.3.3. V - type drive
2.3.4. Jet type drive
More and more civilian and military boats, such as rescue boats, pilot boats, transport boats,
taxi boats, police boats and patrol boats, are using waterjets for propulsion. The waterjet
offers many benefits, which include giving the boat greater availability and maneuverability.
The waterjet also minimizes the draught of the boat and enables it to operate in shallow
water, as well as reducing the risk of personal injury during rescue and diving operations.
To obtain good economy from a waterjet installation, the jet unit and the engine should be
correctly matched to the full load service speed of the boat. The differences between a good
and a poor match are enormous with regard to fuel efficiency and overall performance of the
boat. A correctly sized and installed waterjet gives very small torque variations and creates
no engine overload, regardless of the boats loading conditions and speed. The jet is always
rotating in one direction, but the reversing of the boat is done by changing the jet stream
direction with a reverse deflector, which further reduces the load variations on the engine.
Waterjet has excellent characteristics when it comes to general maneuverability and comfort.
Superior control of the boat is achieved across the complete speed range, with small turning
radius and quick stops. The boat can rotate within its own length and with two jets the bot
can also move sideways. The lack of need of underwater rudder can make the vessel less
course stable, it is therefore important that the hull and the maneuvering control system is
correctly designed for the use of waterjets.
A waterjet installation has no underwater appendages which reduces the drag of the hull and
increases the overall efficiency in speeds above 20 - 30 knots depending on type of waterjets.
In comparison with conventional propeller system the inboard noise, vibration and the hydro-
acoustic noise is reduced as well.
Jet-type drives work according to the principle of jet propulsion. A jet of water is generated
whose thrust sets the vessel in motion.
- 10 -
Engine and reduction gear is installed
separately and connected via an elastic
coupling and a universal joint. If universal
joints are to be used, it is important to
remember that it is necessary to have the
exact same angle at each joint under all
operating conditions in order for the
system to work properly.