Page 256 - Introduction to Naval Architecture

PROPULSION 241

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been concluded that they can be useful in large tankers where by
using slow running contra-rotating propellers the quasi-propulsive
coefficient can be increased by up to 20 per cent. In high speed dry
cargo ships, where propeller diameter may be restricted by draught,
propeller efficiency may be increased by 12 per cent Like CPPs, contra-
rotating propellers introduce mechanical complications.

Azimuthing propellers
These are propellers mounted on a housing which can rotate through
a full circle to give thrust in any direction. Drive must be through bevel
gearing and the transmissable power is limited. The usual application is
to tugs for good manoeuvrability.

Vertical axis propeller
This is essentially a horizontal disc, rotating about a vertical axis, which
carries a series of vertical blades which can rotate about their own
vertical axes. The individual vertical blades have aerofoil sections and
generate lift forces by the same principles as those described for the
screw propeller. By controlling the angle of the blades as the horizontal
disc turns, a thrust can be produced in any desired direction. Vertical
axis propellers are fitted in tugs and ferries for good manoeuvrability.
Drive again is usually through bevel gears with a limitation on the
power, see Figure 10.10.

Water jet propulsion
This type of propulsion has become more common in recent years for
high speed craft. Water is drawn into the ship and then pushed out at
the stern to develop thrust. The ejecting unit can be steerable to give
a varying thrust direction. It is attractive for craft where it is desired to
have no moving parts outside the hull. For this reason early
applications were for craft operating in very shallow water. The water jet
can be discharged either above or below water. Some hydrofoil craft use
the system, discharging above water.

Paddle wheels
A paddle wheel is a ring of paddles rotating about a horizontal
transverse axis. In very simple craft the paddles are fixed but in craft
requiring greater efficiency their angle is changed as the wheel rotates.
When fitted either side of a ship they can exert a large turning moment
on the ship by being run one ahead and the other astern. Unfortu-
nately this leads to a wide vessel. For use in narrow waterways the
paddle wheel is mounted at the stern giving rise to the stern wheeler on
the rivers of the USA.