Page 256 - Practical Ship Design

Powering II 215

There was a good deal of interest some years ago in the use of nozzles on large
tankers and a number of ships had installations of this sort, although its use never
really caught on.
More recently with a general acceptance of the advantage which can be obtained
from a slow revving propeller, nozzles are now only likely to be considered when
for any reason the propeller diameter must be limited. e.g., Great Lakes Bulk
Carriers with a small draft in relation to their size.
Both full nozzles and half nozzles can help to reduce propeller excited vibration
by improving the flow into the propeller.
Steering nozzles, with or without fins allow the propeller to be moved aft
increasing the usable length of the ship but can be liable to maintenance problems.

7.4.8 Propeller position

All the propellers discussed so far can only be fitted at the stern of the ship. This is
not necessarily the best place for the thrust to be developed. For tugs there is a very
great advantage in the thrust being developed forward, so that the tug is pulled into
line with the tow rope, and the risk of capsize thereby greatly reduced or indeed
eliminated. Two types of propeller can be fitted in this way: the Voith Schneider
and the Steerable Thruster. The former was the first in the field and has the
advantage that it does not project so far below the ship to which it is fitted. It has
been used in many double ended ferries with considerable success. The latter is
cheaper, but projects well below the hull of the vessel to which it is fitted increas-
ing the draft to an extent that may be unacceptable on the shallow draft craft to
which it might be applied.
Propellers which can be fitted forward totally within the ship’s hull are the
Schottel cone jet and the Gill axial flow propellers. Most uses of these types are as
a bow thruster or a get-you-home auxiliary as the greatest powers so far made
would only be suitable as a main propulsion unit for small/slow ships.

7.4.9 Paddle wheels

A propulsion unit from the past is the paddle wheel. Paddle wheels can be fitted at
the sides or at the stern. A feathering paddle can have an efficiency under certain
conditions as high as that of a screw propeller, but its vulnerability to damage and its
limitation to operation at a fixed draft mean that it is no longer a serious competitor.

7.4.10 Water jet propulsion

A comparative newcomer for serious consideration at quite high powers is water
jet propulsion. This system has been used for many years for small craft, but