Page 326 - Practical Ship Design

284 Chapter 9

solids, it must have large hatches which will not be easy to marry in with a
sail rig. On the other hand, the carriage of bulk liquids does not present the
same difficulty and a small tanker with auxiliary sail has now been
operating successfully for a number of years.
(iii) The crew required must not be significantly more than that needed on a
conventional ship. To achieve this a high degree of automation of rig
handling must be provided. Speedy reefing (or its equivalent) must be
possible for safety when strong winds are encountered.
(iv) The automation must not have such a large auxiliary power demand nor
incur such high costs as to negate the savings made by the use of “free”
energy.
An interesting problem associated with the “wind assist” mode is the need for
the ship’s propeller to be able to adapt to negative slip, with a controllable pitch
propeller providing the answer.
Wind power does not seem likely to return as the major energy source on ships. Its
use as an auxiliary may be another matter and a number of recent installations appear
to be providing satisfactory economies in trades in which the wind spectra is
suitable.

9.7.2 Wave power
Wind is not the only free energy source available to ships and a system invented by
a Norwegian engineer utilises wave power. This device is said to be limited to
ships of up to 50 m in length and it must be admitted that its extension to larger ships
would seem unlikely. The device consists of a moveable foil placed horizontally on
an axis beneath the vessel and arranged to have an angle of attack which results in
its vertical movement as the ship moves in the waves providing a horizontal force
imparting forward movement.

9.8 FUEL ECONOMY

Much of this chapter has been concerned with fuel economy and it seems right to
conclude it with the summary of the various contributors to fuel economy given in
Table 9.2.
Whilst it is not possible to add together all the fuel savings attainable by the
various methods mentioned in this and previous chapters, there is no doubt that
large aggregate savings can be made.
Some savings reduce the possibilities of further savings; for example, it has
already been noted that the improved efficiency of diesel engines leaves less waste
heat to be reclaimed. On the other hand, there is the odd case where one saving may