Page 257 - Practical Ship Design

216 Chapter 7

generally has had low efficiency except at high ship speeds. Recent developments
have greatly improved the efficiency at lower speeds and at the same time there has
been a big increase in the power to which these units are manufactured and an
application to warships of frigate size is now a possibility.
The success of “Destriero”, crossing the Atlantic at 56 knots will no doubt boost
interest in water jet propulsion.

7.4. I1 The future

Looking into the future (possibly to quite a distant future) electromagnetic propul-
sion, a radically different type of propulsor now the subject of theoretical work and
small-scale experiments, may turn out to have both the higher efficiency and the
good control characteristics being sought.

7.4.12 General

Most of the types of propulsors mentioned above are shown in outline in Fig. 7.1 1.
These by no means exhaust the whole field of propulsors for marine vehicles, but
most of the others do not apply to displacement ships. Transcavitating, super-
cavitating and surface-piercing propellers apply to fast planing craft; air propellers,
fans or jets have their application on hovercraft. There are a few other types which
might potentially be used for displacement ships, but these have hardly progressed
beyond small scale prototypes.

7.4. I3 Open water efliciency

Curves of the open water efficiency of a number of propulsors are shown in Fig.
7.12 which indicates quite clearly the superiority of the conventional propeller
from a straight propulsive efficiency point of view, although as already stated this
may sometimes be overridden by other considerations.

7.5 PROPULSIVE EFFICIENCY

In the last section in which various types of propulsors were discussed it was
shown that a conventional propeller provided the highest propulsive efficiency,
although other propulsors might have other advantages that outweighed this. In
this section attention will be confined to conventional propellers.