Page 249 - Practical Ship Design

Powering II 21 I

avoidance of any vibration is of course absolutely vital if the passengers are to be
kept happy. They are also, not infrequently, retrofitted as a cure for vibration. The
tip clearances for a propeller of this type can be less than for a conventional
propeller permitting a larger diameter propeller to be fitted with a small gain in
efficiency.
The astern thrust of a fixed pitch highly skewed propeller has to be limited by
blade strength problems, but there is of course no problem with the astern power of
a controllable one.

7.4.5 Self-pitching propellers
At present these are limited to relatively small powers. The concept is highly
attractive, particularly when they are used as an auxiliary on sailing yachts in place
of a folding propeller but increasing mechanical problems for large sizes seem likely
to prevent the application to large ships. An interesting feature is the fact that the
open water efficiency, not allowing for the wake, is the same astern as it is ahead.

7.4.6 Other propeller types
Tip Vortex Free (TVF) and Balanced Thrust Loading (BTL) propellers are notable
amongst the many attempts made by propeller manufacturers to improve efficiency,
but the claims made are not generally regarded as fully substantiated.
The advantage of a contra-rotating propeller is based on the improvement in
efficiency which reducing the rotational losses in the propeller race should bring.
Quite considerable claims have been made for the improvement in efficiency on
the relatively small number of ships so far fitted and these seem credible. The
mechanical complexity and high cost of the gearbox, shafting and propellers limit
applications at present but developments in hand may result in this type becoming
much more widely accepted. This type of propeller would appear to have particular
application to direct drive electric propulsion systems where the gearbox problem
can be avoided.
The same principle is used when a rotating thruster on the rudder operates in the
propeller wake. This is probably only suitable as the provider of a small augment-
ation of power and when the improved steering performance is the most important
factor.

7.4.7 Nozzles
Nozzles shrouding the propeller, notably the Kort nozzle, have as their primary
aim the improvement of the efficiency of heavily loaded propellers such as those
on tugs when towing or of fishing vessels when trawling. In this condition they are
very effective but usually at the expense of the free running speed.