Page 248 - Practical Ship Design

210 Chapter 7

7.4.2 Single or twin screw

If the speed and power are high in relation to ship size and in particular to the
maximum allowable draft, it may be necessary to have twin propellers (or triple or
quadruple) to enable the required power to be absorbed by propellers whose
diameter can be accommodated within the draft.
Twin or multiple screws are generally chosen for ships requiring a high degree
of reliability and where the cost of immobility from a breakdown for even a short
time is high or verges on being unacceptable - cruise liners and ferries.
Multiple screws are also chosen when there is a particular likelihood of a set of
machinery being put out of action - warships by enemy action or ice breakers by
ice (although see $16.5.2), with in both cases there being the possibility of either
damage to a propeller or the flooding of a machinery compartment.
Twin screws used to be chosen for the better manoeuvrability they provide but
the provision of a bow thruster and/or a high performance rudder can now so
improve this aspect of the performance of single screw ships that this is no longer
the case in relation to slow speed manoeuvring in docks canals and rivers. Where
there is a particular requirement for high speed manoeuvring as applies to a
warship, twin screws continue to be the best choice.

7.4.3 Controllable pitch propellers

These are often selected in spite of their slightly poorer efficiency because of the
contribution they can make to the ease of manoeuvring particularly in ships which
have to operate frequently in confined waters. They can also be chosen to match
the characteristics of an engine which is best run at one speed all the time and in
other cases may be chosen for fuel efficiency reasons if the ship is intended for
operation at more than one significantly different speed or displacement. On
warships where the cruising endurance speed is often less than half the full speed, a
controllable pitch propeller avoids the over-torquing of the cruising engine that
might be caused by a fixed pitch propeller.
An alternative to a C.P. propeller in such a case would be a two speed gearbox.
It has been suggested that the hub drag mentioned in $7.3.1 if correctly included
would further reduce the efficiency of these propellers.

7.4.4 Highly skewed propellers

Propellers with highly skewed blades can be either fixed pitch or controllable. The
skewing makes little difference to the efficiency, but reduces the propeller-induced
forces on the hull. Propellers of this type are quite often fitted on cruise liners and
ferries on which there appears to be a possibility of vibration and on which the