Page 318 - Practical Ship Design

276 Chapter 9

Bringing all the electrics together in one system - along with some reduction in
the relative cost of electric propulsion systems - have combined to reduce the
extra cost of today’s type of electrical propulsion and it is now the favoured system
for large cruise liners, on which its many operational advantages outweigh any
residual extra cost.
A system of this sort has also become widely accepted for specialist ships such
as research vessels, ice breakers, cable ships, fish factory ships, oil production
vessels.
In general, electric propulsion is attractive either:
- where there is a large non-propulsion electrical load as on a passenger ship, or
- where there are a number of propulsion devices spread throughout the ship,
such as the azimuth thrusters of a dynamically positioned offshore vessel.

Amongst the advantages which electrical propulsion has for a cruise liner are:
(i) the possibility of maximum attenuation of noise and vibration;
(ii) the ease of providing the large, but very occasional, electrical demands of a
bow thruster without the need of a special system;
(iii) an ability to operate at very low ship speeds;
(iv) a high degree of redundancy giving good reliability.

A good description of a modem diesel electric system which explains in some
detail the reasons for its adoption and the alternatives considered is given in a paper
entitled “Fantasy and reality” presented by J.W. Hopkins to the Institute of Marine
Engineers in 199 1.
The prime movers for a diesel-electric installation of this sort can be either
medium- or high-speed diesels, the former being the preferred choice on cruise
liners and the latter on smaller specialist vessels.
The number and size of generators must be arranged to suit a scenario of
different loadings and there will generally be advantage in having two different
powers of engine to match these as closely as possible. “Fantasy” for example has
four 12-cylinder engines, each developing 7920 kW and two 8-cylinder versions of
the same engine, each developing 5280 kW.
The advantage of having all the engines on the ship of the same type starts with
the price advantage from bulk buying and continues with such benefits as the need
for only one fuel, a reduced requirement for spares, easier maintenance and repairs.
If medium speed engines are chosen there is the advantage of using a cheaper fuel
for all purposes instead of this being used only for propulsion.
The high powedweight and more importantly power/space ratio of gas turbines
is now starting to be appreciated by the owners of large cruise liners who can see a
change to this type of prime mover helping them to achieve a worthwhile increase
in the number of passengers carried in a particular size of ship.