trial turbines, but they were replaced with me-
visions, in the form of controllable-pitch propel-
dium-speed diesel engines because of high fuel
lers or reversing gears, are used for astern op-
consumption.
erations. Typical specific fuel consumption of the
Turbines are unidirectional engines, and the
engines is between 170 and 200 g/kWh, and the
astern operations must be provided by the trans-
consumption of lubricating oil is between 1.5 and
mission, usually through an electrical system, a
3 g/kWh. Most medium-speed diesel engines for
reversing gear or a controllable-pitch propeller. The
icebreakers use turbochargers to improve their fuel
advantages of gas turbines over other prime mov-
efficiency in open water. Diesel engines are basi-
ers are their high power-to-weight ratio and their
cally constant torque machines in the 50100%
compactness. Their main disadvantages are the
range of speed. At a given load, torque may ex-
high fuel consumption and maintenance re-
ceed the rated capacity by about 10%. The flexibil-
quirements.
ity of diesel engines is acceptable because they can
operate between 25 and 35% of their rated speed,
Steam turbines
depending upon the characteristics of a particular
Only the Russian nuclear-fueled icebreakers and
engine. It is expected that medium-speed diesel
icebreaking cargo ships are fitted with modern
engines will continue to be the preferred prime
steam turbines. The Canadian icebreaker Louis S.
movers for polar ships of all sizes in the near fu-
St. Laurent was fitted with a steam-turbineelec-
ture (Dick et al. 1987).
tric system, but a diesel-electric system was in-
stalled during the ship's major reconstruction pro-
Slow-speed diesel engines
gram, completed in 1993. The efficiency of a steam
The Russian LASH ship Alexey Kosygin is the
turbine is about 20%, compared to 50% for mod-
only polar ship fitted with two slow-speed diesel
ern marine diesel engines (Dick et al. 1987). Simi-
engines, each delivering 13.4 MW to directly drive
lar to gas turbines, steam turbines are unidirec-
fixed-pitch propellers (Dick et al. 1987). This type
tional engines, and astern operations must be
of engine was specifically developed for ship pro-
handled by the transmission. Turbines can oper-
pulsion. They operate on the two-stroke cycle, are
ate at any power level, but the fuel efficiency is
reversible, and are directly coupled to propellers,
poor at reduced power levels.
mostly of the fixed-pitch type. The range of their
rotational speed is between 60 and 225 rpm. The
Medium-speed diesel engines
range of cylinder bore diameter is from 250 to 900
Medium-speed diesel engines have most com-
mm. The maximum power per cylinder is about
monly been used as prime movers for the propul-
3.7 MW. This type of engine is large and heavy,
sion of polar ships because of their compactness,
and it can only be fitted to vessels that can provide
light weight, fuel efficiency and good reliability
a large engine room and carry the extra weight:
(Dick and Laframboise 1989). Their disadvantage
bulk cargo ships, oil tankers and container ships.
for use as prime movers is their lack of significant
Ferries, Ro/Ro ships and barge carriers have lim-
over-torque capacity. However, this shortcoming
ited head room and are generally fitted with me-
is overcome by using an electrical transmission,
dium-speed diesel engines. These engines are not
which damps out the high torque transients and
suitable for polar ships because of their poor ma-
stops them from being transmitted to the engine.
neuverability and flexibility.
A few icebreakers are fitted with these engines
Developments in the last 15 years include the
driving controllable-pitch propellers through
use of constant pressure turbocharger technology
gears. Some of the direct drive systems consist of
and the adoption of extra-long strokes. This has
fluid couplings to prevent engine stall under the
enabled slower propeller speeds without the use
most severe propeller overloads.
of gears, resulting in higher propulsion efficiency
In the past 15 years, medium-speed diesel en-
in large bulk carriers and oil tankers. The specific
gines have undergone developments that have
fuel consumption of these engines is below 160 g/
allowed them to have better fuel economy, burn
kWh for large economical engines, and about 175
heavier grades of fuel, increase routine mainte-
g/kWh for small engines.
nance intervals and increase the power per cylin-
der. Some of the largest engines of this type can
Combined prime movers
generate about 22 MW at 400 rpm in 18 cylinders
The reason for combining two different prime
arranged in a vee form (Dick et al. 1987). The en-
movers in a ship is to improve the overall fuel
gines operate in one direction, and separate pro-
economy. This is done by either recovering the
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