60
a
Medium Speed Diesel
50
Slow Speed Diesel
Steam Turbine
Gas Turbine
40
30
20
10
0
Diesel-mechanical Propulsion System
35
b
30
25
20
15
10
5
0
1940
1950
1960
1970
1980
1990
Azimuth Propulsion System
Year of Construction
Figure 18. Prime movers installed on icebreaking ships:
Figure 17. Differences between diesel-
(a) total power vs. year of construction, and (b) power
mechanical and azimuth installations
per shaft vs. year of construction (after Dick and
(after Kvrner Masa-Yards and ABB,
Laframboise 1989).
no date).
neuverability, robustness and over-torque capabil-
4. Reduces noise and vibrations.
ity (Dick and Laframboise 1989). These character-
5. Provides operational flexibility, resulting in
istics have been discussed earlier for the propul-
lower fuel consumption, reduced maintenance
sion system. The prime movers used currently in
costs, fewer exhaust emissions and adequate re-
polar ships do not have all these characteristics,
dundancy with less installed power.
but in combination with a suitable transmission,
In late 1990, the propulsion system of the Finn-
the overall propulsion system can approach the
ish waterway service vessel Seili was converted
above-mentioned ideal characteristics.
from diesel-mechanical propulsion to azimuth
Figure 18 shows two plots of total installed
(Azipod) propulsion. The performance of this ves-
power and power per shaft versus the year of con-
sel was tested in 65-cm-thick, level ice in the Gulf
struction. In Figure 18 different types of prime
of Bothnia. Laukia (1993) reported that, besides
movers have been identified. Each type is briefly
good maneuverability and icebreaking capability
discussed in the following.
in level ice and first-year pressure ridges, the ves-
sel broke ice better when moving astern than while
Gas turbines
moving ahead. There are unconfirmed reports
Only two icebreakers, the USCG Polar Star and
that new vessels with two types of hulls at each
Polar Sea, are fitted with gas turbines. Each ship
end are on the drawing boards of shipyards: a
has three aero-engine derivative gas turbines, each
smooth bow for moving forward in open-water,
driving a controllable-pitch propeller through a
and an icebreaking stern for moving astern
gearbox. These turbines are used only for heavy
through first-year ice in sheltered areas.
icebreaking, and a medium-speed diesel-electric
propulsion system is used for cruising and light
Prime movers
The characteristics of an ideal prime mover for
icebreaking. The Canadian icebreaker Norman
McLeod Rogers was initially fitted with two indus-
an icebreaking ship are reliability, flexibility, ma-
19
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