waste heat and converting it to mechanical work,
and this would wear off during first few days of
or by operating each prime mover according to
icebreaking. In the early 1970s, the importance of
load demands to obtain better fuel economy. The
hullice friction on the ice resistance was demon-
first option has not been used in icebreakers so far.
strated through full-scale and laboratory tests. A
The USCG icebreakers Polar Sea and Polar Star
measure of the force attributable to static friction
are the only polar vessels fitted with two types of
acting on a hull can be obtained by gradually in-
prime movers. In these ships, there are three gas
creasing the level of power to initiate forward
turbines (total 45 MW or 60,000 shp) and three die-
motion of a ship that was stopped in ice and then
sel-electric propulsion systems (total 13.4 MW or
measuring the steady-state velocity at that same
18,000 shp) for each of the three controllable-pitch
power level. For ships having uncoated hulls, this
propellers. Each shaft can be turned either by the
power level corresponds to a 3-knot (1.5-m/s)
diesel-electric or the gas turbine power plant. Ei-
speed of advance, whereas for a ship with low-
ther one or two 2.24-MW (3000-shp) diesel-elec-
friction coating, the initiating power levels are
tric drive units, or a single 15-MW (20,000-shp) gas
equivalent to a speed of 0.5 knots (0.26 m/s) (Voelker
turbine, can be used to drive each shaft. For ex-
1990). The power required for an icebreaker with a
ample, diesel engines could supply power to the
low-friction coating to become unstuck is much
wing shafts, while a gas turbine could turn the
lower than that for ships without any coating.
center shaft. Gas turbines are used for heavy ice-
Mkinen et al. (1994) have given an historical
breaking, whereas the diesels are used for cruis-
account of the development of low-friction coat-
ing and light icebreaking. This is a good example
ings in Finland, where the first effective hull coat-
of combining two different systems to meet widely
ings were developed by Wrtsil Shipyard (now
differing load demands for the sake of fuel
Kvrner Masa-Yards). Liukkonen (1992) devel-
economy.
oped a theoretical understanding of hullice fric-
tion and found a functional relationship between
the coefficient of friction and the normal force. This
functional relationship was verified by full-scale
AUXILIARY SYSTEMS
measurements of normal and frictional forces with
There have been other developments to improve
the help of instrumented panels installed in the
the performance of polar ships besides those in
bow and the sides of icebreakers.
propulsion systems and hull shapes, such as the
Mkinen et al. (1994) have listed the require-
use of low-friction coatings on the hull, air-bub-
ments of a good low-friction coating. A few of these
blers to lubricate the ice/ship interface, air-bub-
are reasonable cost, high bond strength with and
blerwater-injection systems, and the water-del-
good corrosion protection for the base material,
uge (or wash) system to pump a large volume of
and resistance to all of the following: wear, high
water on the ice ahead of the vessel. These im-
normal pressure, low temperatures and changes
provements have also contributed to increase the
in temperature. Tests were conducted on many dif-
icebreaking capability of polar ships beyond the
ferent coatings; Inerta 160 and stainless steel were
limit for which they were designed. A brief ac-
selected for full-scale testing and further devel-
count of each auxiliary system follows.
opment. Another coating by the name of Zebron
was also found to be suitable, but its use has de-
creased with time, perhaps because of lower resis-
Low-friction hull coating
Depending on the age of a vessel, the coefficient
tance to wear.
of friction between ice and unpainted hull plating
Inerta 160 has been applied to hundreds of ships
can be in the range of 0.2 to 0.3, which is high in
currently in service (Mkinen et al. 1994). It is ap-
comparison to the friction coefficient in the range
plied with a two-component spray gun, which has
of 0.05 to 0.17 between ice and a low-friction coat-
heating equipment to keep the temperature of the
paint between 40 to 50C. Two problems associ-
ing. As discussed later, the factor to account for
the friction of old steel in the expression for ice
ated with the application of Inerta 160 were corro-
resistance of an icebreaker is twice that for Inerta-
sion of cast iron propellers and corrosion of hull
coated steel plates (Keinonen et al. 1991).
surfaces. These problems were corrected by using
Prior to the 1970s, there was no suitable coating
stainless steel propellers and cathodic corrosion
available that could withstand interaction with ice.
protection.
Only anti-fouling paint was applied to the hulls to
An important property of a coating is to with-
minimize biological growth on the hull surface,
stand the deformation of the base material. In the
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