design of an ice-strengthened propeller must meet
One of the major problems found with large
the dimensions and the strength requirements of
vessels is the misalignment of the shaft bearings.
the classification societies.
The sources of the misalignment problem are (Dick
The material used for the propeller blades of
et al. 1987):
polar ships must have high stress and impact re-
1. Deflections in the hull.
sistance qualities. Stainless steel and bronze are
2. Eccentric thrust on the propellers, which
commonly used for ice-strengthened propeller
causes bending moments in the shaft.
blades. Because stainless steel has a higher erosion
3. Insufficient axial and radial bearing flexibility.
resistance and higher ultimate and yield strengths
4. Changes in the height of bearings, gear case
than does bronze, stainless steel propellers have a
or the engine because of thermal expansion.
slender and efficient blade profile. Most of the ex-
Dick et al. (1987) have discussed other elements
isting bronze controllable-pitch propellers are op-
of the shaft line components, such as couplings,
erating in nozzles, whereas most stainless steel
seals and bearings.
controllable-pitch propellers fitted to icebreakers
are open propellers. For example, bronze has been
Mechanical transmission
selected for the propellers of recent Canadian ice-
components
breakers, and the open propellers of the U.S. ice-
The operating speed of steam reciprocating en-
breakers Polar Star and Polar Sea are made of stain-
gines and slow-speed diesel engines is low enough
less steel.
that the power can be transmitted directly through
Propeller nozzles are used to increase the thrust
a shaft between the engine and a propeller. This is
over a range of ship speed, and to protect the pro-
the most efficient form of transmitting power to a
peller from ice. Thus, the nozzles have an indirect
propeller, because the only losses incurred are at
influence on the design of a propeller by reducing
the bearings. However, most prime movers, such
the load levels and thereby reducing the strength
as medium-speed diesel and steam and gas tur-
requirements. Ships equipped with nozzles, e.g.,
bines, have an output speed that is too high to
Kigoriak and Arctic, have operated successfully in
obtain the best propeller efficiency. A speed-reduc-
ice with very few problems. Some of the shallow-
ing transmission must be used to deliver power to
draft vessels, however, have occasionally experi-
the propellers at the optimum speed.
enced clogging of their nozzles in rubbled or
As shown in Figure 15b, many icebreakers and
ridged ice. Nozzles have been installed on the azi-
icebreaking cargo vessels have been fitted with
muth-mounted propellers of Finnica and Nordica,
mechanical transmission of power since 1966. Most
and these are being considered for future high-
of these vessels are driven by one or more medium-
powered ships.
speed diesel engines and a set of single-reduction
gears, except the Russian LASH, which is driven
by a steam turbine. A clutch or fluid coupling is
Shafting
For large icebreaking ships, the diameters of
used between an engine and a gear system. In a
propeller shafts are large because of high power
few icebreakers, flywheels have also been used to
and high torque requirements. The range of diam-
smooth out the transient, ice-induced torque.
eters of the shafts installed in existing icebreakers
The gearboxes that are installed on polar ships
is from 380 mm in Polar Stern to 980 mm in the
are within the experience of the manufacturers. The
Russian SA15 cargo ships. The basis for designing
largest gearboxes installed on any icebreaker are
shaft diameter is that the propeller blade should
those on the U.S. icebreakers Polar Sea and Polar
fail before the shafting. The method to calculate
Star, which are powered by combined gas turbine
the shaft diameter depends on the modulus of the
and diesel-electric systems. The Russian SA15
propeller section and on the ratio of the ultimate
cargo ships have been fitted with large gearboxes
strength of the propeller blade material to the yield
with twin inputs, each delivering 7.5 MW, and con-
strength of the shaft material. The requirements of
nected through fluid couplings to limit overload
hydrodynamic torque and ice-induced torque are
torque.
specified by the classification societies. Shafts are
generally made of forged carbon steel, although
Electrical transmission systems
in some cases low alloy steel forgings are also used.
Four types of electrical transmission systems are
There is considerable saving in weight when high-
available for polar ships. These systems are listed
strength steel is used.
according to their chronological order of develop-
17
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