National Archives. Via Wildenberg (1996)
National Archives. Via Wildenberg (1996)
Ship machinery refers to the boilers and engines
that drive a ship's
propellers, together with the associated pumps,
gearboxes, condensers, and other auxiliary machinery. For most
purposes, the details of the machinery are less important than the
maximum speed of the ship and its fuel consumption curve
(equivalent to its range at various speeds.) The number of shafts and
boilers does have some bearing on the ability of the ship to survive
flooding or other damage to the machinery spaces, propellers and
propeller shafts, and steering. The more, the better, so far as
survivability is concerned.
By the time of the Pacific War, all ships of any
military significance were fueled either with coal, heavy fuel oil, or diesel oil.
Coal was widely used in steam ships at the start of the twentieth
century, and it has the advantage of being found in abundance
throughout the world. However, it is a dirty fuel that produces a
considerable volume of ashes (clinker), and, as a solid fuel, it is
difficult to handle. Fuel oil burns
cleaner than coal without producing clinker, yields 40% more energy per
unit weight, and is a liquid that is relatively easy to store and
handle. However, it is scarcer than coal. Diesel oil, being more highly
refined, burns cleaner than fuel oil, but is also more expensive.
Unlike coal or fuel oil, which are burned in a boiler to produce
steam which is then fed to an engine, diesel fuel is burned in the
engine itself (which is then known as an internal combustion engine.)
Steam engines.
Most of the warships of the Pacific War burned oil in a boiler to
produce hot steam. The chemical energy released when the fuel was
burned was captured in the steam in the form of heat energy, which
imparted enormous pressure to the steam. This pressure was used to
perform mechanical work, converting part of the heat energy into
mechanical energy to drive the propellers. The engines that performed
this conversion were designed to convert the energy as rapidly and
efficiently as possible, so that the engines could produce high power
without wasting fuel.
The laws of thermodynamics governing steam engines
are beyond the scope of this Encyclopedia. It is sufficient to know
that it is physically impossible to convert all the heat energy of hot steam
into mechanical work. All engines work by tapping a flow of heat from a
high temperature source (the hot steam) to a low temperature sink
(ocean water pumped through a condenser), and at least some of the
energy must reach the low temperature sink to maintain the energy flow.
The greater the difference in temperature between the source and the
sink, the smaller the fraction of the energy flow that must reach the
sink and the greater the fraction that can be diverted to do mechanical
work.
Thus, the higher the temperature of the steam, the
more efficiently its heat energy could be converted into mechanical
energy to drive the propellers. Modern ships used superheated steam,
with a temperature far above the normal boiling temperature of water.
Unfortunately, this meant that if battle damage to the machinery
allowed the steam to escape, it could kill the engine room crew in
seconds. The boilers were designed with a maze of small tubes
containing the water to be boiled, which provided a very large surface
area through which to absorb heat from the burning fuel. These tubes
also rapidly filled with scale from any impurities in the water,
necessitating laborious boiler maintenance.
The oldest warships, as well as a sizable
fraction of auxiliaries and merchant ships, fed the superheated steam
into a set of cylinders with pistons resembling those of most
automobile engines. The pistons drove a crankshaft that was connected
to the propeller shaft. As the steam drove the piston, converting heat
energy into mechanical work, the steam rapidly cooled, and the
resulting temperature fluctuations in the piston and cylinder wasted
energy. To improve efficiency, the steam usually passed through a set
of three cylinders. Each extracted only part of the energy,
reducing the temperature fluctuations and the resulting energy loss.
These so-called triple-expansion reciprocating engines were still
relatively
inefficient, converting less than 30% of the heat energy into
mechanical work; they were noisy and produced heavy vibrations; and
they took up a great deal of space. Most of the warships of the Pacific
War used turbine engines instead.
A turbine engine consists of a rotor mounted in a
heavy outer casing. Attached to the rotor are a set of wheels, each of
which carries a set of blades that somewhat resembles a large fan.
These are known as moving blades. The case also has sets of blades
mounted in its interior that are known as stationary blades.
A turbine typically has several stages, or sets of
blades, each of which extracts part of the heat energy of the steam.
These stages typically increase in diameter in the direction of the
steam flow, allowing the steam to expand as it moves through the
turbine. There are several distinct types of turbine stage,
corresponding to efficient operation at different pressure levels.
A modern warship typically had three or four
turbines on each shaft. At a minimum, there was a high-pressure
turbine, a low-pressure turbine, and an astern turbine. There might
also be a cruising turbine. The astern turbine was often installed in
the same turbine case as the low-pressure turbine, while the cruising
turbine was often installed at the front of the high-pressure turbine.
During normal cruising, steam passed from the boiler through the
cruising turbine, then the high-pressure turbine, and finally the
low-pressure turbine before entering the condenser, returning to the
liquid state, and being pumped back to the boiler. When maximum power
was required, the cruising turbine was bypassed, so that steam passed
directly from the boiler to the high-pressure turbine. This gave
maximum power at a significant cost in efficiency. The direction of
propulsion could be reversed by feeding the steam directly to the
astern turbine.
The high-pressure turbine usually began
with one or more Curtis stages. A Curtis stage consists of a nozzle
diaphragm, a set of moving blades, a set of stationary blades, and a
second
set of moving blades. Steam passing through the nozzle is accelerated
to a high velocity while losing pressure, as part of its heat energy is
converted to kinetic energy (energy of motion). The jet of fast-moving
steam strikes the first row of moving blades, losing velocity as part
of its kinetic energy is transferred to the blades, which turn the
rotor. The steam "ricochets" off the first set of moving blades into
the stationary blades, which redirect the steam back at the second set
of moving blades. Here the steam again loses velocity as it transfers
kinetic energy to the second set of blades and from there to the rotor.
Because heat energy is converted to kinetic energy only in the nozzle,
and not in the blades, the Curtis stage is classified as an impulse
stage. Typically the entire high-pressure turbine is composed of
impulse
stages, separated from one another by nozzle diaphragms.
The final stages of the high-pressure turbine are
typically Rateau stages. These are very simple stages consisting of a
nozzle diaphragm and a single set of moving blades. Like the Curtis
stage, the Rateau is classified as an impulse stage.
The low-pressure turbine typically consists of several sets of Parsons
stages. The Parsons stage is classified as a reactive stage rather than
an impulse stage, because the blades themselves convert heat energy to
kinetic energy
rather than relying on a separate nozzle to perform this function. This
is more effective at lower steam pressures than an impulse stage. The
Parsons stage consists of a set of stationary blades followed by a set
of moving blades, all shaped like tiny airfoils. Steam passing over the
stationary blades is accelerated as its pressure drops. The steam then
passes over the moving blades, creating "lift" and losing both pressure
and velocity as its thermal and kinetic energy is transferred to the
blades.
When space was at a premium, the low-pressure turbine was split into
two halves facing each other inside the turbine case.
Turbine engines operate best at high rotation
speeds. This requires very large turbines if the rotor is connected
directly to the propeller shaft, since propellers do not function well
at very high rotation speeds. The alternative is to use smaller
turbines operating at very high speed and connected to the propeller
shaft through a complicated reduction gearbox. The majority of warships
that saw service in the Pacific War used these geared turbine engines.
Well-designed turbines could achieve 40%
efficiency. This is close to the practical limit of turbine
technology: The 50% efficiency barrier remains unbroken at the start of
the 21st century.
Turboelectric
Engines. A number of American
battleships and aircraft carriers
were equipped with turboelectric drive. Instead of driving the
propellers directly, the turbines drove large electrical generators.
The electrical power was then bussed to large motors that drove the
propeller shafts. This eliminated the need for reduction gearing and
allowed the machinery to be spread out over a
larger number of compartments, improving subdivision. It allowed
considerable flexibility in how the engines were operated. Some of the
carriers with turboelectric drive could even be propelled astern at
full power, allowing flight operations to take place in either
direction of the flight deck. There was some energy lost in the
conversion from mechanical to electrical and back, but since electrical
generators and motors could be designed with better than 85%
efficiency, this was not critical. However, turboelectric drives proved
less robust than expected
under battle conditions, and later American
warship designs reverted to direct drive.
Diesel Engines.
Although diesel fuel is more expensive than fuel oil, and
diesel engines have some of the faults of reciprocating steam engines,
a well-designed diesel engine could achieve efficiencies approaching
60%. As a result, smaller warships requiring long cruising ranges,
particularly submarines, were
equipped with diesel engines.
References
Federation
of American Scientists (accessed 2009-2-3)
Lacroix
and Wells
(1997)
The Pacific War Online Encyclopedia © 2009 by Kent G.
Budge. Index
