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RECIPROCATING STEAM ENGINES
New York Central No.16 was a retired steam tug that
was salvaged from a marine scrapyard and installed next to a restaurant
on the traffic circle next to the Bourne Bridge. The tug was a landmark
for travellers to and from Cape Cod until its demolition in 2007
to make way for a CVS Pharmacy. Photo by Preston Cook.
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Above: A small single cylinder simple reciprocating
steam engine from the 1890s. This would be a typical launch or small
tug engine in the time period. Simple engines work directly off
the live steam pressure from the boiler, with the exhaust steam
generally being vented directly to atmosphere.
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The application of reciprocating steam engines to
vessel propulsion was a natural extension of their early use in
stationary and locomotive service. Thousands of tugs, from the earliest
tug known, Scotland’s little Charlotte Dundas, were built
with such engines. The few remaining original steam tugs are significant
historical artifacts of a departed era.
In the beginning, the fuel used was coal, or sometimes wood, but
about the time of the first World War, oil gradually began replacing
coal because more fuel could be carried per cubic volume. Oil was
far cleaner, with no coal dust when being loaded and no ashes to
be hoisted from the engine room and dumped overboard. In oil burning
tugs fuel was being consumed only while underway, and a smaller
crew was possible because there were no standing fires to tend in
the boilers.
Marine steam engines can be described according to their basic
design. In “simple” engines, the working steam pressure
from the boiler goes directly to one or more cylinders. In contrast,
in “compound” steam engines the working steam from the
boiler goes to one or more high pressure cylinders, where it expands
and cools while surrendering part of its pressure as work. The lower
pressure exhaust steam is then sent through one or more larger cylinders
to obtain additional work and improve propulsive efficiency. The
type of compound engine can further identified by a name such as
“triple expansion”. In these engines the steam is allowed
to expand three times before being exhausted from the lowest pressure
cylinder. In smaller tugs there was little advantage to using a
compound engine, as the engine was much more complex than a simple
steam engine, and steam had to be admitted to the low pressure cylinder
for starting which negated the advantage of a compound if the tug
had to reverse frequently. Many New York shipdocking tugs of the
World War I. era had a simple one-cylinder engine generating perhaps
300 horsepower.
Two terms were used to describe what happened to the exhaust steam.
In a “condensing” steam system, the exhaust is routed
through a condenser that converts the steam back to water. Once
any carryover of cylinder lubricating oil in the water is removed,
this water can then be reused as boiler feedwater. In a “non-condensing”
system the exhaust is simply disposed of, usually through a trumpet
pipe adjacent to the smoke stack. Non-condensing tugs were easily
recognizable because of the white plumes of vapor they trailed.
Operating reciprocating steam engines took considerable foresight
on the part of the topside personnel and quick reactions in the
engine room crew. The primary means of communication between the
wheelhouse and the engine room were a bell and gong. Various combinations
of “boings” and “dings” told the engineer
in what direction his engine should be turning and at what speed.
He had to turn valves to change speed and use levers to move valve
chains in order to reverse the direction of engine rotation. And
there was always the threat present in any one-cylinder steam engines
that it could stop on “dead center”, where the connecting
rod is lined up with the crankshaft throw and has no mechanical
advantage to get the engine rotating again. The engine room crew
had to watch carefully when stopping the engine to let the freewheeling
propeller drag the engine to a rest position that was not likely
to result in a dead center stall on reversal.
Steam tugs were not very efficient. Thermal efficiency
is expressed as a percentage of the heat energy of the fuel that
actually is used to generate work, the rest being lost in exhaust,
internal friction, and heat radiation. In small vessels like tugs,
steam propulsion systems typically had an overall thermal efficiency
of 6% for non-condensing engines and compounds were not a lot better.
Even the earliest diesels were in the range of 30% thermal efficiency
(nowadays a few are around 50%), and required much less down time
for fueling and maintenance. Consequently the application of steam
machinery to new construction tugs virtually disappeared in the
1950s, just about the same time that the major builders of steam
locomotives all turned to production of diesels. Vast numbers of
suitable diesel engines became available after the end of World
War Two, and many tugs promptly lost their steam plants and got
a diesel from an LST or other warship. Diesel power has remained
the predominant tug propulsion system for more than fifty years,
although early forms of hybrid power systems are now appearing in
marine and industrial applications.
The write-up above was prepared by Preston Cook
and Hugh Ware
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Above: An 1890s manufacturers view shows the valve
gear and reversing lever of a reciprocating steam engine. The notches
in the flywheel for a locking pawl to hold the engine during reversals
are also visible.
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Above: Two cylinder compound steam engines did not
always have the cylinders side by side. This compound has the high
pressure cyinder on the center crankshaft throw while the low pressure
cylinder is directly above it, the motion being transmitted to the
crankshaft through a pair of piston rods, crossheads, and connecting
rods to the first the third crankshaft throws. This engine uses
a "D" valve, or slide valve, to control the steam admission
to the low pressure cylinder, while the high pressure steam passes
through a spool valve.
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Above: These side and front views show a typical
triple expansion steam engine. This one is a conventional inline
arrangement with the high, medium, and low pressure cylinders in
a row from the front to the rear of the engine.
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Illustrations on this page are from 1890's era manufacturers
catalogs.
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