Tug Propulsion Machinery


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.


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.


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


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.


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.


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.


Illustrations on this page are from 1890's era manufacturers catalogs.


© 2007. Tugboat Enthusiasts Society of the Americas