The triple expansion engine is a stage in the evolution of the steam engine which began with the Newcomen atmospheric engine. In this type of engine the steam was introduced in to a cylinder at low pressure and temperature and condensed. This formed a vacuum in the cylinder which allowed the weight of air above the piston to force the piston down. The next development was to use the steam positively in association with the condensing action. Again the steam pressure was relatively low as was the temperature. Over the succeeding decades the steam pressures and temperatures increased. This led to lighter , faster engines which were more efficient and used less fuel. The increasing steam pressure required a concomitant improvement in the method of construction of steam boilers. However, the use of high pressure steam was still generally considered unsafe and in the late 19th century a member of the British Board of Trade expressed concern for steam pressures above 70 lbs per square inch in merchant ships.
Even so, the need for greater economy in the operation of steam engines; especially on ships where coal bunkerage was limited, required an increase in steam pressure. In the late 18th century experiments had been carried out with the use of high pressure steam on existing low pressure engines by placing a high pressure cylinder alongside the low pressure one on the engine. The experiments were successful with increases in power output.
With a different understanding of steam ; as an 'elemental force' instead of a vapour, innovators in the 19th century began to use steam expansively, first in the high pressure cylinder and then exhausted into a low pressure cylinder where it continued to expand. This process was known as compounding. This was a very successful way of using more of the power of high pressure steam to effect economies in fuel and water use. Compounding was used on river vessels as early as the 1820s but it was not until the 1870s that compounding became popular on sea-going ships. The next evolutionary step was to introduce a third, and intermediary, cylinder between the high pressure and the low pressure cylinders to use more of the expansive power of the available steam. Known as triple expansion, this type of engine appeared in 1880. The final evolutionary stage in reciprocating steam engine development was the introduction of quadruple expansion in the 1890s.
R&W Hawthorn began the manufacture of triple expansion marine engines in 1885 and in 1886 joined with the Hebburn shipyard of Andrew Leslie to form R&W Hawthorn-Leslie Ltd. It may be coincidental that in the year prior to the manufacture of the 'Pheasant' engine (1889) the colonial navy had fitted Hawthorn-Leslie made triple expansion engines in three of their vessels under construction by Armstrong's. The vessels were: Katoomba (engine number 2125); Mildura (engine number 2126); Wallaroo (engine number 2127).
The use of the Parramatta River and Sydney Harbour as a means of communication since the late 18th century began with the 'Rose Hill Packet' and has continued to the present day. The engine draws together the period of the private company operating the river service with a large capacity ferry and its assoociated steam tram service from the wharf to the heart of Parramatta, to the establishment of Sydney Ferries Ltd., and its takeover by the New South Wales State Government in 1951.