The Marine Triple-Expansion Engineafter the practice of Harland & Wolff · Belfast, 1912
By 1912 the reciprocating engine had gone to sea and grown to its final form: three cylinders on three cranks set 120 degrees apart, the steam expanded in three stages through two receivers, the condenser fed by the ocean itself. Three cranks mean there is no dead centre anywhere on the circle — she starts from any position, reverses at a lever, and needs no flywheel. Nor a governor: the propeller resists as the square of its speed and holds the engine steady by itself, though a screw lifted clear by a heavy sea will let her race — mind the wreck speed. The Titanic carried two of these giants, four storeys tall, flanking a turbine that drank their exhaust; it is the arrangement this exhibit follows. Within a decade the turbine took the whole engine room, and the piston story that began with Newcomen ended here, at sea, at its best.
Key to the plate
a. — from the stokehold. Scotch boilers by the row, each a drum of furnaces and fire-tubes, worked at over two hundred pounds. At sea the boilers outnumber the engines many times over, and the stokehold gangs shovel around the clock — the fire, not the iron, is what limits a steamer.
b. — high-pressure cylinder. The small one takes the boilers’ full steam and uses only the top of its expansion before exhausting into the first receiver. The hand lever sets its cutoff; everything downstream follows.
c. — intermediate cylinder. The middle stage, breathing from the first receiver and exhausting to the second. Its pressure is set by no valve — the receivers float to wherever the neighbouring cylinders’ appetites balance.
d. — low-pressure cylinder. The giant, sized by the ratio lever, expanding the second receiver’s mild steam down onto the condenser vacuum. Splitting the temperature fall three ways is the compound’s argument made a third time — each cylinder’s walls swing less, and less charge dies on the iron.
e. — three cranks at 120°. The three cylinders drive one shaft with their cranks set a third of a turn apart, so some cylinder is always at full push: no dead centre, no barring over, no flywheel — and a turning moment smooth enough to live below passenger cabins.
f. — condenser & the sea. The ocean is the cold body: sea water floods the condenser tubes and pulls the exhaust down to a deep vacuum, and the condensed boiler water is pumped back to the feed. A marine engine always condenses — fresh water is too dear to throw overboard.
g. — shaft & screw. The propeller resists as the square of its speed, which governs the engine better than flyballs ever did — until a heavy sea lifts the screw clear and the load vanishes at a stroke. Engine-room practice was a hand on the throttle in weather, for exactly that reason.