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The Job Steam Does Inside a Cylinder

Every reciprocating marine engine, from a little launch plant to the great triple-expansion sets in coastal paddlers, works on the same simple idea. Steam at pressure enters a cylinder and pushes a piston along its bore. The piston is connected by a rod to a crosshead, then through a connecting rod to a crank, and the crank turns a shaft. That is the whole trick, repeated several hundred times a minute.

What matters is that the steam is admitted at full boiler pressure for only part of the stroke. Once the piston has travelled perhaps a quarter or a third of its journey, the inlet closes and the trapped steam continues to push by expanding. This is called cut-off, and it is where the efficiency of the engine lives. A charge of steam that expands to three or four times its original volume gives far more work per pound than one admitted for the whole stroke.

When the piston reaches the end of its travel — top or bottom dead centre — the exhaust valve opens and the spent steam escapes. On the return stroke the same sequence happens on the other side of the piston if the engine is double-acting, which nearly all marine engines are.

Expanding in Stages: Simple, Compound and Triple

A single cylinder cannot usefully expand steam too far before the metal condenses it or the cylinder becomes absurdly large. So marine designers split the expansion across two or three cylinders. Each one takes steam at a lower pressure and a larger volume than the last.

• Simple — one cylinder, one expansion. Fine for small launches, typically taking steam at 60 to 100 psi.
• Compound — high-pressure and low-pressure cylinders, often at 90 degrees to each other. Common in river and estuary craft.
• Triple expansion — three cylinders in series, taking steam at 180 psi or more. The standard for serious paddle and screw steamers of the late Victorian era.

The low-pressure cylinder of a triple is enormous compared with the high, sometimes four or five times the swept volume. Watching one of these sets turn over slowly, you can hear the difference between the sharp exhaust beat of the high-pressure cylinder and the softer, breathier note of the low.

Valve Timing: The Heart of the Matter

Valve gear decides when steam enters and leaves, and getting it right is what separates a sweet-running engine from a lumpy one. On the simplest engines a slide valve on the back of the valve chest does the work, driven by an eccentric on the crankshaft. Better engines use piston valves, which wear more evenly and seal better at high pressure.

Timing follows four events, and none of them happens exactly at dead centre:

• Admission begins a few degrees before dead centre, so the steam is already pressing as the piston turns over.
• Cut-off closes the inlet at the chosen fraction of the stroke.
• Release opens the exhaust slightly before the end of the stroke, letting pressure drop early.
• Compression closes the exhaust before the piston finishes its return, cushioning the moving parts and warming the incoming steam.

Reversing is handled by shifting the eccentric setting. Stephenson link motion, with its curved slotted link, does this elegantly: move the link one way for ahead, the other for astern, and somewhere in the middle the engine will run very slowly or not at all. The notched reversing quadrant beside the helmsman's position is a familiar sight on any preserved steam launch.

Paddle Wheels, Propellers and What They Demand

A paddle wheel is happiest turning slowly — often between 20 and 45 rpm — and it wants torque rather than speed. Its floats dip into the water and push aft, and if the wheel spins too fast the water simply leaves the float behind. A paddler, therefore, suits a big, slow-turning engine driving the shaft directly, with the wheel diameter and float area matched to the hull.

A propeller is a different creature altogether. A screw works best at higher revolutions, commonly 80 to 200 rpm in the steam era, and it prefers a slim, fast-turning shaft. That is why screw steamers often ran their engines faster, or used gearing, and why the propeller's pitch, diameter and blade area all had to be chosen together with the engine's normal running speed. Too fine a pitch and the engine races without driving the hull; too coarse and it labours and stalls.

Condensers and the Value of a Vacuum

Exhausting steam to atmosphere wastes most of the pressure difference you could be using. A condenser changes that. Exhaust steam passes into a vessel where it meets either a spray of cooling water (a jet condenser) or a nest of tubes through which seawater circulates (a surface condenser). The steam collapses into water, and the volume drops so sharply that a partial vacuum forms — typically 26 to 28 inches of mercury on a good plant.

That vacuum is worth real power. The piston now has atmospheric pressure helping it on the exhaust side instead of fighting it, and the effective pressure difference across the piston might be increased by a third. The condensed water, plus any feed make-up, collects in the hotwell and is pumped back to the boiler as feed, still warm. This saves fuel and, importantly, keeps dissolved salts out of the boiler.

Steady Pressure: The Engine Driver's Real Skill

An engine cannot run smoothly on a wandering steam supply. If boiler pressure sags when the throttle opens, the cut-off point that worked beautifully at 150 psi behaves quite differently at 120. The result is a rhythmic surge and fall in revolutions, uncomfortable for the paddler and hard on the shaft bearings.

So the fireman tends the boiler steadily, the safety valve is set and left alone, and the water level in the gauge glass is watched constantly. Trained crews on preserved plant still do this, matching firing rate to the engine's demand as the boat comes off a lock or meets a headwind. Get it right and the engine ticks over with a steady, unhurried beat, the vacuum holds, and a modest boiler will drive a surprisingly heavy boat along with quiet, dependable efficiency.