Keith Sharpe’s


KEITH Sharpe’s model is of a typical triple expansion, double acting, condensing marine steam engine, with Stephenson’s link reversing valve gear, of the kinds introduced in the 1860s and which continued in widespread use for marine propulsion until the late 1940s. The engine is not a scale model of any particular engine, but is intended to visually represent the type used in ships and tugs, as distinct from boats and launches.

The model was originally intended to be a scaled-down version of the O.B. Bolton design, as later modified by J.P. Bertinat. However, it soon became clear that various design features could not be sensibly reduced in size to the required degree and, coupled with the necessity of making the engine without the use of castings, many variations from the original design became necessary such as the single piece cylinder block and the use of a piston valve for the intermediate pressure cylinder instead of a slide valve sandwiched between two cylinder blocks.

The bedplate assembly has two main parts, the bedplate and sub-bedplate. The bedplate incorporates integral horns for the main bearings and wells to clear the cranks/big ends and eccentric straps. The stands which support the cylinder block are held down on raised pads by studs and nuts and are also dowelled to ensure accurate positioning. A system of sumps and a central drainage channel are also incorporated to conduct oil to a bedplate drain cock. The air pump and holding-down lugs are screwed to the underside of the sub-bedplate.

The crankshaft is carried in seven main bearings and this configuration, as well as being more true to prototypical practice, played an important part in the construction of the crankshaft, which was assembled using the bedplate and main bearings as part of a system of jigs. The crankshaft components are held together by a modern structural adhesive, supplemented by steel taper pins. No finish turning of the crankshaft was necessary after assembly.

The flywheel is made of mild steel as a three layer sandwich and is held onto the crankshaft by a tapered split collet.

The cylinder block was machined from a single piece of phosphor bronze bar, with stainless steel liners being used in the HP cylinder and IP piston valve bores to facilitate the machining of the steam passages. Although it was tempting to omit cylinder drain valves, which are difficult to produce in an appropriately small size, it was realized that internal condensation would be a significant factor in the operation of this engine, as the steam progresses from the HP inlet on its lengthy path through the cylinder block and pipework to the LP exhaust. Without a means of removing condensation, the engine would be impossible to operate on steam.

Working drawings were prepared using CAD.  All components were made from scratch, from bar stock, except for a one-way roller clutch used in the mechanical lubricator, a small number of socket head screws and some springs and stainless steel balls. Commercial nuts varying in size from 6BA to 16BA were also used but all sizes above 16BA were modified to reduce their hexagon sizes for the sake of an improved ‘scale’ appearance. The engine contains 2097 components and, some 48 jigs, fixtures and special tools were made. All machining was carried out under manual control.

The engine runs well on compressed air and on superheated steam and is the culmination of four years of “fairly intense effort.”

It was photographed at the Midlands Model Engineering Exhibition.


HP Cylinder Diameter: 13mm

IP Cylinder Diameter: 18mm

LP Cylinder Diameter: 24mm

Piston Valve Diameter: 8mm

Stroke: 16mm

Valve Travel: 4mm

Main Bearings Diameter: 6.35mm

Crankpin diameter: 5mm

Overall Dimensions (L x W x H): 178 x 118.6 x 132.5mm

Weight: 3.08Kg