Marine Compound Engine

January 2010.      

John R. Bentley 2010.

Constructing a
Marine Compound Launch Engine
-  in miniature  -

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The finished engine
June 20, 2011

As a result of a most welcome and unexpected gift at Christmas 2010 from a very good friend across the Ocean, this engine was machined and erected using a castings set made by Stuart Models. I hope in these pages to convey my experiences in the workshop as the engine took shape.

The largest of the raw castings contained in the set

Two of the above castings after partial machining

The Stuart Compound Launch engine is a venerable old design which has been offered for many years. It is a miniature working version of a typical full size receiver compound engine. The engine is almost identical to the Stuart Twin Launch version except, instead of having two equal cylinders, the Compound has a small high pressure cylinder and a much larger low pressure cylinder. The Compound Launch is designed to be more efficient than the Twin Launch, in that steam is only supplied to the small cylinder and the exhaust steam from that cylinder then travels to the large cylinder. On both engines the cranks are spaced 90 degrees apart so there is always a power stroke across dead centres.

In the years before today's modern sophisticated remote control systems were available to model enthusiasts, compound steam engines weren't popular because they were deemed "non-self-starting". Of course this is nonsense as they could easily be made so at the tap of a button, however the primitive remote control systems of the day struggled to control a throttle, rudder and reverse linkage - let alone a starting valve!   Therefore the much more efficient compounds gave way to simple twins in remote controlled model boats. Sadly this misconception persists to this day when there is little difference in operating either type.

Occasionally when starting a compound engine there can be dead spots at the top and bottom of the small piston's travel because at the outset it is the only one supplied with steam. Once the engine is running there are no dead spots. During each revolution expanded exhaust steam from the small cylinder is being supplied to the large cylinder. The large piston is in mid-stroke when the small piston is at either end of its run.

During starting, the dead spots can be easily eliminated by the addition of a "simpling" valve which temporarily converts the machine to a "simple" single-cylinder engine (verses a compound). Also called a "starting" or "impulse" valve, it admits steam directly to the large cylinder. This piston is in mid-stroke when the small one is resting at the top or bottom. If a simpling valve is included in the piping, the compound engine offers the best of both worlds, allowing for higher efficiency and insuring remote starting from any position of the crankshaft. In practical terms all that is required is a small momentary valve that externally bypasses the small (HP) cylinder.

Physically, adding this valve is a tiny job when piping the engine and surely not a valid reason to reject this Compound in favor of the less efficient simple Twin Launch version of this engine.

Both of these versions of the Stuart launch engines put out approximately the same amount of power to the propeller shaft without a condenser. However a compound can make better use of a simple condenser as the natural exhaust pressure is much lower than that of the (simple) Twin Launch engine and hence is already nearly condensed. The condenser vacuum also has a larger effect as the Stuart Compound Launch LP piston is larger in area than the Twin Launch's two cylinders combined. The Compound is designed to operate at somewhat higher pressure with a much lower volume of steam.

Fore and aft views of the Stuart Compound Launch engine
(before cylinder lagging)


This engine has lots of torque to drive a large propeller with a steep pitch
It was recommended for displacement hulls up to six feet in length

All the pictures on this page are of the same engine and in some - like this one below - it can appear quite large!

Although powerful, the four-pound engine is actually quite compact and will easily sit on its side on my hand

In recent times most of my models have been scratch built, so I thought this would be a good time to show my approach to machining a castings set. Of course I am overjoyed at this prospect as this is refreshing change for me. For those of you who have never tried a castings set, but wondered exactly what to expect I hope this will help to answer some of those questions. This is not my first experience at this aspect of making model steam engines - I have made some from castings as well as a number of models built from scratch over the years. However I don't claim to be any kind of an expert in the machining of castings and will simply proceed at the best of my abilities using common sense as my guide.

Please note that constructing this engine involves the use of metal cutting tools and machines which can easily cut people equally well. Everyone should be confident that they will be safe before attempting any of these operations. Use common sense where safety is concerned and be prepared to accept the responsibility alone for any mishaps you cause. Simply thinking out carefully what is going to happen - before you hit the switch - can go a long way toward helping to prevent damage to yourself, the machine or the workpiece.

As always on this website, I am describing how I did things - not necessarily how someone else should.

Perhaps you may have wondered, like I did years ago, exactly how rough "raw castings" actually are... That much I think I can answer with the pictures on the next page. An honest attempt was made to photograph all the lumps of metal in the set as they looked before I performed any machining operations.

Engine Construction Pages:

Castings, Materials and Fastenings


Cylinder Block

Top Covers

Bottom Cylinder Covers

Steam Chests

Crosshead Guides and Bracket




Connecting Rods and Crossheads

Main Bearings


Fittings: Oil Cups

Fittings: Drain Cocks

Fittings: Exchange Pipe, Flanges and Glands

Stephenson Link Reversing Gear (5 pages)

Completing and Erecting the Compound Launch Engine

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© John R. Bentley 2010-2011.