Stoking the steam, one engine at a time…
Steam Pinnace
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Pinnace Named after a boat type

Type: Vertical, twin cylinder, compound, double acting, slide valve.
Bore 1 1/4 in and 1 3/4 in
Stroke: 1 in
Weight: 11 lbs.
Availability 1924-1928
Design: 5 column marine engine

1 1/4-in. and 1 3/4-in. Bore x 1-in. Stroke. (Weight, 11 lbs.)

Stacks Image 98
CAST IRON.- 1 Cylinder Casting with steam ports cast in, 2 each top and bottom Cylinder Covers, 2 each Valve Chests and Covers, 2 Pistons, 1 Bed Plate, 2 Double Eccentric Sheaves, 4 Counterbalance Weights, 1 Disc Wheel.
GUNMETAL.- 2 each Piston and Valve Rod Glands, 1 Guide Bar Bracket, 3 pairs Main Bearing Brasses, 2 pairs Big End Brasses, 2 pairs Eccentric Rod Straps, 2 Slide Valves, 1 each Feed Pump Body and Gland, 1 each Valve Box and Cover, 2 Crosshead Slides with Pump Arms, 4 Pipe Flanges, 3 Air Pump Castings, 1 Hand Wheel, 2 Reversing Shaft Brackets.
STEEL.- 1 Crank Shaft, 2 Connecting Rods, 4 Eccentric Rods, 2 Piston Rods with Crossheads.
SUNDRIES.- All materials in steel and brass with all necessary Studs, Nuts, Bolts and Screws, Lagging, etc., with two sheets of fully detailed working Drawings.

Price, £2 10s. (Weight packed, 14 lbs.).

Set of 4 Drain Cocks, 8/-; 2 Cylinder Lubricators, 4/6


Another Marine style compound engine from Stuart, why? Well this engine is certainly an attempt to produce an engine for the model boat enthusiast. The smaller size and weight make is more suitable for a model that the No.3 & No.6. The design has been modified to fulfil this new use. Called the COmpound Pinnace Rngine , it is the word 'Pinnace' that is important. A Pinnace is a small boat used by large warships to transport select crew to shore or other boats.The pinnace was in the time of steam, powered by small economical marine engines of this multicolumn design. Engines for this type of boat are often much smaller than expected, in the range 3-5ft tall. Often called "six Column engines" due to the number of columns supporting the cylinder block, stuart removed the middle one at the front to beter show the workings, thus a 5 column. The move away from cast iron standards significantly reduces weight. It also simplifies construction as producing multiple columns to the same dimensions is pretty simple on a lathe. The sole plate has been made wide to allow for easy and stable mounting into a model boat. The bearing size also apprear quite large and the description mentions counterweights for the crankshaft. The lack of a box bed or similar means that the engine cannot be mounted on a flat surface as the cranshaft and flywheel rotate substantially bellow the mounting points, again typical in a model boat. The reduced cylinder capacity and compound nature of the engine redice the boiler size requirements. The pictures below are the only ones I have managed to track down of this elusive engine which was shortly to be supercedded by two of stuarts best engines , The compact Launch, available as simple or compound (identical except for the cylinder block, covers and valve chests. The other is the pinnacle of stuart designs, the Triple compound engine. As can be seen by comparison with the third picture, the Stuart is a scaled down version of the full sized engine.

Stacks Image 117
Pinnace Compound, showing missing middle column to be found on full size engine.

Stacks Image 127
Pinnace Compound rear view.

Stacks Image 147
6 Column Compound steam engine. Typical of that found in a Pinnace or similar boat.

Video Of
please let me know if you come accross video of this rarity running.

July 28th, 1921.

A.G.O. writes:-
"I have received the Castings for the Pinnace Engine, and I must say that they are much above my expectations ; indeed, I have never seen such a highly satisfactory set of small iron castings before.

Simpling Valve.

Compound engines have a dead spot where the engine is not self starting. This is a problem with marine engines as having to push over an engine to get it running is not acceptable in a boat or ship. There is a tendency when going into reverse that the engine stops at least power to the screw, dead spot. To overcome this problem a simpling valve is often included. This specialized valve converts the compound engine to simple operation. Compound is where steam exhaust from the high presure cylinder feeds into the low pressure second cylinder. Only the high pressure cylinder is supplied from the steam source so at top or bottom dead centre the engine wont start.
The simpling valve disconnects the HP cylinder exhaust from the LP cylinder inlet and instead connects high pressure steam to the LP cylinder. With both cylinders receiving high pressure steam the fact that the two cranks are at 90 degrees to each other means that one of the cylinders will be at a starting position at all times. So when the engine stops, a quick blast on the simpling valve will get the engine running again. The valve must not be left active once the engine is running as the much larger LP cylinder reciving high pressure steam will mismatch with the small HP cylinder giving a heavy pulcing power output, potentially causing damage.
In full size engines the complexity of the simpling valve is offset by the usefullnes. In a model engine there is no way anyone other than a masochistic perfectioninst is going to machine one. SO for models a simpler option is often used, an impulse valve. This will kick the engine into starting if the engines has stopped dead. It simply connect high pressure steam directly into the LP cylinder valve chest. This will have the effect of connecting high pressure steam to the Inlet and exhaust of the HP cylinder rendering it inoperable, this is of no importance because the LP cylinder will kick overto its dead spot and sit their. Closing the impulse valve you can open up the main steam valve as the HP cylinder is now in starting position. Thuse a short Impulse is used if needed.

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