Stoking the steam, one engine at a time…
Triple Compound Marine Steam Engine.
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No.6 No.3's Big Brother

Type: Vertical twin cylinder, double acting, slide valve,
standard & rod,Compound / Simple Working.
Bore 2 1/2 in & 4 in
Stroke: 3in.
Compound Working
Height: 18 in
Availability 1906-today
Design: Twin Standard & Rod.

Stuart No. 6
= high speed =
Compound Engine.
Cylinders 2 1/2in. and 4in. x 3in. Stroke With Balanced Cranks.


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The enormous interest occasioned by my No. 3 Compound, and numerous subsequent enquiries for a large size, has prompted me to design this engine.

The materials and parts herien listed are absolutely the best obtainable, no expense has been spared to produce a first-class set at a moderate price.

Working drawings may be obtained on approval on receipt of P.O. 10/6 which amount will be refunded on return of Drawing in good condition. If a set is ordered mount may be deducted when remitting.

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No. 6

2 1/2in. and 4in. by 3in. Stroke

Finished Engine,
without Reversing Gear

(as shown on page 13,

Price = £25 0s. 0d.

With Reversing Gear,

Price = £33 0s. 0d.

List of Castings and Parts.

CAST IRON.-Cylinders and Valve Chests in one casting, with steam ports cast in, 2 Pistons, 2 Covers, 2 Bottoms, 2 Chest Doors, 2 Slide Valves, 2 Standards, 1 Sole Plate, 1 Box Bed, 1 8in. Disc or 10in. by 2in. wheel, 4 Counterbalance Weights, 2 Eccentric Sheaves; 2 Valve Stuffing Boxes.
Gun-Metal.- 2 Piston Glands, 2 Valve do, ; 2 Valve Rod Guides, 2 Pairs Eccentric Straps, 4 Con. Rod Brasses, 6 Main Bearing Brasses; 4 Flanges, 2 Crosshead Bushes.
Mild Cast Steel.- 2 Connecting Rods, 2 Eccentric Rods, 3 Main Bearing Caps.
Forged Steel, etc.- 1 Crank Shaft, 2 Piston Rods with Crossheads, Steel for Front Columns - Crosshead, Connecting Rod and Eccentric Strap Bolts ; 4 steel Guide Plates, 6 Finished Steel Piston Rings, CopperTube for Eduction Pipe. Steel Sheet for Lagging; 2 Bright Steel Valve Rod Heads.
A large sheet with every part fully dimensions is included.
Price, packed and Delivered, Henley Station, G.W.R. £6 0s. 0d.


All Studs, Bolts and Nuts, Split Pins, etc.,£1 10s. 0d. Reversing Gear Castings and Parts £1 10s. 0d. Set of Cylinder Drain Cocks with Unions and Pipes, £1 5s. 0d.
These latter have been specially made to suit this Engine since the photo on page 13 was taken.
The overall dimensions are : Height, 18in. ; Length, 14in. ; Width, 10in. exclusive of Box Bed. Weight about 150 lbs. Speed up to 750 revolutions per minute, Steam pressure 120 lbs., but extra drawing may be obtained showing Valve setting for 150 lbs.
Packing for export and delivery, London Shippers, 10/- extra.


This is the next evolution of the No.3 Compound. The power of this engine is enough to propel a full size launch up to about 25' length. This made the engine very popular. A few small changes have been made to accomodate the increase in size. The most noticable are the Valve rod support brackets being mounted in front of the rods, on the front of the valve chest, instead of beside them mounted inboard of the valve chest. The reversing lever is no longer mounted on the valve chest cover moving up and down, but is now mounted to a pivot point situated at the bottom and back of the soleplate and moves backwards and forwards. Much easier to operate when sitting in ones' steam launch.

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Stuart No.6 in need of a makeover.

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Makers plate on the engine.

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Stuart No6 with boiler feed pump.

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The real thing

No.6a running.


I couldnt find any video of a No.6 running. So I have included this video of a No.6a an improvement made when the engine was reintroduced in 1980.
Improved piston rod glands, and inclusion of a vacuum pump to maintain a vacuum if a steam condensor is fitted to the at the low pressure exhaust. This allows fresh water recovery to increase boiler runtime. It also increases power by lowering the pressure at the low pressure cylinder exhaust. Reducing the exhaust pressure to 0 bar 15 p.s.i. below atmospheric pressure has a similar effect to increasing boiler pressure by a similar amount. The vacuum pump is connected to the low pressure cylinder crosshead and mounted behind the engine.

A client writes from Tasmania :-
" I received the No.6 Castings and Forgings in good condition, the packing of same could not have been better. Fully expected, from what I had read of your work in the MODEL ENGINEER, to get good castings, but I must say that what you sent exceeded my anticipations. Better Castings have never been seen in this Colony. The Drawings are exceptionally detailed."
A large number of these Castings have already been sold and have given great satisfaction.
G.N writes "It is with very great pleasure that I acknowledge receipt of your letter, and photos of the engine and boiler you are dispatching per S. S. Kinsfaun Castle. After examining the pictures all I can say is that I am more than satisfied. I I have heard a great deal about your work, but never in my wildest hopes did I expect to be the possessor of such an Engine and boiler, and at such a reasonable price. I can now realise the difficulties encountered in order to turn out such a work of art, it is well worth waiting for. One thing I must specially compliment you for is the excellence of detail." (This refers to the direct coupled Compound engine and dynamo shown on page 17).

Steam Condensor

The steam condensor is fitted to the exhaust of a compound engine. (Labeled Exhaust steam). Cold water from the water tank is pumped into the many cooling pipes, The exhaust steam flows around these pipes and heat is exchanged the steam cooling, condensing and contracting. The condensate and cold air is pumped out the bottom by the steam condensate pump which maintains a lower than atmospheric pressure. The heated water is fed into the boiler requiring less energy to bring to the boil.Condensate is fed back to the water tank. This is especially important with marine engines as sea water cannot be used in a boiler.

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