The Cylindrical Return-Tube Marine Boiler
Or "Scotch Boiler"

Contains excerpted material from Wikipedia.

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The cylindrical or Scotch boiler was the most common marine fire-tube boiler in the world during the Age of Steam. The earliest models were probably developed from Lancashire boilers on land -- cylindrical units mounted on brick foundations, and used to power factory machinery in Britain's industrial belt. The first recorded marine installation was by Randolph, Elder & Co. in the SS McGregor Laird of 1862; this precedent and the predominance of Scots in steam engineering accounted for the sobriquet. With the long history of the type, there was ample opportunity to work out the bugs in the design and optimize the boilers' performance, bringing it from 50 - 80 psi in earlier models to 215 psi post-1900. Royal Navy ships powered by Scotch boilers included the Royal Sovereign and Majestic classes of the 1890s. These boilers were used in British battleships as recently as 1904, with some of the "Wobbly Eight" partially equipped with Scotch boilers in that class's peculiar, mixed-type boilering scheme. We hasten to add that the mixed-boiler experiment was not considered a success. Scotch boilers perhaps achieved their greatest moment in the gigantic ocean liners of the 1900 - 15 period, providing the high-pressure steam that drove the Atlantic speed queens' advanced triple expansion and turbine engines. -- Ed.

The general layout of this boiler is that of a squat horizontal cylinder. One or more large cylindrical furnaces are in the lower part of the boiler shell. Above this is a large number of small-diameter fire-tubes. Gases and smoke from the furnace pass to the back of the boiler, then return through the small tubes and up and out of the chimney. The ends of these multiple tubes are capped by a smokebox, outside the boiler shell.

Side elevation of Scotch boiler showing combustion  and circulation patternsThe Scotch boiler is a fire-tube boiler, in that hot flue gases pass through tubes set within a tank of water. As such, it is a descendant of the earlier Lancashire boiler (c. 1830) and, like the Lancashire, it uses multiple separate furnaces to give greater heating area for a given furnace capacity. It differs from the Lancashire in two aspects: a large number of small diameter tubes (typically 3 or 4 inches diameter each) are used to increase the ratio of heating area to cross-section. Secondly the overall length of the boiler is halved by folding the gas path back on itself.

The far end of the furnace is an enclosed box called the combustion chamber which extends upwards to link up with the firetubes.

The front wall of the combustion chamber is supported against steam pressure by the tubes themselves. The rear face is stayed by rod stays through the rear shell of the boiler. Above the combustion chamber and tubes is an open steam collecting space. Larger long rod stays run the length of the boiler through this space, supporting the ends of the boiler shell.

With multiple furnaces, there is a separate combustion chamber for each furnace. A few small boilers did connect them into one chamber, but this design is weaker. A more serious problem is the risk of reversing the draught, where exhaust from one furnace could blow back and out of the adjacent one, injuring the stokers working in front of it.

Some Scots Boiler Images

Boiler of USS KEARSARGE lifted by dockyard crane

Double-ended boiler of the U.S. battleship Kearsarge during construction of the ship, c. 1900. Three large circular holes are the furnaces. Fire tubes perforate the boiler end above the furnaces, emptying into what will be the funnel uptake. They are grouped in three columns corresponding to three combustion chambers within the boiler. Bolt heads studding steam collector at boiler's top (nearly upside down in the slings) are the outside portions of rod stays strengthening the ends of the boiler shell.

Diagram of a fire-tube boiler
Cutaway diagram shows the workings of an oil-fired Scotch boiler.

RMS OLYMPIC's boilers in the builder's shop, 1911

Some of the Olympic's 29 boilers awaiting installation at Harland & Wolff, 1911; The White Star ships' steam plants performed at a working pressure of 215 psi. The Olympic was the Titanic's nearly identical sister ship, in service through 1936. The 24 double-ended boilers in each ship were 15 feet 9 inch diameter and 20 feet long; the five smaller single-ended units were 11 feet 9 inch diameter. All had three cambered Morrison furnaces of 3 feet 9 inch diameter -- 159 furnaces total. The steam fed two of the largest reciprocating engines ever made, plus a low-pressure turbine that ran off the exhaust steam from the piston engines; top speed was around 23½ knots. System schematic from Samuel Halpern's Prime Mover site

Plan of a double-ended Scotch Boiler

Elevation and profile of a double-ended Scotch marine boiler. Burly lengthwise expansion bolts, known as boiler stays, tied the assembly together from end to end, particularly in the steam collecting chamber at top. Furnaces on either end shared a common combustion chamber in the middle. Much longer than single-ended units, these were awkward to install in the ship's interiors, hence less common in service.

Elevation of a double-ended Scotch boiler for RMS LUSITANIA, mfd by John Brown & Co., Clydebank

End elevation of one of the Lusitania's Scotch boilers. These were an extra large model with four cambered furnaces, and four combustion chambers, instead of the usual three. The pioneer turbine liner carried 24 of these monsters in four firerooms: 22 double-ended and 2 single-ended. An army of 102 stokers labored around the clock, shoveling in an average 910 tons of coal a day to produce the 215 psi steam that maintained the ship's 26-knot service speed. Picture History of the Lusitania   Of course, battleships employed fewer stokers and consumed far less fuel than these oceangoing behemoths -- 372 tons a day at full power for the King Edwards, for example. Warship boiler rooms were nevertheless dusty, sweaty sites of continuous backbreaking labor little different from their counterparts in the merchant fleet, save that military discipline was added to unremitting physical toil.

Lascars stoke Scotch boiler furnaces on P&O steamer
Lascar firemen stoke the furnaces aboard a turn-of-the-century P&O liner.

Section of the HMS VICTORIA of 1888

Deck plan and inboard profile of HMS Victoria, ill-fated Mediterranean fleet flagship, showing layout of boiler rooms. Her cylindrical boilers were arranged transversely with uptakes grouped on either side, venting to side-by-side funnels. Twin funnels abreast was to remain the preferred British practice for the next decade.   Enlarge plan