Scotch marine boiler

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.

It differs from the Lancashire in two respects: many small-diameter tubes (typically 3 or 4 inches [75 or 100 mm] diameter each) are used to increase the ratio of heating area to cross-section.

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

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.

This allowed the entire assembly of outer tubeplate, furnace tube, combustion chamber and firetubes to all be removed from the boiler shell as one unit, simplifying manufacture and maintenance.

Various makers had their own particular ways of making these corrugations, leading to their classification for maintenance purposes under the broad titles of, Leeds, Morrison, Fox, Purves or Brown.

[3] The typical design is the "wet back", where the rear face of the combustion chamber is water-jacketed as a heating surface.

This design saves some structural weight, but it also makes the boiler longer and more difficult to install into a ship.

[4] The "Inglis"[8] modification adds an extra combustion chamber where an additional single large flue returns from the rear to the front of the boiler.

[4] The major advantage of the Inglis is the extra heating area it adds, for a comparable shell volume, of perhaps 20%.

These can now run the full length of the boiler shell, rather than just the rather shorter distance from the inner combustion chamber to the front tubeplate.

The Scotch marine boiler achieved near-universal use throughout the heyday of steam propulsion, particularly for the most highly developed piston engines such as the triple-expansion compounds.