A launch-type, gunboat or horizontal multitubular boiler[1] is a form of small steam boiler. It consists of a cylindrical horizontal shell with a cylindrical furnace and fire-tubes within this.
Their name derives from the boiler's popular use at one time for small steam yachts and launches. They have also been used in some early Naval torpedo boat destroyers.
Contents |
The boiler has similarities with both the locomotive boiler (the multiple small fire-tubes), and the Scotch marine boiler (the short cylindrical furnace). As a fire-tube boiler it has generous heating area and so is an effective steamer. Firebox construction is also simpler, thus cheaper, than for the locomotive firebox.
The firebox is of limited size though, and unlike the locomotive boiler cannot expand beyond the size of the boiler shell. This limits the sustained output that is possible. The grate and ashpan are also limited in size, the grate being a set of bars part-way across the furnace tube and the ashpan the restricted space beneath this. These features limit the boiler's ability to burn hard bituminous coal and they require a supply of Welsh steam coal, or similar, instead. The small ashpan also restricts their ability to steam for long periods.
One drawback of the boiler was the large diameter of the furnace relative to the boiler shell, and thus the small steam space above the crown of the furnace. This made the boilers prone to priming, particularly on a rough sea, where water could be carried over into the steam pipe.
A more serious danger was the limited reserve of the water level, where the water level had only to drop by a small amount owing to inattention before the furnace crown would be exposed, with likely overheating and risk of boiler explosion. The boiler was safe when correctly fired, but could not be left unattended.
The boiler did see some popularity in mainland Europe, as a boiler for small portable engines. A similar boiler, but arranged with return fire-tubes, was built in America as the Huber boiler.
The boiler was only rarely used for railway locomotives, although they were notably used by Sir Arthur Heywood for his 15" minimum gauge railways at Duffield Bank and Eaton Hall.[2]
Other minimum gauge railways, notable the 18 inch gauge works railways at Crewe, Horwich and the Guinness brewery in Dublin, also used lauch-type boilers, owing to the limited space between the frames for a conventional firebox.
A large launch-type boiler with a corrugated furnace, described as the Lentz boiler, was fitted to the Heilmann steam-electric locomotives of 1890.[3] The boiler design was German in origin. A similar boiler, the 'Vanderbilt' was used in the USA.[4]
The Lancashire & Yorkshire Railway suffered problems with firebox stays, leading to a boiler explosion with an 0-8-0 Class 30 near Knottingley in 1901[5][6] Their Chief Mechanical Engineer Henry Hoy, sought to avoid the problems of the stayed firebox altogether and so developed an alternative boiler and firebox. This used a corrugated tubular furnace and cylindrical outer firebox, as for the Lentz.[4] The furnace was also of steel, rather than the copper used for fireboxes at this time.[4] Hoy's involvement was ironic, as a major cause of the original accident had been Hoy's invention of a new brass alloy for firebox stays, an inelastic alloy that turned out to have serious drawbacks.[6][7] One Class 30, 396, was rebuilt in 1903 and 20 more were built new with this boiler.[6] The new boiler design did not last long in service and the locomotives were rebuilt with conventional boilers after ten years.[note 1][6] Hoy's successor, George Hughes, described these boilers unfavourably in papers read to the I. Mech E..[8][9]