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. Firebox capacity is further restricted by the space used for the ashpan and also by the dry-back furnace. 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. These water level restrictions became even more troublesome when the boiler was tilted, even by as little as a steep railway line. An unusual rate of wear and number of replacement furnaces supplied for the Heywood locomotives has been put down to this cause.
The boiler was only rarely used for railway locomotives, although they were notably used by Sir Arthur Heywood from 1874 for his 15 in (381 mm) minimum gauge railways at Duffield Bank and Eaton Hall.
Other minimum gauge railways, notable the 18 in (457 mm) gauge works railways at Crewe, Horwich and the Guinness brewery in Dublin, also used launch-type boilers, owing to the limited space between the frames for a conventional firebox.
In 1888 the Hohenzollern Locomotive Works delivered the first two narrow gauge 600 mm (1 ft 11 5⁄8 in) feldbahn locomotives for the Prussian Army. These used a conical development of the launch boiler. A backplate of enlarged diameter and a greatly reduced smokebox tubeplate were fitted into a steeply conical shell. This was installed with the upper edge[note 1] of the cone horizontal. The purpose of the conical shape was to increase water depth over the furnace, the hottest part of the evaporative surface. The furnace and tubes were moved to the lower part of the shell, with the tubes running upwards parallel to the lower edge of the cone. A difficulty was the boiler's lack of steam space, requiring an enlarged dome, of almost as much capacity as the main shell. As a major virtue of the launch boiler is the simplicity of its construction, rolling a conical shell and fitting a large dome represented a considerable increase in their complexity and cost.
These locomotives, and their boilers, were a complete failure. They were undersized and underpowered for the task, with tiny wheels prone to derailment on uneven track and (for the first 2-2-2 locomotive) limited adhesion from its single driver. Despite being some years after Heywood and the publication of his Minimum Gauge Railways, they ignored almost all of Heywood's principles. The boilers lacked evaporative capacity and could not support sustained running.
A large launch-type boiler with a corrugated furnace, described as the Lentz boiler, was fitted to the Heilmann steam-electric locomotives of 1890. The boiler design was German in origin. A similar boiler, the 'Vanderbilt' was used in the USA.
Lancashire & Yorkshire Railway
- or 'generatrix'
- The eight to ten year interval before rebuilding would be a typical service life for such a boiler. It indicates that the boilers were adequate, and were not withdrawn from service merely to replace them, but also that the experiment was not considered a success and so they were not continued with.
- Harris, K. N. (1974). Model Boilers and Boilermaking. MAP. p. 50.
- Mosley, David; van Zeller, Peter (1986). Fifteen Inch Gauge Railways.
- Fach, Rüdiger; Krall, Günter (2002). Heeresfeldbahnen der Kaiserzeit. Kenning.
- "Heilmann Locomotives". Loco locos. 19 Oct 2007.
- Hewison, Christian H. (1983). Locomotive Boiler Explosions. David and Charles. pp. 110–111.
- Cook, A.F. (1999). Raising Steam on the LMS.
- Hewison, Locomotive Boiler Explosions, p. 111