Bagnall boiler

A launch-type, gunboat or horizontal multitubular boiler 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.

The cylindrical furnace or firebox fits entirely within the boiler's outer shell. Unlike the locomotive boiler, there is no firebox grate emerging beneath the main boiler. 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.

...
Wikipedia