Launch-type boiler
This also allowed the boiler to be made with a bolted joint in the outer shell and so the whole furnace and tube nest could be withdrawn for inspection and maintenance.
This made the boilers prone to priming, particularly on a rough sea, where water could be carried over into the steam pipe.
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.
As the wider grate allowed the burning of poor fuels, such as straw or sugarcane waste, it was favoured for agricultural use and was widely known as the 'colonial' type.
[4] This was also offered in a lengthened form as a log-burning furnace, particularly for use in Australia and Africa where forest land was being clear-felled for agriculture.
A limitation of this design for steam locomotives was the need to fit fire, grate and ashpan all within the confined circular furnace tube.
A backplate of enlarged diameter and a greatly reduced smokebox tubeplate were fitted into a steeply conical shell.
The purpose of the conical shape was to increase water depth over the furnace, the hottest part of the evaporative surface.
A difficulty was the boiler's lack of steam space, requiring an enlarged dome, of almost as much capacity as the main shell.
[8] 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.
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.
It has been suggested that they would have been more successful with oil firing,[ii] as this would have allowed the whole furnace diameter to have been used and would have avoided ash build up.
[iii][12] Hoy's successor, George Hughes, described these boilers unfavourably in papers read to the I. Mech E.[14][15] The single NZR E class of 1906 was an experimental Vauclain compound articulated 2-6-6-0T Mallet, intended for working the Rimutaka Incline.
Compounding encouraged the choice of the then remarkably high boiler pressure of 200 psi (14 bar; 1,400 kPa), which required a strong firebox construction.
The NZR chief draughtsman G. A. Pearson chose a corrugated furnace design in a tapered boiler, similar to the Vanderbilt.
