Sentinel boiler

The boiler was carefully designed for use in a steam wagon: it was compact, easy to handle whilst driving, and its maintenance features recognised the problems of poor feedwater quality and the need for it to be maintained by a small operator, rather than a major locomotive works.

Sentinel boilers are vertical, as was common for many designs of steam wagon, so as to reduce the effects of tilting due to hill climbing or uneven roads disturbing the water level.

These tubes are short, straight and pass between the flat faces of the squared section in a grid pattern.

Above the tube nest the water space widens to form an increased reservoir, protecting against tilting.

Both of these disturb circulation and risk local overheating and damage, scale also reduces boiler efficiency and wastes fuel.

Regular servicing (depending on water conditions) was to separate the water-tubes in their drum from the boiler outer shell, so that they could be cleaned.

Sludge dropped free on opening the shell and the short, straight tubes could easily be cleaned with brush or scraper.

The Sentinel though left the outer shell in place and instead dropped the tube nest downwards (having first removed the ashpan).

[1] This had the advantages that it required simpler lifting gear: the wagon would be raised on ramps or over a pit, the bolts removed and then the tube bank lowered with a block and tackle from a fixed beam, without requiring a mobile crane that could lift it and then move it sideways.

Dropping the firebox was not required at every washout and was recommended at intervals of 2 to 12 months, depending on water quality.