Haycock boiler

A haycock boiler is an early form of steam locomotive boiler with a prominently raised firebox of "Gothic arch", "haystack", or "coppernob" shape. The term haystack is most commonly used, but is avoided here as it is confusingly used for three quite different forms of boiler. This particularly large outer firebox served as the steam dome and was often highly decorated with polished brass. These were popular for early railway locomotives, from 1840 to the 1850s.

The prominently raised firebox first appeared in 1830, in Bury's 0-4-0 locomotive Liverpool. This was the progenitor of his bar-frame locomotives and shared their distinctive boiler design. The inner firebox was D-shaped in plan, with a flat tubeplate. Fireboxes of this time did not yet have a brick arch and so the Bury firebox was relatively short in length but tall, to give an adequate length of combustion path. The outer firebox was a vertical cylinder, formed into a tall hemispherical dome above it. Later Bury designs were flattened on top and became known as "haystacks".

A regular problem with early steam locomotives was that of priming, the carry-over of water with the steam. Many varieties of steam dome on the boiler barrel were tried to avoid this, by taking the steam outlet from as high as possible above the waterline. Stephenson's Rocket of 1829 had used such a small steam dome. In the 1830s, domes became extravagantly large. A drawback to fitting such large domes was the weakening of the boiler shell where such a large hole was cut into it.

In 1840, Stephenson produced their 2-2-2 design which avoided the dome altogether, in favour of a raised firebox in the Bury style of ten years earlier. Boiler power had increased considerably over the decade, now requiring a larger fire grate area. The inner and outer fireboxes were square in plan, with flat sides that required staying. At the top these four sides were vaulted inwards to a point, having a profile approximating a then-fashionable Gothic arch. These provided a large steam space above the waterline, but their flat surfaces limited working pressure.

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