I'm looking for a word for 'boiler' as in the phrase 'the ship's boiler nearly burst', but I can't think of anything remotely suitable.
Perhaps there is something obvious that I can't bring to mind? Can anyone help?
This kind of boiler is also known as a steam generator, and direct translations from relevant words in other languages are mostly "steam generator" or "steam pot". One could take for example the Italian expression "generatore di vapore" and adapt to Latin. The result, generator/generatrum vaporis, should be easily understood and descriptive. I find the neuter agent noun derivative most suitable here, but some might prefer the masculine.
However, I would prefer to start with the verb vaporare, which means just the kind of things a boiler would do: emit steam, glow, heat… Therefore my suggestion is vaporatrum. This is admittedly new coinage to the best of my knowledge, but I find the word descriptive, neat, and understandable.
The classical author Vitruvius was familiar with a basic steam turbine, which he describes in De architectura I.VI.2 thus:
Fiunt enim aeoli pilae aereae cavae, - hae habent punctum angustissimum - quae aqua infunduntur conlocanturque ad ignem; et antequam calescant, non habent ullum spiritum, simul autem ut fervere coeperint, efficiunt ad ignem vehementem flatum.
You could follow him in using aeoli pila.
You've probably already checked here, but Smith's Copious and Critical English-Latin Dictionary offers fornax vaporifer as a "furnace emitting steam", citing Statius' Silvae 1.3.45:
An quae graminea suscepta crepidine fumant
balnea et impositum ripis algentibus ignem,
quaque vaporiferis iunctus fornacibus amnis
ridet anhelantes vicino flumina Nymphas?
Or [shall I sing] of the steaming baths taken up by their [the silent woods'] grassy ledge and fire imposed on chilly banks, where the river linked to a vaporous furnace laughs at the Nymphs as they pant, though the stream be hard by?
Translation: D.R. Shackleton Bailey, Loeb's Classical Library.
That's, er, not exactly an allusion to a steam engine. Maybe it's supposed to make the reader think of the kind of boiler used to heat a Roman bath. If so, that's not a bad classical precedent to start from. I don't know a lot about either, but I understand both the Roman boiler at a therma and the boiler for a steam engine to work on the same principle: a vessel containing water is exposed to fire, the water turns to steam, a pipe leads the steam up to some form of heat exchanger where the heat does something useful, and a pipe returns the resulting cold water back down to the boiler.
Some quick googling suggests that my guess about how the heat was transferred in a Roman bath might be wrong: some sources suggest that they heated hypocausts with air or smoke rather than by steam turning to water. But I'm not sure. This extraordinary commentary on Vitrivius's explanation of how the baths were heated explains more, and it mentions many ancient words for the boiler or closely related things, including vaporarium, caldarium vas, miliarium, and fornacula. Have a look; it may give you a new idea.
Anyway, I was trying to follow C.M. Weimer's suggestion to see what's been written in Latin about steam ships.
Could you use vaporifer substantively? Forcellini defines vaporifer as an adjective meaning vaporem et calorem emittens and gives a few more classical citations. The calorem gives me some hope. Nothing about steam engines there, but steam engines were just getting started in Forcellini's time. A quick look at Google Books, though, suggests that a few people have invented the word vaporifer to mean various things, but none for steam boilers.
Here is an article explaining how steamboats work in a four-language version of Comenius's Orbis Pictus updated by J.G. Gailer in 1842. It says:
Quae vis ut efficiatur, ignis sub aheno vaporario perpetuo alendus est.
The fire must in order to produce this power be continually kept up under the steam boiler.
Translation: J.G. Gailer, apparently a native speaker of German.
So, it appears that ahenum vaporarium has one precedent specifically for a steam boiler on a steam ship—in a children's book. This is the only instance I found. I can't tell if the substantive is ahenum modified by vaporarium or the other way around. Either way, ahenum, suggesting copper, seems unnecessarily narrow. Google Books reveals that vaporarium as a substantive by itself has other established meanings, most notably a greenhouse (in botanical Latin) and (I think) a sauna. Classically, the vaporarium seems to be the pipe carrying steam to the hypocaust (hey, maybe my first guess was right). Cicero uses it in Q. fr. 3.1.2.
A suitable word in Spanish is caldera, which sounded pretty Latin to me. So looking at L&S I finally converged to caldaria. One of its meanings is:
A pot for boiling
Even if you were not looking for a word for pot, I think the analogy remains valid.
A caminus (from Greek κάμινος) is a furnace, so another possibility is a caminus vaporalis.