Good pic Mr Greenhaus, sir.
The boiler is of the vertical variety and not at all large. The sea going versions would likely be even smaller.
And before anyone argues against the use of fires aboard sailing ships, might I point out that whaling ships had large fire heated cauldrons for boiling down the blubber to obtain oil etc.
The point of the shanty is that 'proper sailors' would sneer at those that used the new fangled contraption, while those who sailed in ships so fitted would be saying "At least we are on our way matey, while you poor buggers are waiting a favourable wind or the use of a steam tug" - a sort of 'put that in your pipe & smoke it' retort.
Way, hey and away WE go - riding on a donkey.
It meant fewer hands necessary to work ship - but it was still the beginning of the end for the shantyman.
Schooners were so rigged as to need fewer hands anyway, and were smaller and handier than ship-rigged vessels, so no donkey was required on those fore n' afters.
All references to infernal combustion engines are redundant as the song refers to a practise that hails from before the days of regular steamship usage.