but what happens if a fife &c is not available? What little we do have says we revert to the less desirable but still acceptable a cappella for the duration.

EHmmm No it does not Phil, or if it does please show me where it says that. Fife & Drum - ONLY on larger men o'war that carried a full company of Marines - the Royal Navy itself NEVER had any bandsmen or musicians. They would always have bosuns and bosuns mates with bosun's pipes, the Royal Navy would never, ever have need to, as you put it - revert to the less desirable but still acceptable a cappella

Please tell me just WTF are " instrumental shanties".

Work on warships was continuous NOT staged the fifes and drums on larger ships, or fhipf if you prefer, 3rd rates and above were played not to work to but to lighten the monotony. The same work would have been done irrespective and it was on the smaller vessels in commission in the Royal Navy.