How far back could a modern-looking warship travel before people interpreted it as witchcraft, and what ship could keep operating there?
Short answerThe 1700s might produce supernatural interpretations, but a coal-fired ship with reciprocating machinery would be more sustainable than a turbine-powered vessel; HMS Dreadnought is already near the limit.
NotesA strange warship might pass without accusations of witchcraft in the 1800s, while a modern vessel in the 1700s could be interpreted as something sent by God or the devil. The answer then shifts from spectacle to sustainment. A useful ship would need fuel that the earlier economy could supply, which favors coal over modern fuels. It would also benefit from reciprocating machinery because turbines demand more specialised maintenance. HMS Dreadnought is therefore a more plausible boundary case than a later ship even though keeping it operational would still be extraordinarily difficult. The time-travel joke becomes a lesson on the logistical foundations of naval technology.