My grandfather was at sea from the age of 14 (1924 ) till he retired at 65, I remember him telling me that his job was a Donkeyman / Greaser but in the Royal Navy he would have been called a Stoker and that Stoker was the Navy's term for Engine room ratings.
He was under RN terms a few voyages during the war and as such classed as a naval rating for the trip.
Still remember seeing his seamans book, with the stamps of all the ships he served on, plus 3 with "Discharged at sea" stamped across the entry. They were the ships that were torpedoed from under him, and his wages stopped as soon as the ship sank!!!!!!!!!!