Ship's Corporals


Doing a bit of family history research and I discover that my wife's great-great-grandfather was a Ship's Corporal in the RN back in the 1890s/early 1900s.

What was a Ship's Corporal and how did a man become one? The GGF was in the Royal Marines Light Infantry, but in 1896 transferred to the Royal Marines Police, then passed for Ship's Corporal, and finally transferred to the RN in 1897. Did he have to take some written examination, or just go before a promotion board, or what?

Grateful for the hivemind's collective wisdom on this.