'Ship's corporal' definitions:

Definition of 'Ship's corporal'

From: GCIDE

Corporal \Cor"po*ral\ (k[^o]r"p[-o]*ral), n. [Corrupted fr. F. caporal, It. caporale, fr. capo head, chief, L. caput. See Chief, and cf. Caporal.] (Mil.) A noncommissioned officer, next below a sergeant. In the United States army he is the lowest noncommissioned officer in a company of infantry. He places and relieves sentinels. [1913 Webster]
Corporal's guard, a detachment such as would be in charge of a corporal for guard duty, etc.; hence, derisively, a very small number of persons.
Lance corporal, an assistant corporal on private's pay. --Farrow.
Ship's corporal (Naut.), a petty officer who assists the master at arms in his various duties. [1913 Webster]