Dictionary.net

(Naut.) A man-of-war, stationed in a harbour to superintend marine affairs there, and inspect nightly vessels not in commission. In fleets, each ship takes the guard in turn for twenty-four hours, commencing at 9 a.m., and during her tour of duty hoisting the Union Jack at the mizzen.

n. A vessel of war to superintend the marine affairs in a harbour or river—used formerly to receive impressed seamen;—care; protection.