A guard ship was a form of naval reserve assigned to an important port, commonly using a third-rate or fourth-rate ship of the line. A first-rate or second-rate could serve at an exceptionally critical port, though this was rare. The vessel remained fully stored but carried only a fraction of its wartime crew, enough to maintain it and provide the core around which a full crew could rapidly form. The ship also supported the port's defences and often housed the port admiral or captain and their staff. Using it as the commander's flagship gave that officer a personal reason to keep it seaworthy, because it might be the ship they took to sea when war came.
Chapter · 1:14-3:50
How a guard ship combined reserve readiness and port defence
Guard ships kept a partly crewed, fully stored warship ready at an important port while also serving as the local commander's headquarters and defensive reserve.