hulk: plural noun: hulks 1. an old ship stripped of fittings and permanently moored, especially for use as storage or (formerly) as a prison.

A hulk is a ship that is afloat, but incapable of going out to sea. It's an old ship that's had it's riggings and internal equipment removed therefore only retaining it's flotational qualities. A prison hulk was used as a floating prison. They were used extensively throughout Great Britain and were decommissioned during the mid-19th century.