46 USC 7308 - Able seamen - limited

The required service for the endorsement of able seamanlimited, qualified for limited service on a vessel on any waters, is at least 18 months service on deck on board vessels of at least 100 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title operating on the oceans or navigable waters of the United States (including the Great Lakes).