Maritime Glossary: Terms Every US Mariner and Employer Should Know
A plain-English reference to the ranks, licenses, vessel types, schedules, documents, and pay terms used across US maritime work — organized so you can find what you need quickly.
Ranks & Positions
Ordinary Seaman (OS) — Entry-level unlicensed deck rating; performs maintenance, line handling, and lookout under supervision. Able Seaman (AB) — Senior unlicensed deck rating earned with sea time and an exam; leads deck work and stands helm/lookout. Deckhand — Common inland/towing term for entry-level deck crew who build and break tow and maintain the vessel. Bosun — Senior unlicensed deck supervisor who directs deck maintenance and the AB/OS crew. Mate — Licensed deck officer who stands a navigational watch; grades run from Third Mate up to Chief Mate. Chief Mate — Senior deck officer and the vessel's second-in-command on deck; the step before Master. Master (Captain) — Officer with ultimate authority and legal responsibility for the vessel, crew, and cargo. Steersman (Apprentice Mate) — Trainee towing officer who operates the vessel under a licensed Master's supervision. Wiper — Entry-level unlicensed engine rating; the engine-room counterpart to an OS. QMED — Qualified Member of the Engine Department; senior unlicensed engine rating with endorsements like Oiler and Junior Engineer. Chief Engineer — Officer with overall responsibility for all machinery and technical systems aboard. DPO — Dynamic Positioning Operator; a deck officer certified to operate a vessel's DP system. Tankerman — Crew member endorsed to conduct or supervise liquid-cargo transfers on tank vessels.
Licenses & Endorsements
Rating — An unlicensed qualification recorded on the MMC (e.g., OS, AB, QMED). License / Officer Endorsement — Authorization to serve as a licensed officer (Mate, Master, Engineer), graded by tonnage/horsepower and route. Tonnage — The vessel-size limit a deck license authorizes (e.g., 100, 500, 1,600 GRT, or Unlimited). Route — The waters a license covers: Inland, Near Coastal, Oceans, Western Rivers, or Great Lakes. STCW — International standard behind much required safety and officer training, endorsed on the MMC. Basic Safety Training (BST) — The four-element foundational STCW course (survival, fire fighting, first aid, personal safety). Radar Observer — Endorsement to interpret radar for navigation and collision avoidance. ARPA — Automatic Radar Plotting Aid; STCW competency for operating automatic target-tracking radar. Tankerman PIC — Person In Charge endorsement authorizing responsibility for cargo transfers, issued by cargo category (e.g., DL, LG). Towing Endorsement — Authorization to serve as Mate/Master of Towing Vessels, tied to a TOAR and route service. DP Operator Certificate — Nautical Institute credential for dynamic positioning operators.
Vessel Types
Towboat / Towing Vessel — Pushes or pulls barges on rivers, the Gulf Intracoastal Waterway, lakes, or coastal routes. Barge — Unpowered vessel carrying dry or liquid bulk cargo, moved by a towing vessel. Harbor Tug — Powerful, highly maneuverable vessel that assists ships docking, undocking, and maneuvering in port. OSV (Offshore Supply Vessel) — Carries cargo, fuel, and equipment to offshore energy platforms. AHTS — Anchor Handling Tug Supply vessel used to move rigs and set anchors offshore. Crew Boat / FSV — Fast vessel moving personnel and light cargo to offshore installations. Tank Vessel — Vessel or barge that carries liquid bulk cargo such as petroleum or chemicals. Ferry — Vessel carrying passengers and/or vehicles on scheduled routes. Passenger Vessel — Vessel carrying passengers for hire, from small tour boats to large excursion vessels. Deep-Sea / Ocean-Going Vessel — Large ship in international or coastwise trade (container, tanker, bulk carrier).
Rotations & Schedules
Hitch — A single continuous period of duty aboard a vessel (the 'on' portion of a rotation). Rotation — The recurring pattern of time on the vessel versus time off, written as days-on/days-off. Even Time — A rotation with equal on/off periods, such as 14/14 or 28/28. Crew Change — The scheduled handoff when the offgoing crew departs and the oncoming crew joins. Watch — A scheduled duty period aboard; deck and engine crews commonly stand rotating watches around the clock. Six-and-Six / Watch-and-Watch — A watch pattern of six hours on and six off (or similar), common on towboats. Day Work — A schedule of daytime working hours rather than round-the-clock watches, typical for some maintenance roles. Standby / On Call — Time when a mariner is available to catch a boat but not yet aboard or earning a day rate.
Documents & Credentials
MMC — Merchant Mariner Credential; the single USCG document listing all your ratings, licenses, and endorsements. TWIC — Transportation Worker Identification Credential; TSA biometric card for secure-area access. CG-719K — USCG medical examination report / medical certificate confirming fitness for duty. CG-719B — The MMC application form submitted to the NMC. CG-719S — Sea Service form documenting days served aboard, signed by the employer. NMC — USCG National Maritime Center, the office that evaluates and issues MMCs. REC — Regional Exam Center where mariners test and submit documents. TOAR — Towing Officer Assessment Record; the practical assessment record for towing officer licenses. Discharge / Discharge Record — Documentation of service aboard used to substantiate sea time. Deficiency Letter — An NMC notice requesting additional information before an application can proceed.
Pay & Benefits Terms
Day Rate — A fixed amount paid for each day worked aboard the vessel. Overtime (OT) — Additional pay for hours beyond a base, common under union contracts. Per Diem — A daily allowance for expenses, sometimes paid during travel or training. Travel Pay — Reimbursement or allowance for getting to and from the crew-change point. Collective Bargaining Agreement (CBA) — A union contract setting wages, overtime, and benefits. Union — A labor organization (e.g., representing deep-sea deck, engine, or licensed officers) that negotiates pay and dispatches members to jobs. Benefits — Non-wage compensation such as medical, dental, retirement contributions, and paid training. Jones Act — US law requiring domestic point-to-point maritime transport to use US-built, US-crewed vessels, supporting demand for US mariners.