Plain-English Definitions

Maritime glossary

Half the battle on the USCG exam is the vocabulary. Set and drift, give-way versus stand-on, GM, leeway — here's what they actually mean, in plain language. Search a term or browse by subject.

Aground: When the hull is resting on the bottom.
Backing wind: Wind shifting counter-clockwise (e.g., W → S) — often precedes worsening weather.
Beam: The vessel's width at its widest point.
Bearing: The direction from you to an object, in degrees.
Center of buoyancy (B): The center of the underwater volume; the upward buoyant force acts here.
Center of gravity (G): The point where the vessel's total weight acts; raising it reduces stability.
Close-quarters situation: When vessels are near enough that risk of collision exists and action is required.
Constrained by Draft (CBD): A power-driven vessel limited by the depth of water relative to her draft (three red all-round lights).
Course: The direction you intend to travel (course to steer / course made good).
Crossing: Two power-driven vessels on intersecting courses; the one with the other on her starboard side gives way.
Dead reckoning (DR): Estimating your position from a known point using course, speed, and time.
Deviation: Compass error caused by the vessel's own magnetic fields; changes with heading.
Displacement: The weight of water the vessel pushes aside — effectively the vessel's weight.
Draft: The vertical distance from the waterline to the bottom of the keel — how deep she sits.
EPIRB: Emergency Position-Indicating Radio Beacon — transmits your distress position to satellites.
Fix: A confirmed position from two or more lines of position (bearings, ranges, GPS).
Free surface effect: Loss of stability from liquid sloshing in a partially filled tank.
Freeboard: The distance from the waterline up to the main deck — reserve buoyancy.
Front: The boundary between two air masses; passage usually brings a wind and weather change.
Give-way vessel: The vessel required to take early, substantial action to keep clear.
Gross tonnage (GRT): A measure of a vessel's internal volume — sets the tonnage limit on your license.
Ground tackle: The anchor and its associated gear (chain, rode, shackles).
Head-on: Two power-driven vessels meeting nearly bow-to-bow; both turn to starboard.
Heading: The direction the vessel's bow is pointing right now, in degrees.
Knot: One nautical mile per hour — the unit of speed at sea.
Leeway: Sideways drift caused by wind pushing on the vessel.
Line of position (LOP): A line you must be somewhere on, from a single bearing, range, or sounding.
List: A persistent athwartships lean from uneven weight (vs. roll, which is temporary).
Metacentric height (GM): A measure of initial stability — the distance from the center of gravity to the metacenter. Bigger GM = stiffer.
MMC: Merchant Mariner Credential — the document holding your officer/rating endorsements.
MOB: Man Overboard — immediately mark, point, and maneuver to recover.
Muster: Assembling crew/passengers at assigned stations for an emergency or drill.
Nautical mile: 1,852 meters (about 1.15 statute miles); one minute of latitude.
NMC: National Maritime Center — the Coast Guard office that evaluates and issues credentials.
Not Under Command (NUC): A vessel that, by exceptional circumstance, can't maneuver as required (shows two red all-round lights).
OUPV: Operator of Uninspected Passenger Vessels — the '6-pack' license, up to six passengers.
Overtaking: Approaching another vessel from more than 22.5° abaft her beam; the overtaker always gives way.
PFD: Personal Flotation Device — a life jacket, rated by type for the conditions.
Port / Starboard: Left (port, red) and right (starboard, green) when facing the bow.
Restricted in Ability to Maneuver (RAM): A vessel whose work (dredging, towing, surveying) limits maneuvering (red-white-red lights).
Safe speed: A speed at which you can take proper, effective action to avoid collision and stop in time (Rule 6).
SART: Search and Rescue Transponder — shows your position on a rescuer's radar.
Scope: The ratio of anchor rode paid out to the depth of water — more scope, better hold.
Sea service: Documented days underway used to qualify for a license; logged on Form CG-719S.
Set and drift: Set is the direction a current is pushing you; drift is its speed in knots.
Stand-on vessel: The vessel that holds course and speed — until it must act to avoid collision.
Trim: The difference between forward and aft draft — down by the bow or stern.
TWIC: Transportation Worker Identification Credential — required before the USCG will issue your license.
Variation: The angle between true north and magnetic north at your location.
Veering wind: Wind shifting clockwise (e.g., S → W) — often follows a passing front.
Windward / Leeward: Windward is toward the wind; leeward (lee) is away from it.

New to the vocabulary? Start with the license coach, then drill the terms in context in the question bank.