Captain's Glossary

A light showing an unbroken light over a 360° arc of the horizon. Used for anchor lights, NUC lights, RAM lights, and other special lights.

Fog formed when warm, moist air moves over cooler water, cooling the air to its dew point. Common along coasts. Can persist in strong winds — unlike radiation fog which dissipates in wind.

The horizontal direction of one point from another, expressed as an angle from 000° to 360° measured clockwise from north (true) or from the vessel's head (relative).

A scale from 0 (calm) to 12 (hurricane force) used to describe wind speed by its observed effects on sea and land. Force 7 (Near Gale) is approximately 28–33 knots.

The forward (front) end of a vessel. The bow cuts through the water as the vessel moves forward.

The width of a vessel at its widest point, or the direction at 90° to the centerline (abeam). A vessel is "on the beam" when it is directly to the side.

The upward force exerted by water on a submerged or floating object, equal to the weight of the water displaced. A vessel floats when its buoyancy equals its weight.

The intended direction of travel of a vessel, expressed as a true, magnetic, or compass bearing. Distinguished from "heading" (actual direction pointed) and "track" (actual path made good).

Course Over Ground — the actual direction of travel of a vessel relative to the earth's surface, as measured by GPS. May differ from heading due to current and leeway.

The total angular difference between true north and compass north. Compass Error = Variation + Deviation. Used to convert between true and compass bearings.

A vessel, due to her draft in relation to the available depth of water, that cannot deviate from the course she is following. Displays three all-round red lights vertically. Only recognized in the International COLREGS.

Estimating current position by advancing a known past position using known or estimated speed, course, and time elapsed — without external references. Always labeled "DR" on a chart.

The angular difference between magnetic north and compass north, caused by the vessel's own magnetic field. Varies with the vessel's heading and must be determined for each vessel.

The speed of a current, expressed in knots. Together with set, it describes the effect of current on a vessel's motion over the ground.

The vertical distance between the waterline and the lowest point of the hull (keel). A vessel with a deep draft needs greater water depth to navigate safely.

The weight of water displaced by a floating vessel, which equals the vessel's total weight. Expressed in long tons or metric tons.

Estimated Time of Arrival — the predicted time at which a vessel will reach a destination, calculated from current position, speed, and distance to go.

The tidal current flowing away from land (out to sea) as the tide falls. At its strongest approximately mid-way through the falling tide.

A position determined from two or more simultaneous lines of position (LOPs). A running fix uses LOPs taken at different times, adjusted for vessel movement between observations.

The tidal current flowing toward land (into bays and harbors) as the tide rises. Opposed to ebb current.

The boundary between two air masses of different temperature and humidity. Cold fronts, warm fronts, and occluded fronts each bring characteristic weather patterns.

A unit of depth equal to 6 feet (1.83 m). Still used on some nautical charts and in traditional soundings. The term comes from the Old English for "outstretched arms."

The vertical distance from the waterline to the main deck. Higher freeboard = more deck above water = greater reserve buoyancy.

The distance wind has blown over open water without obstruction. Greater fetch allows waves to build larger. Important for evaluating sea state in coastal and offshore passages.

In a crossing or overtaking situation, the vessel required to take early and substantial action to avoid collision — typically by altering course to starboard or slowing.

The direction the bow of the vessel is currently pointing, in degrees. May differ from course due to leeway (wind effect) and current.

The sideways inclination of a vessel, caused by wind pressure on sails, wave action, or uneven loading. Excessive heel is dangerous.

A line on a weather map connecting points of equal atmospheric pressure. Closely spaced isobars indicate steep pressure gradient and strong winds.

A unit of speed equal to one nautical mile per hour (1.852 km/h or 1.151 mph). Named after the knotted ropes historically used to measure speed.

A line somewhere on which the vessel is located, obtained from a bearing, range, depth contour, or celestial observation. Two or more LOPs determine a fix.

The sideways movement of a vessel caused by wind pushing the vessel to leeward (downwind). It causes the actual track to differ from the heading.

The downwind side — the direction toward which the wind is blowing. Opposite of windward (upwind). A leeward shore is a dangerous lee shore because the wind pushes a vessel toward it.

A permanent sideways tilt of a vessel caused by unequal loading or flooding on one side. Distinguished from heel (temporary tilt from external forces).

A white all-round light (225° arc) carried forward on the centerline of a power vessel underway. Vessels over 50m carry a second, higher masthead light aft.

The internationally recognized radio distress call (from French "m'aidez" — help me). Declared only when there is imminent danger to life. Spoken three times on VHF Channel 16.

A vessel unable to maneuver as required by the COLREGS due to some exceptional circumstance — such as engine failure or steering casualty. Displays two all-round red lights vertically.

Tides with the smallest range, occurring when the sun and moon are at right angles (quarter moon). High water is lower and low water is higher than average.

A unit of distance equal to 1,852 meters (approximately 6,076 feet), defined as one minute of arc of latitude. Used in navigation worldwide.

A vigilant watch maintained by sight and hearing, using all available means, to make a full appraisal of the situation and risk of collision. Required by COLREGS Rule 5.

The left-hand side of a vessel when facing forward toward the bow. Port navigation light is red. Remembered by "port wine is red."

An urgency radio call indicating a serious situation that does not yet pose imminent danger to life (e.g., medical emergency, person overboard). Transmitted on VHF Channel 16.

A vessel restricted in her ability to maneuver due to the nature of her work — such as cable-laying, dredging, or survey operations. Displays red-white-red all-round lights vertically.

A collision is deemed to exist if the compass bearing of an approaching vessel does not appreciably change as distance decreases. A constant bearing = collision course.

Fog that forms on calm, clear nights over land when the surface cools by radiation and the air above cools to its dew point. Burns off after sunrise.

Speed Over Ground — the actual speed of a vessel relative to the earth's surface, as measured by GPS. May differ from vessel speed through water due to current.

The direction TOWARD which a current flows, expressed in degrees true. A current that "sets" 090° flows to the east.

In a crossing or overtaking situation, the vessel that has the right of way and is required to maintain course and speed. The stand-on vessel must take action only if collision appears inevitable.

A white light carried at the stern, showing over an arc of 135° centered on dead astern. Visible to vessels following from behind.

Navigation lights showing ahead and to the side — red on the port (left) side and green on the starboard (right) side. Each shows over an arc of 112.5°.

The speed at which a vessel can take proper action to avoid collision in the prevailing conditions — visibility, traffic density, maneuverability, sea state. Required by COLREGS Rule 6.

The brief period when tidal current velocity is zero, occurring between the ebb and flood currents. Typically coincides with high water (HW) or low water (LW).

Tides with the greatest range, occurring near new and full moon when the gravitational pull of sun and moon are aligned. From Anglo-Saxon "springan" — to rise, not the season.

A sudden, brief increase in wind speed, often associated with heavy rain or thunderstorms. Common ahead of cold fronts. Mariners should reduce sail and prepare before a squall line arrives.

The right-hand side of a vessel when facing forward toward the bow. Starboard navigation light is green.

The aft (rear) end of a vessel. The sternlight is mounted here, visible to vessels following from behind.

Long-period waves generated by distant storms that have traveled far from their source. Swells are smooth and regular, distinct from locally generated wind waves.

A radio safety call broadcast to warn other mariners of a navigational hazard (e.g., debris, unlit vessel). Transmitted on VHF Channel 16.

The difference in height between consecutive high water and low water. Spring tides (near new/full moon) have the greatest range; neap tides (near quarter moon) have the smallest range.

The difference in draft forward and aft. A vessel trimmed by the stern (deeper aft) is normal; trimmed by the head (deeper forward) is generally undesirable.

The angular difference between true north and magnetic north at a given location, caused by the Earth's magnetic field. Changes over time and differs by location. Found on the chart's compass rose.

Same as CBD — a deep-draft vessel that cannot maneuver freely due to the shallow depth of the channel relative to her draft. Other vessels should avoid impeding her passage.

A specific geographic position defined by latitude/longitude coordinates, used in GPS and chart plotting for route planning. Vessels navigate from waypoint to waypoint.

The upwind side — the direction from which the wind is blowing. A windward shore allows the wind to carry a vessel away from land.