Marine Navigation Glossary
A comprehensive glossary of marine navigation and boating terms you may encounter on BoatRoutes.
Anchorage
Section titled “Anchorage”A sheltered area of water where boats can anchor. Good anchorages have protection from wind and waves, adequate depth, and a bottom that provides reliable holding for an anchor (mud and sand are preferred over rock or kelp).
The width of a vessel at its widest point. Beam is relevant when assessing whether a boat can fit into a slip or navigate a narrow passage.
The forward end of a vessel. “Bow-to” mooring means tying the bow to the dock or buoy.
Chart Datum
Section titled “Chart Datum”The reference level from which depths are measured on nautical charts. In US waters, Chart Datum is typically MLLW. In Canadian waters, it is the lowest normal tide. Charted depths represent the minimum expected water depth under normal tidal conditions.
Course Over Ground. The actual direction a vessel is traveling relative to the earth’s surface, as measured by GPS. COG may differ from heading due to current, wind, and leeway.
Current
Section titled “Current”The horizontal movement of water, usually caused by tidal forces in coastal areas. Tidal currents in the Pacific Northwest can exceed 7 knots in narrow passages. See also Ebb, Flood, Slack Water.
A reference point or surface used for measurements. In marine navigation, the most important datum is the vertical reference for depth (see Chart Datum) and the horizontal reference for position (typically WGS84 for GPS coordinates).
Deadhead
Section titled “Deadhead”A partially submerged log floating vertically in the water, with one end below the surface. Deadheads are a significant hazard in Pacific Northwest waters, especially near river mouths and logging operations. They can be very difficult to spot and can cause serious hull damage.
Displacement
Section titled “Displacement”The weight of water displaced by a vessel, equal to the vessel’s total weight. Displacement hulls push through the water rather than planing on top of it, and have a maximum speed determined by waterline length.
The vertical distance between the waterline and the lowest point of a vessel’s hull (usually the keel). Draft determines the minimum water depth a boat can safely navigate. A sailboat with a 6-foot draft needs at least 6 feet of water at low tide to avoid grounding.
Ebb {#ebb}
Section titled “Ebb {#ebb}”The tidal current flowing seaward (outgoing tide). In Puget Sound, ebb currents generally flow south and west. The ebb phase occurs as the tide drops from high to low water. See also Flood.
Electronic Navigational Chart. A vector-format digital chart published in the S-57 or S-100 international standard. ENCs contain structured data (coastline, depths, aids to navigation) that can be processed by software. BoatRoutes uses NOAA ENCs to build its navigation grid. See also Electronic Navigational Charts.
Fathom
Section titled “Fathom”A unit of depth equal to 6 feet (1.83 meters). Many printed US nautical charts display depths in fathoms, especially in deeper water. The electronic chart tiles shown in BoatRoutes render depths in meters, the standard for electronic navigational charts (ENCs).
The unobstructed distance over which wind can blow across water to build waves. A long fetch with strong wind creates larger waves. Protected routes minimize fetch by staying near shorelines and behind islands.
Flood {#flood}
Section titled “Flood {#flood}”The tidal current flowing inland (incoming tide). In Puget Sound, flood currents generally flow north and east. The flood phase occurs as the tide rises from low to high water. See also Ebb.
Global Positioning System. A satellite-based navigation system providing position, velocity, and time data. GPS is the primary means of electronic position-fixing for marine navigation.
Harbor
Section titled “Harbor”A sheltered body of water where vessels can anchor or dock. Harbors may be natural (formed by geography) or artificial (protected by breakwaters or jetties). In Canadian English, spelled “harbour.”
The mechanism for steering a vessel — either a wheel or tiller. “At the helm” means the person steering the boat. “Helm” is also used colloquially to mean the steering position or station.
A unit of speed equal to one nautical mile per hour (approximately 1.15 statute miles per hour or 1.85 kilometers per hour). A boat traveling at 7 knots covers 7 nautical miles in one hour. Do not say “knots per hour” — the rate is already implied.
The side sheltered from the wind. A “lee shore” is a shoreline downwind of your position — potentially dangerous because wind pushes you toward it. A “lee anchorage” is one protected from the prevailing wind.
Marina
Section titled “Marina”A facility with docks and slips for mooring recreational boats, typically offering services such as shore power, water, fuel, pump-out, restrooms, and other amenities.
MLLW {#mllw}
Section titled “MLLW {#mllw}”Mean Lower Low Water. The average height of the lower of the two daily low tides, used as the vertical reference datum for US nautical charts. When a chart shows a depth of 12 feet, that means 12 feet at MLLW. At higher tides, there will be more water; at extreme low tides, there may be slightly less. See also Understanding Tide Overlays.
Mooring
Section titled “Mooring”A permanent anchor or heavy weight on the seabed with a line or chain to a surface buoy, used to secure a vessel. Mooring buoys in marine parks eliminate the need to anchor and protect the sea floor. “Mooring” also refers to the act of securing a boat to a dock, buoy, or anchor.
Nautical Mile
Section titled “Nautical Mile”A unit of distance used in marine navigation, equal to one minute of latitude (approximately 1.15 statute miles or 1.852 kilometers). All distances on BoatRoutes are measured in nautical miles.
National Oceanic and Atmospheric Administration. The US federal agency responsible for ocean and atmospheric science. NOAA produces nautical charts (ENCs), tide and current predictions, and weather forecasts that BoatRoutes relies on.
Passage
Section titled “Passage”A navigable channel between two bodies of water, often narrow. Examples include Deception Pass, Active Pass, and Dodd Narrows. Many passages in the Pacific Northwest have strong tidal currents requiring careful timing. Also used generally to mean a voyage from one port to another.
The left side of a vessel when facing the bow (marked with red lights). Also, a harbor town or the waterfront facilities where vessels load and unload. Context determines the meaning.
Radio Detection And Ranging. An electronic system that uses radio waves to detect objects and measure their distance and bearing. Essential for navigation in fog, which is common in Pacific Northwest waters, especially in the Strait of Juan de Fuca.
Slack Water {#slack-water}
Section titled “Slack Water {#slack-water}”The brief period when tidal current speed is at or near zero, occurring between flood and ebb tides. Slack water is the safest time to transit narrow passages with strong currents. Windows are typically 15-30 minutes at minimum current. See Slack Water & Passage Timing.
Speed Over Ground. A vessel’s actual speed relative to the earth’s surface, as measured by GPS. SOG differs from speed through the water when currents are present. A boat moving at 7 knots through the water with a 2-knot favorable current has an SOG of 9 knots.
Starboard
Section titled “Starboard”The right side of a vessel when facing the bow (marked with green lights). In narrow channels and when meeting other vessels head-on, the convention is to keep to the starboard (right) side.
The rear end of a vessel. “Stern-to” mooring means backing the stern toward the dock or shore and setting an anchor from the bow — common in tight anchorages like those in Desolation Sound.
The periodic rise and fall of water levels caused by gravitational forces of the moon and sun. The Pacific Northwest typically experiences mixed semidiurnal tides — two high tides and two low tides per day, with unequal heights. Tidal range in Puget Sound can exceed 14 feet during spring tides.
Tide Rip
Section titled “Tide Rip”A visible disturbance on the water surface caused by opposing tidal currents meeting, or currents flowing over an uneven bottom. Tide rips can create confused seas with steep, short waves. Common near headlands and in passages like Cattle Pass and Active Pass.
Waves created by a moving vessel. Large wakes can disturb anchored boats, erode shorelines, and swamp small craft. Many harbors and marinas have “no wake” zones requiring vessels to slow down to minimize wake.
Waypoint
Section titled “Waypoint”A specific geographic position (latitude/longitude) used in navigation planning. A route is composed of a series of waypoints connected by course lines. BoatRoutes generates waypoints automatically when computing routes.