Collision regulations
- Underway
- Not at anchor, made fast to the shore, or aground (Rule 3).
- Making way
- Underway and moving through the water under your own power or sail.
- Power-driven vessel
- Any vessel propelled by machinery, including a sailing yacht with her engine running.
- Sailing vessel
- Any vessel under sail, provided propelling machinery, if fitted, is not being used.
- Not under command (NUC)
- A vessel that, through some exceptional circumstance such as a steering failure, can't manoeuvre as the rules require and so can't keep out of the way.
- Restricted in her ability to manoeuvre (RAM)
- A vessel whose work, such as dredging, surveying, cable laying or servicing a navigation mark, restricts her ability to keep out of the way.
- Constrained by her draught (CBD)
- A power-driven vessel severely restricted in her ability to deviate from her course by her draught in relation to the available depth and width of water.
- Engaged in fishing
- Fishing with nets, lines, trawls or other gear that restrict manoeuvrability. Trolling lines that don't restrict manoeuvrability don't count.
- Restricted visibility
- Any condition in which visibility is reduced by fog, mist, falling snow, heavy rainstorms, sandstorms or similar.
- Give-way vessel
- The vessel required to keep out of the way of another.
- Stand-on vessel
- The vessel required to keep her course and speed while the other gives way.
- Short blast
- A blast of about one second (Rule 32).
- Prolonged blast
- A blast of four to six seconds (Rule 32).
Go deeper: COLREGS steering and sailing rules explained · COLREGS Rule 19: conduct in restricted visibility
Tides
- Chart datum
- The level charted depths and drying heights are measured from. On UK charts it's approximately Lowest Astronomical Tide.
- Lowest Astronomical Tide (LAT)
- The lowest tide level that can be predicted under average weather conditions.
- Height of tide
- The height of the sea surface above chart datum at a given time.
- Charted depth
- The depth of water below chart datum shown on the chart.
- Drying height
- The height above chart datum of ground that covers and uncovers with the tide, underlined on the chart.
- Range
- The difference in height between successive high and low waters.
- Springs
- Tides with the largest range and strongest streams, a day or two after new and full moon.
- Neaps
- Tides with the smallest range and weakest streams, around the quarter moons.
- MHWS, MHWN, MLWN, MLWS
- Mean high water springs, mean high water neaps, mean low water neaps and mean low water springs.
- Standard port
- A port with its own full daily predictions of the times and heights of high and low water.
- Secondary port
- A port whose tides are found by applying time and height differences to a standard port.
- Tidal diamond
- A magenta diamond on the chart, keyed to a table giving the tidal stream's direction and rate hour by hour, relative to high water at a standard port.
Navigation and chartwork
- Variation
- The angle between true north and magnetic north. It changes with place and over time.
- Deviation
- The compass error caused by the boat's own magnetism. It changes with the boat's heading.
- Set and drift
- The direction a tidal stream flows towards, and how far it moves you in a given time.
- Leeway
- The sideways movement of a boat downwind of the course steered.
- Dead reckoning (DR)
- A position worked out from the course steered and distance run alone.
- Estimated position (EP)
- A dead reckoning position corrected for tidal stream and leeway.
- Course over ground (COG)
- The direction the boat is actually moving over the seabed.
- Course to steer
- The heading worked out to allow for tidal stream and leeway so that you track along your intended line.
- Transit
- Two charted objects in line, giving a position line with no compass error.
- Clearing bearing
- A bearing on a charted object that keeps you clear of a danger.
Lights
- Isophase (Iso)
- A light with equal periods of light and darkness.
- Occulting (Oc)
- A light that is on for longer than it is off.
- Flashing (Fl)
- A light that is off for longer than it is on.
- Quick flashing (Q)
- 50 to 60 flashes a minute.
- Very quick flashing (VQ)
- 100 to 120 flashes a minute.
- Sector light
- A light showing different colours over different arcs, to mark a safe channel or a danger.
Go deeper: Cardinal marks: topmarks, colours and lights · IALA buoyage: lateral marks explained
Radio and safety
- DSC
- Digital Selective Calling: sends automated digital alerts and calls on VHF channel 70.
- MMSI
- Maritime Mobile Service Identity: the nine-digit number that identifies your radio for DSC and AIS.
- EPIRB
- Emergency Position-Indicating Radio Beacon: a 406 MHz satellite distress beacon.
- NAVTEX
- Automatic text broadcasts of navigational and weather warnings on 518 kHz.
- SRC
- The Short Range Certificate, needed to operate a marine VHF radio.
Go deeper: How to send a Mayday or Pan-Pan on VHF
Weather
- Veering
- A wind shifting clockwise, for example from south-west to west.
- Backing
- A wind shifting anticlockwise, for example from west to south-west.
- Beaufort scale
- The 0 to 12 scale of wind force, from calm (0) to hurricane force (12).
- Sea breeze
- An onshore wind that builds on sunny afternoons as the land heats up faster than the sea.