RYA Day Skipper Theory key terms

Angular distance north or south of the equator, measured in degrees from 0 at the equator to 90 at each pole. On a chart it is read from the side scale and written before longitude in a position.

Angular distance east or west of the Greenwich meridian, measured 0 to 180 degrees. It is read from the top or bottom scale of a chart and written after latitude in a position.

A line of constant longitude running north-south between the poles. On a Mercator chart meridians appear as straight vertical lines, which is why true north is always straight up the chart.

A chart projection where meridians are parallel vertical lines and the scale of latitude increases towards the poles. A straight line drawn on it is a rhumb line, so compass courses can be plotted directly.

A line crossing every meridian at the same angle, so it represents a constant compass course. On a Mercator chart it plots as a straight line, but it is not the shortest route over the ground.

The shortest route between two points on the Earth's surface, part of a circle whose plane passes through the Earth's centre. On a Mercator chart it curves towards the pole and is approximated by a series of short rhumb lines.

A confirmed position obtained by crossing two or more position lines, for example bearings, ranges, GPS or a sounding. It is marked on the chart with a circle and the time beside it.

A line on the chart along which the vessel must lie, derived from one observation such as a compass bearing or a measured distance. Two or more crossing position lines produce a fix.

Two charted objects seen exactly in line, giving a single very accurate position line along that line of sight. A transit is often the most reliable fix available in pilotage waters.

The direction of an object from the vessel, expressed in degrees. A hand-bearing compass gives a compass bearing, which must be corrected for variation and deviation before being plotted as a true bearing.

An estimated position found by advancing a known position along the course steered, for the distance run through the water, allowing for leeway but not for tide. It is shown with a half-circle symbol.

The best position estimate, obtained by applying an estimate of tidal stream and leeway to the dead reckoning position. It is marked with a square or triangle symbol and is less reliable than a fix.

The compass course that must be held so that tide and leeway carry the vessel along the intended track over the ground. It is found graphically with a tidal vector triangle.

The actual direction the vessel travels over the seabed, combining the course steered with the effect of tide and leeway. It is shown on a GPS and differs from the heading and from the course steered.

The low-water level to which depths and drying heights on a chart are referenced, usually close to the lowest astronomical tide. Charted depths are therefore normally greater than the actual depth of water.

The depth of water printed on the chart, measured below chart datum and shown in metres and decimetres. The actual depth is the charted depth plus the height of tide at that moment.

The height above chart datum of a feature that is uncovered at low water, such as a rock or sandbank. It is shown on the chart in a different style from a depth and is added to chart datum, not subtracted.

The height of the sea surface above chart datum at a given place and time, found from tide tables and adjusted for the port and for atmospheric pressure. It is added to charted depth to give the actual depth of water.

A large-range tide occurring shortly after new and full moon, when the sun and moon pull together. Spring tides give higher high waters and lower low waters and therefore the strongest tidal streams.

A small-range tide occurring shortly after the moon's first and last quarters, when the sun and moon pull at right angles. Neap tides give lower high waters, higher low waters and weaker tidal streams.

The difference in height between consecutive high and low waters at a place. Range varies through the spring-neap cycle and with the height of the tide, and it controls how fast the tidal stream runs.

A rough method of finding the height of tide between high and low water. The range is divided into twelve parts that run in the ratio 1:2:3:3:2:1 over the six hours of the tide.

A port for which full daily tide tables are printed, giving times and heights of high and low water. Predictions for nearby secondary ports are obtained from it by applying time and height differences.

A port whose tide predictions are worked out from a standard port by applying tabulated time and height differences. The differences may themselves vary with the height of the tide at the standard port.

The horizontal movement of water caused by the tide, described by its set (direction) and drift (speed). It is shown on charts by arrows with letters, and in atlases by hourly vectors.

The two components of a tidal stream: set is the direction towards which the water flows, given in degrees true, and drift is its speed, usually in knots. Both are needed to calculate a course to steer.

The brief period when the tidal stream is weakest, roughly at the turn of the tide, before it reverses direction. It is not the same as high or low water, which are vertical tide events.

The moment when the tide reaches its greatest height at a place, followed by a period of slack before the ebb. Tide tables list the times and heights of high and low water for each day.

The ratio between a distance on the chart and the real distance it represents, such as 1:25,000 for a harbour plan. Larger-scale charts cover smaller areas and show more detail useful for pilotage.

A written plan for a voyage, prepared from charts, pilot books and tide tables, covering the route, waypoints, tides, hazards, contingencies and abort ports. It is briefed to the crew before departure and reviewed as the passage proceeds.

A chosen position on the planned track, marked on the chart and often entered into a GPS, at which the vessel alters course. Consecutive waypoints define the legs of the passage.

Navigation in confined waters close to hazards, using transits, bearings, soundings, buoys and leading lines rather than open-water passage techniques. It requires a prepared plan and frequent position checks.

A line shown on the chart, often marked by two beacons or buoys, which a vessel follows to pass safely through a channel. Keeping the marks in line confirms the vessel is on the leading line.

Buoys and beacons marking the sides of a navigable channel, red to port and green to starboard when entering harbour in the IALA A region. Their colour, shape and topmark indicate which side to pass.

Buoys indicating the safe side on which to pass a hazard, named north, east, south and west. Each is identified by its colour pattern, cone topmarks and a flashing light whose group matches a clock face.

The angle between true north and magnetic north at a place, changing slowly from year to year. It is shown in the compass rose and must be applied to convert between true and magnetic directions.

The error in a magnetic compass caused by the vessel's own magnetic field, varying with the heading. It is recorded on a deviation card and applied together with variation to convert compass to true.

The international rules for preventing collisions at sea, defining conduct in sight of another vessel, in restricted visibility, and the lights and shapes vessels must show. A candidate must know the steering and sailing rules and the risk-of-collision duties.

The vessel required to keep out of the way of another under the collision regulations, taking early and substantial action. In a crossing situation the vessel with the other on her starboard side gives way.

The vessel that must keep her course and speed while the give-way vessel manoeuvres. She must still take action if it becomes clear the other vessel is not doing so, and in extremis act to avoid collision.

The speed at which a vessel can take proper and effective action to stop within a safe distance, judged for visibility, traffic, draught and sea state. It is a duty under the collision regulations, especially in fog.

Any condition such as fog, mist or heavy rain that limits the distance a vessel can be seen. In these conditions the collision regulations require a safe speed, sound signals and radar or other lookout where fitted.

A rehearsed procedure for recovering a person who has fallen into the water, including shouting the alert, throwing buoyancy, marking the position and turning the boat back. It is practised regularly so the crew act without hesitation.

Personal flotation worn to keep a person's head above water and turn them face up if unconscious. Checks before use include fit, buckles, gas cylinder and crotch strap; it should be worn on deck at night or in rough weather.