Admiralty charts use WGS84 datum, same as GPS, so plotted positions match your GPS fix directly.
Depths are in metres and reduced to Chart Datum, usually close to Lowest Astronomical Tide (LAT) - the lowest the tide is predicted to fall.
Heights of bridges, cables and lit structures are given above Mean High Water Springs (MHWS), the opposite reference to depths.
Always check the chart's scale and the units panel (metres, fathoms or feet) before using it - not every chart is metric.
Latitude is read from the side (vertical) scale, longitude from the top or bottom (horizontal) scale - never mix them up when plotting.
One minute of latitude always equals one nautical mile, anywhere on the chart, so use the latitude scale level with your position for distance measurement.
Positions are written as degrees, minutes and decimals of a minute, eg 50 degrees 42.5 minutes N, 001 degrees 58.3 minutes W.
A magnetic compass points to Magnetic North, not True North; the difference is variation, shown on the chart's compass rose and changing slowly year on year.
Deviation is a separate error caused by the boat's own metal and electronics, and it changes with the boat's heading - correct with a deviation card if fitted.
The memory aid 'Compass to True: Add East' (CADET) converts a compass reading to true: True = Compass + Easterly error, subtract if Westerly.
Working the other way, from true to compass, reverse the rule: subtract Easterly, add Westerly.
Always combine variation and deviation into one total error before converting, and always state whether an error is East or West.
A three-point fix using bearings of three identified objects gives the most reliable position; a small triangle (cocked hat) is normal, plot your position in the middle unless closer to danger.
A GPS fix is instant and usually most accurate, but always cross-check it against a visual or chart-based fix, especially close to hazards.
A transit (two objects in line) gives an exact position line with zero compass error, useful for checking compass deviation too.
Forgetting to apply variation, or applying it the wrong way round, is the classic exam trap - always double-check East adds, West subtracts when going compass to true.
Mixing up the latitude and longitude scales when plotting a position.
Using the wrong depth reference (Chart Datum for depths, MHWS for heights) leads to wrong under-keel clearance and air-draft calculations.
Tides drive two things you must plan around: how much water is under the keel, and how the water itself is moving (tidal streams). Get either wrong and you run aground or fight a foul current all day.
UK tides are semi-diurnal - two high waters and two low waters roughly every 24 hours 50 minutes (the extra 50 minutes because the moon orbits with the earth's rotation). One tidal cycle (high to high) is about 12 hours 25 minutes.
Springs and neaps run on a roughly 14-day cycle tied to the moon's phase:
Use the Rule of Twelfths to estimate height at a given time when you have no curve: in each hour after HW or LW the tide moves 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the total range. This is an approximation for a roughly sinusoidal tide - it is not accurate for rivers or ports with unusual curves.
Chart datum (Lowest Astronomical Tide, LAT) is the reference for charted depths and drying heights. To find depth of water: Charted depth + height of tide (from tide tables, corrected for the day) = actual depth. Always check drying heights (underlined figures) separately - they show ground that uncovers.
Secondary ports use time and height differences applied to a nearby Standard Port - always check whether you interpolate for springs/neaps using the range at the Standard Port, not guesswork.
Streams are the horizontal movement of water, shown as tidal diamonds on charts cross-referenced to tidal stream atlases or almanac tables (direction in degrees true, rate in knots, for springs and neaps each hour referenced to HW at a Standard Port).
The UK uses IALA Region A. The golden rule is 'red to port' when returning from sea (entering harbour).
Cardinal marks tell you where the safe water is relative to a danger, named after the compass point on the safe side.
Preferred channel marks (where a channel splits) are the opposite colour with a small band of the other colour, and a composite group flashing light such as Fl(2+1)R or Fl(2+1)G.
Pilotage is planning and executing a passage in confined or unfamiliar waters using landmarks, transits, clearing lines and depths rather than open-water navigation.
The International Regulations for Preventing Collisions at Sea (Colregs) are the 'rules of the road' at sea. As Day Skipper you need the steering and sailing rules cold - Rules 4 to 19 cover this ground, but the exam focuses on a practical subset.
When any vessel is under power, a simple hierarchy applies. Sailing vessels give way to vessels not under command, restricted in ability to manoeuvre, fishing vessels engaged in fishing, and vessels constrained by draught. Power-driven vessels give way to all of the above plus sailing vessels.
The give-way vessel must take early and substantial action to keep well clear. The stand-on vessel should hold course and speed, but must take avoiding action if it becomes clear the give-way vessel is not acting, or if collision cannot be avoided by the give-way vessel alone.
Most UK weather arrives from the Atlantic, driven by the jet stream steering low-pressure systems (depressions) eastward. Depressions bring wind and rain; highs (anticyclones) bring settled, lighter weather. A skipper needs a forecast before every passage - never rely on how it looks out the window right now.