Day Skipper theory: navigation, tides, passage planning and safety
What this domain tests
Day Skipper theory tests practical navigation: chart reading, tide height calculation, course and speed, passage planning and safety planning. You need to show you can turn a chart and a forecast into a safe plan, not just recite definitions.
Core concepts
- Charts: depths are usually in metres below chart datum, the lowest astronomical tide; a charted 2.5 m is not a promise of 2.5 m at all times. Latitude and longitude give position; scale tells you how much detail is shown.
- Tides: use tide tables for high and low water, then interpolate. The rule of twelfths gives a quick height estimate: in six hours the rise is 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range.
- Course, speed and time: distance = speed × time; time = distance ÷ speed. Use speed over ground when tidal stream affects you.
- Passage plan: study the chart for hazards, depths, entrances and no-go areas; check weather and tides; set waypoints and alternates; brief crew; leave a float plan; plan arrival at a bar or harbour entrance near slack water.
Worked example
A passage is 12 nautical miles. Your boat makes 6 knots through the water, and you expect 1 knot of fair tide, so speed over ground is 7 knots. Time = 12 ÷ 7 = 1.71 hours = 1 hour 43 minutes. If you leave at 09:00, ETA is 10:43. If the tide turns against you at 10:30, do not assume the ETA still holds - replan.
Exam traps
- Using charted depth as actual depth without adding tide height.
- Forgetting that chart datum is not mean sea level.
- Using boat speed through water when the question gives a tidal stream.
- Planning a bar crossing at maximum ebb without checking local advice.
- Leaving no alternate if weather or tide changes.