RYA Day Skipper Tidal Calculations 2 — Questions and Answers
Question 1: What is a 'secondary port' in tidal calculations?
- A port whose tidal times and heights are calculated by applying differences to a standard port (Correct answer)
- A minor harbour with no tidal data
- A port only used in emergencies
- A port where tides are unpredictable
Correct answer: A port whose tidal times and heights are calculated by applying differences to a standard port
Secondary ports are listed in tide tables with 'seasonal differences' relative to a named standard port. You correct the standard port's predictions using these differences to find the secondary port's tides.
Question 2: At a secondary port, the time difference on HW is listed as −0h 15min. Standard port HW is 1200. What is the secondary port HW time?
- 1145 (Correct answer)
- 1215
- 1200
- 1315
Correct answer: 1145
Secondary port HW = 1200 − 15 minutes = 1145. The negative sign means the secondary port HW occurs before the standard port HW.
Question 3: Which tidal level is used as the datum for heights of objects (e.g., cliff heights) on UK charts?
- Mean High Water Springs (MHWS) (Correct answer)
- Chart Datum
- Mean Sea Level
- Highest Astronomical Tide (HAT)
Correct answer: Mean High Water Springs (MHWS)
Heights of landmarks, bridges, and obstructions on UK Admiralty charts are measured above MHWS, which represents approximately the highest regular tidal level.
Question 4: A tidal stream rate at springs is 3.0 knots. At neaps it is 1.2 knots. On a day with a range that is 70% of the spring range, the approximate rate is:
- 2.46 knots (interpolated between neap and spring rates) (Correct answer)
- 3.0 knots
- 1.2 knots
- 2.1 knots
Correct answer: 2.46 knots (interpolated between neap and spring rates)
Interpolate: rate = neap rate + 0.7 × (spring rate − neap rate) = 1.2 + 0.7 × (3.0 − 1.2) = 1.2 + 1.26 = 2.46 knots.
Question 5: What is 'height of tide'?
- The vertical distance from chart datum to the sea surface at a given time (Correct answer)
- The height of a wave
- The height above sea level of a charted landmark
- The tidal range over a full cycle
Correct answer: The vertical distance from chart datum to the sea surface at a given time
Height of tide (HoT) is the vertical distance from chart datum (CD) to the actual water surface at the time in question. It varies throughout the tidal cycle.
Question 6: A marina gate opens when there is at least 2.5 m of water over the sill. The sill is charted at 0.2 m above CD. What height of tide is needed?
- 2.3 m (Correct answer)
- 2.5 m
- 2.7 m
- 0.2 m
Correct answer: 2.3 m
Height of tide needed = required depth − sill height above CD = 2.5 − 0.2 = 2.3 m. (Since sill is 0.2 m above CD, the water only needs to reach 2.3 m HoT to provide 2.5 m over the sill.)
RYA Day Skipper Tidal Calculations 2
What is a 'secondary port' in tidal calculations?