Tidal Calculations Flashcards

What is a 'secondary port' in tidal calculations?

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.

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?

Answer: 1145

Secondary port HW = 1200 − 15 minutes = 1145. The negative sign means the secondary port HW occurs before the standard port HW.

Which tidal level is used as the datum for heights of objects (e.g., cliff heights) on UK charts?

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.

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:

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.

What is 'height of tide'?

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.

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?

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.)