Tidal Streams & Secondary Ports Flashcards

In a tidal stream atlas, the figures '08,15' alongside an arrow mean:

Answer: Mean neap rate 0.8 knots; mean spring rate 1.5 knots

UK tidal atlases express rates in tenths of knots: the first figure is the neap rate, the second is the spring rate. '08,15' = 0.8 kn neaps, 1.5 kn springs.

Why does tidal range vary between springs and neaps on a fortnightly cycle?

Answer: The relative positions of the Sun, Earth, and Moon — at new and full Moon, solar and lunar tidal forces combine (springs); at quarter Moon they partially cancel (neaps)

Spring tides occur when the Sun and Moon are aligned (new/full Moon), adding their gravitational effects. At first/third quarter the Sun and Moon are at right angles, partially cancelling each other, producing neaps.

At a secondary port, HW time difference is +0h 45min and LW time difference is +1h 10min. Standard port HW = 0800, LW = 1400. What are the secondary port HW and LW times?

Answer: HW 0845, LW 1510

HW: 0800 + 0h45 = 0845. LW: 1400 + 1h10 = 1510. Apply each difference independently to the respective HW and LW times.

The height of HW at a standard port is 5.8 m at springs and 4.2 m at neaps. Height difference for a secondary port is +0.3 m (springs) and +0.2 m (neaps). Today's HW height is between springs and neaps (range factor 0.6). What is the secondary port HW height (approx)?

Answer: 4.2 + 0.6(5.8−4.2) + [0.2 + 0.6×0.1] ≈ 5.22 m

Interpolated standard height: 4.2 + 0.6×1.6 = 5.16 m. Interpolated difference: 0.2 + 0.6×0.1 = 0.26 m. Secondary HW height ≈ 5.16 + 0.26 = 5.42 m (approximately 5.4–5.5 m). The calculation is complex but the method is the key point.

A vessel needs to cross a bar with 1.8 m draft plus 0.5 m clearance. The charted depth at the bar is 0.4 m. The current tide height is 1.5 m. Can the vessel cross?

Answer: No — required depth = 2.3 m; available depth = 0.4 + 1.5 = 1.9 m — 0.4 m short

Required = draft + clearance = 2.3 m. Available = charted depth + tide height = 0.4 + 1.5 = 1.9 m. Shortfall = 0.4 m. The vessel cannot safely cross at this state of tide.

When using the Yachtmaster method to estimate arrival time at a tidal gate, which tidal atlas pages are used?

Answer: Pages referenced to hours before and after HW at the tidal atlas standard port for that area

UK tidal atlases have 13 pages: 6 hours before HW, HW, 6 hours after HW at a named standard port. The skipper must calculate HW time at that standard port to use the correct page.