Maritime Questions › Day Skipper Charts Tides

When do neap tides occur and why are they smaller than spring tides?

Q: On a UK Admiralty chart, depths are measured from which reference level?

Lowest Astronomical Tide (LAT) UK charts use LAT as chart datum — the lowest level the sea is predicted to fall under average meteorological conditions. All charted depths are below this, so actual depth is always at least the charted depth.

Q: Using the Rule of Twelfths, how much of the tidal range rises or falls in the SECOND hour of a tidal cycle?

2/12 of the range The Rule of Twelfths: 1st hour 1/12, 2nd hour 2/12, 3rd hour 3/12, 4th hour 3/12, 5th hour 2/12, 6th hour 1/12. The tidal flow is fastest mid-cycle.

A. Neaps occur monthly when the Earth is at aphelion

B. When the Moon is at perigee (closest to Earth)

C. At new and full moon, when Sun and Moon align and pull together

D. At first and third quarter moon, when the Sun and Moon are at 90° — their gravitational pulls partially cancel

On a UK Admiralty chart, depths are measured from which reference level?

A. Lowest Astronomical Tide (LAT)

B. Mean High Water Springs (MHWS)

C. Mean Low Water Neaps (MLWN)

D. Mean Sea Level (MSL)

Using the Rule of Twelfths, how much of the tidal range rises or falls in the SECOND hour of a tidal cycle?

A. 2/12 of the range

B. 1/12 of the range

C. 4/12 of the range

D. 3/12 of the range

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