Maritime Questions › Day Skipper Charts Tides

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

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.

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.

A. 2/12 of the range

B. 1/12 of the range

C. 4/12 of the range

D. 3/12 of the range

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)

What is the difference between magnetic variation and compass deviation?

A. Both are the same error applied to the same compass bearing

B. Variation applies only to electronic compasses; deviation to magnetic compasses

C. Deviation is the angle between true and magnetic north; variation is caused by the vessel's steel

D. Variation is the angle between true and magnetic north (Earth's field); deviation is caused by the vessel's own magnetic influences on the compass

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