Tidal Calculations Flashcards

The Rule of Twelfths is used to estimate the rise or fall of the tide. In the 3rd hour of a 6-hour tide cycle, what fraction of the range rises?

Answer: 3/12 of the range

The Rule of Twelfths: hour 1 = 1/12, hour 2 = 2/12, hour 3 = 3/12, hour 4 = 3/12, hour 5 = 2/12, hour 6 = 1/12. The 3rd hour contributes the largest share.

The tidal range at springs is 5.0 m and at neaps is 2.0 m. The range today is 3.5 m. What does this suggest?

Answer: The tide is between neaps and springs, closer to springs

3.5 m is midway between 2.0 and 5.0, but slightly closer to springs (halfway = 3.5 m exactly). This represents a mid-range tidal coefficient.

A tidal atlas shows an arrow with '15,28' for an hourly position. What do these numbers mean?

Answer: Neap rate = 1.5 knots; spring rate = 2.8 knots

In UK tidal atlases, the two numbers alongside stream arrows represent the mean neap rate (in tenths of a knot) and mean spring rate. '15,28' = 1.5 knots neaps, 2.8 knots springs.

To find the height of tide at an intermediate time using Admiralty tables, a mariner uses:

Answer: The tidal curve for that port (springs or neaps curve as appropriate)

Admiralty method uses the tidal curve (a graphical interpolation tool specific to each standard port) along with the predicted times and heights of HW and LW to find height at any intermediate time.

Chart datum is 0.5 m below MLWS at a particular port. The charted depth at a rock is 1.2 m. The height of tide is currently 0.8 m. What is the actual depth over the rock?

Answer: 2.0 m

Actual depth = charted depth + height of tide = 1.2 + 0.8 = 2.0 m. The height of tide is added directly to the charted depth.

A vessel draws 2.0 m. She needs 0.5 m clearance. The charted depth at the approach is 1.0 m. What minimum height of tide does she need?

Answer: 1.5 m

Required water depth = draft + clearance = 2.0 + 0.5 = 2.5 m. Height of tide needed = required depth − charted depth = 2.5 − 1.0 = 1.5 m.