RYA Day Skipper Tidal Calculations

Question 1

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?

Accepted Answer

3/12 of the range

Answer

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.

Question 2

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?

Accepted Answer

The tide is between neaps and springs, closer to springs

Answer

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.

Question 3

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

Accepted Answer

Neap rate = 1.5 knots; spring rate = 2.8 knots

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

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?

Accepted Answer

2.0 m

Answer

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.

RYA Day Skipper Practice Test

RYA Day Skipper Practice Test

RYA Day Skipper Tidal Calculations