RYA Day Skipper Tidal Calculations — Questions and Answers
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?
- 3/12 of the range (Correct answer)
- 1/12 of the range
- 2/12 of the range
- 4/12 of the range
Correct 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.
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?
- The tide is between neaps and springs, closer to springs (Correct answer)
- The tide is at neaps
- The tide is at springs
- The range is abnormally small
Correct 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.
Question 3: A tidal atlas shows an arrow with '15,28' for an hourly position. What do these numbers mean?
- Neap rate = 1.5 knots; spring rate = 2.8 knots (Correct answer)
- 15 degrees set; 2.8 knots rate
- 1.5 NM drift; 28 minutes duration
- Neap set = 15°; spring set = 28°
Correct 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.
Question 4: To find the height of tide at an intermediate time using Admiralty tables, a mariner uses:
- The tidal curve for that port (springs or neaps curve as appropriate) (Correct answer)
- Only the Rule of Twelfths
- A GPS receiver
- The barometric pressure
Correct 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.
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?
- 2.0 m (Correct answer)
- 1.2 m
- 0.8 m
- 1.7 m
Correct 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.
Question 6: 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?
- 1.5 m (Correct answer)
- 2.5 m
- 0.5 m
- 3.0 m
Correct 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.
RYA Day Skipper Tidal Calculations
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?