RYA Day Skipper Tides and Tidal Streams 2 — Questions and Answers
Question 1: How would you find the height of tide at a secondary port?
- Use the same data as the standard port
- Apply the time and height differences listed in the tide tables to the standard port predictions (Correct answer)
- Estimate based on the weather forecast
- Use a depth sounder only
Correct answer: Apply the time and height differences listed in the tide tables to the standard port predictions
Secondary port predictions are calculated by applying published time differences and height differences (or ratios) to the predictions for the associated standard port, as listed in nautical almanacs.
Question 2: What is a 'tidal stream atlas' used for?
- Predicting tidal heights
- Showing the direction and rate of tidal streams at hourly intervals relative to high water at a standard port (Correct answer)
- Identifying fishing grounds
- Plotting weather systems
Correct answer: Showing the direction and rate of tidal streams at hourly intervals relative to high water at a standard port
A tidal stream atlas shows arrows indicating the direction and rate (in knots, for springs and neaps) of tidal streams at hourly intervals, referenced to HW at a standard port.
Question 3: What does 'drying height' on a chart indicate?
- The height of a lighthouse above sea level
- The height of the seabed above chart datum — these areas are exposed (dry) at low water (Correct answer)
- The maximum wave height
- The height of the dock wall
Correct answer: The height of the seabed above chart datum — these areas are exposed (dry) at low water
A drying height (shown underlined on charts) indicates how far the seabed rises above chart datum. These areas are covered at high water but exposed at low water.
Question 4: When using tidal diamonds on a chart, what do the columns for spring and neap rates tell you?
- Wind speeds at different times
- The speed of the tidal stream in knots at mean springs and mean neaps for each hour relative to HW at the reference port (Correct answer)
- The height of tide
- The wave period
Correct answer: The speed of the tidal stream in knots at mean springs and mean neaps for each hour relative to HW at the reference port
Tidal diamond tables give the direction and rate of the tidal stream for each hour relative to HW at the standard port, with separate rate columns for mean spring and mean neap conditions.
Question 5: What is the 'range' of a tide?
- The distance the tide travels horizontally
- The vertical difference between high water and low water (Correct answer)
- The speed of the tidal stream
- The time between high and low water
Correct answer: The vertical difference between high water and low water
Tidal range is the vertical difference in height between consecutive high water and low water. Spring tides have a larger range than neap tides.
Question 6: At what point in the tidal cycle does the tidal stream typically turn?
- Exactly at high and low water
- Approximately at high and low water, though the exact time varies by location (Correct answer)
- At mid-tide only
- Tidal streams are constant
Correct answer: Approximately at high and low water, though the exact time varies by location
Tidal streams typically change direction (turn or 'slack') approximately at high and low water, but the exact timing varies by location. In some areas, the stream may turn well before or after HW/LW.
RYA Day Skipper Tides and Tidal Streams 2
How would you find the height of tide at a secondary port?