RYA Day Skipper Tides and Tidal Streams — Questions and Answers
Question 1: What causes spring tides?
- The spring season
- The gravitational pull of the sun and moon acting in alignment (at new and full moon) (Correct answer)
- Strong winds from the west
- The Earth's rotation speeding up
Correct answer: The gravitational pull of the sun and moon acting in alignment (at new and full moon)
Spring tides occur when the sun and moon are in alignment (syzygy), at new moon and full moon. Their combined gravitational pull produces the highest high tides and lowest low tides.
Question 2: What is the 'Rule of Twelfths' used for?
- Calculating boat speed
- Estimating the rise or fall of tide during each hour of a roughly six-hour tidal cycle (Correct answer)
- Measuring wind speed
- Calculating fuel consumption
Correct answer: Estimating the rise or fall of tide during each hour of a roughly six-hour tidal cycle
The Rule of Twelfths estimates tidal rise/fall each hour: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. It assumes a sinusoidal curve over approximately six hours between high and low water.
Question 3: What is 'chart datum' in relation to tides?
- The average sea level
- The reference level on a chart below which depths are shown and above which tidal heights are measured (Correct answer)
- The highest tide level
- The depth at which anchoring is safe
Correct answer: The reference level on a chart below which depths are shown and above which tidal heights are measured
Chart datum is the reference level used on charts, typically LAT (Lowest Astronomical Tide) in UK waters. Depths shown on the chart are measured below this level, and tidal heights are measured above it.
Question 4: What is the difference between 'height of tide' and 'depth of water'?
- They are the same
- Height of tide is the vertical distance from chart datum to the water surface; depth of water is chart depth plus height of tide (Correct answer)
- Height of tide is always greater
- Depth of water is measured from the surface down
Correct answer: Height of tide is the vertical distance from chart datum to the water surface; depth of water is chart depth plus height of tide
Height of tide is measured from chart datum upwards to the water surface. Actual depth of water at any point = charted depth + height of tide (both measured from chart datum).
Question 5: What is the approximate interval between successive high waters at a given location?
- 6 hours
- 12 hours 25 minutes (Correct answer)
- 24 hours
- 8 hours
Correct answer: 12 hours 25 minutes
In a semidiurnal tidal pattern (as in most UK waters), high water occurs approximately every 12 hours and 25 minutes, reflecting the lunar day of about 24 hours 50 minutes.
Question 6: What does 'neap tides' refer to?
- The largest tidal range
- Tides with the smallest range, occurring when the sun and moon are at right angles to each other (first and last quarter moon) (Correct answer)
- Tides caused by storms
- Tides that only occur in estuaries
Correct answer: Tides with the smallest range, occurring when the sun and moon are at right angles to each other (first and last quarter moon)
Neap tides have the smallest tidal range and occur at first and last quarter moon when the sun and moon are at 90 degrees to each other, partially cancelling their gravitational effects.
RYA Day Skipper Tides and Tidal Streams
What causes spring tides?