Height of tide and depth of water
Tidal stream at a diamond
How to think it through
- 1The diamond table gives, hour by hour, the set and two rates: springs and neaps.
- 2Today's tide is somewhere between: compare today's range with the neap and spring ranges.
- 3With the same fraction, find today's rate between the two in the table.
Worked example
Every step says why you do it, which formula you use, the working and the answer.
Tidal stream at a diamond, between springs and neaps
At diamond F, 4 hours before HW, the table gives set 190°, 2.4 kn at springs and 1.2 kn at neaps. Today's range is 2.6 m (springs 4.5 m, neaps 2.1 m). What stream do you get?
- Diamond F, 4 hours before HW
- 190° · 2.4 / 1.2 kn
- Range springs · neaps
- 4.5 m · 2.1 m
- Today's range
- 2.6 m
Where are you between neaps and springs (fraction from 0 to 1)?
Why: The diamond gives two rates, springs and neaps: today's depends on where today's range sits between them.
Working: (2.6 − 2.1) ÷ (4.5 − 2.1) = 0.21
Answer: 0.21
What is today's rate?
Why: Start from the neap rate and add the share of the difference given by the fraction.
Working: 1.2 + 0.21 × (2.4 − 1.2) = 1.4 kn towards 190°
Answer: 1.5 kn