Worked tide example
How to Calculate the Time of High Water at a Secondary Port
Find where today's standard port high water falls between the times in the secondary port table, and interpolate the time difference in proportion. With HW at the standard port at 0930 and differences of −0010 at 0600 and +0020 at 1200, the difference is +0008, giving HW at the secondary port at 0938 UT (1038 BST).
The question
HW at the standard port is 0930 UT. The secondary port's time differences for high water are −0010 when standard port HW is at 0600 or 1800, and +0020 when it is at 0000 or 1200. When is high water at the secondary port, in British Summer Time?
| Item | Value |
|---|---|
| Standard port HW | 0930 UT |
| Difference at 0600 / 1800 | −0010 |
| Difference at 0000 / 1200 | +0020 |
Working, step by step
- 0930 lies between 0600 and 1200, 3 h 30 min after 0600: 0.58 of the 6 hours.
- The difference changes from −10 to +20 minutes, a change of 30 minutes.
- Difference = −10 + 0.58 × 30 = 7.5, so +0008.
- Secondary port HW = 0930 +0008 = 0938 UT.
- Tide tables are in UT, so add one hour for BST: 1038 BST.
Answer
High water at the secondary port is at about 0938 UT, which is 1038 BST.
The tides in this example are made up for practice. For real passages, use official predictions such as ADMIRALTY EasyTide or a nautical almanac, and the port's own tidal curve.
Try it yourself
Put your own numbers into the free tide calculator, which uses the same Rule of Twelfths and works out under-keel clearance.
Learn the method
More worked tide examples
- Height of tide at a given time
- Time the tide reaches a height
- Depth from charted depth
- Under-keel clearance
- Crossing a drying height
- Bridge clearance
- Anchoring depth and scope
- Springs or neaps from the range
- Tidal stream rate from a diamond
- Secondary port: height of HW