Worked tide example
How to Calculate the Height of High Water at a Secondary Port
Compare today's standard port high water with its MHWS and MHWN, then interpolate the secondary port's height differences in proportion. A 5.3 m high water with differences of −0.4 m at springs and −0.2 m at neaps gives a difference of −0.36 m and high water of 4.9 m at the secondary port.
The question
Today's HW at the standard port is 5.3 m. The standard port's MHWS is 5.5 m and MHWN 4.4 m. The secondary port's height differences are −0.4 m at MHWS and −0.2 m at MHWN. What is the height of HW at the secondary port?
| Item | Value |
|---|---|
| Standard port HW | 5.3 m |
| MHWS / MHWN | 5.5 m / 4.4 m |
| Difference at MHWS / MHWN | −0.4 m / −0.2 m |
Working, step by step
- Today's HW is 0.9 m above MHWN, out of 1.1 m between MHWN and MHWS: 0.82 of the way.
- The difference changes from −0.2 m to −0.4 m, a change of −0.2 m.
- Difference = −0.2 + 0.82 × (−0.2) = −0.36 m.
- Secondary port HW = 5.3 m − 0.36 m = 4.9 m.
Answer
High water at the secondary port is about 4.9 m. Some almanacs also list a seasonal change in mean level; add it if given.
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: time of HW