Module 11 - Tides & Tidal Streams
The Tidal Curve Method
What you will learn
Use the supplied port curve to calculate an intermediate height with visible range, factor, direction and bounds checks.
A tidal curve is a port- or exercise-specific model, not a universal six-hour shape. Keep every input and intermediate value visible so another navigator can audit the result.
Start with the bracketing HW and LW and calculate range = HW height - LW height. Express the required time as an offset before or after HW, then use the curve supplied for that port or exercise to read the fraction of range.
When the factor represents rise above LW, calculate height = LW + (range × factor). Some diagrams use a different construction or plot fall from HW, so follow the printed axis and method rather than memorising one formula.
Check that the result lies between the bracketing heights and moves in the expected direction. A correct prediction still needs draught, under-keel, squat, wave and meteorological allowances before it supports a transit.
Use the curve supplied for the port or exercise; this visual supports the worked arithmetic and is not a universal port curve.
Worked example
HW is 4.9 m at 1200, LW is 1.3 m at 1800, and the supplied curve gives a rise factor of 0.50 at 1500.
- 1Range = 4.9 m - 1.3 m = 3.6 m.
- 2Rise above LW = 3.6 m × 0.50 = 1.8 m.
- 3Predicted height = 1.3 m + 1.8 m = 3.1 m.
- 4A 2.4 m sounding gives 2.4 m + 3.1 m = 5.5 m estimated depth before margins.
Sense check: The tide is falling and 3.1 m is halfway between 4.9 m and 1.3 m, so both bounds and direction are plausible.
Convert height into a timed go/no-go limit
The curve gives adequate depth at 1500, but the tide is falling and the entrance is exposed to swell.
1. Define vessel need
Combine draught with chosen under-keel, squat and sea-state allowances.
2. Find the limiting time
Use the curve to find when predicted depth approaches that requirement, not just one comfortable instant.
3. Brief an earlier abort
Leave decision and manoeuvring time before the calculated limit and name the safe alternative.
Sense check: A depth at one instant is not a usable passage window until trend, uncertainty and turn-away time are included.
Common mistake or limitation
- A 0.50 factor at three hours is not universal; it is valid only when the supplied curve says so.
- Using rise-from-LW arithmetic on a curve that expresses fall-from-HW reverses the construction.
Recap
- Show source values, range, time offset, factor and final height.
- Read the exact curve axes and direction.
- Add operational margins and a timed abort before acting.
Optional quick check
Section 5 of 13
HW is 4.9 m, LW 1.3 m and the supplied rise factor is 0.50. What is the predicted height?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
This public lesson is part of Revision Module 11. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.