Module 11 - Tides & Tidal Streams
The Tidal Curve Method
What you will learn
Use a supplied tidal curve to calculate an intermediate height with visible working and a reasonableness check.
The curve converts a time before or after high water into a fraction of the tidal range. Keep the high water, low water, range, time offset and curve factor visible so the result can be checked.
Start with the relevant high-water time and height and the bracketing low-water height. Calculate range = HW height - LW height. Express the required time as hours before or after high water, then use the curve supplied for that port and exercise to read the corresponding fraction of range.
Calculate height at time = LW height + (range × curve factor). The factor describes how far the water has risen above low water for the selected point on this curve. Some port diagrams or training exercises use a different construction, so follow the published method rather than forcing every tide into a smooth curve or a rule-of-twelfths approximation.
Write units and intermediate values. Check that the answer lies between the bracketing low and high waters and that it moves in the expected direction. The result is still a prediction and needs a prudent allowance for meteorological effects and the vessel's required clearance.
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 factor of 0.50 at 1500.
- 1Range = 4.9 m - 1.3 m = 3.6 m.
- 21500 is three hours after HW; supplied curve factor = 0.50.
- 3Rise above LW = 3.6 m × 0.50 = 1.8 m.
- 4Predicted height = 1.3 m + 1.8 m = 3.1 m.
Sense check: Three hours into a six-hour fall, 3.1 m is halfway between 4.9 m and 1.3 m and remains inside the bracketing heights.
Turn predicted height into a depth decision
The curve gives 3.1 m and the charted sounding on the intended track is 2.4 m.
1. Combine like datums
Estimated water depth = 2.4 m sounding + 3.1 m height of tide = 5.5 m.
2. Subtract the vessel need
Compare 5.5 m with draught plus the chosen under-keel allowance and any squat or wave allowance.
3. Challenge the prediction
Consider pressure, wind, surge and source uncertainty before choosing the latest safe time.
Sense check: A depth answer without draught and an operating margin is not yet a go/no-go decision.
Common mistake or limitation
- The curve factor is not assumed to be 0.50 at three hours for every port; use the curve supplied.
- Adding the factor directly to low water, instead of multiplying it by the range first, mixes unlike quantities.
Recap
- Show range, time offset, factor, rise and final height.
- Use the published curve or method for the port.
- Check bounds and add navigational margins before acting.
Optional quick check
Section 5 of 8
HW is 4.9 m, LW is 1.3 m and the supplied factor is 0.50. What is the predicted height?
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.