Module 11 - Tides & Tidal Streams
Worked Height and Secondary-Port Examples
What you will learn
Complete and sense-check a linked secondary-port, intermediate-height and available-depth calculation without losing datum, sign or margin.
A reliable tidal calculation leaves a visible chain from source values to corrected port values, curve factor, predicted height and navigational margin. Each line should be independently checkable.
Worked height: HW is 4.9 m at 1200, LW is 1.3 m at 1800, and the supplied curve factor at 1500 is 0.50. Range = 4.9 - 1.3 = 3.6 m. Rise above LW = 3.6 × 0.50 = 1.8 m. Predicted height = 1.3 + 1.8 = 3.1 m.
Worked depth: a charted sounding of 2.4 m and predicted tidal height of 3.1 m give 2.4 + 3.1 = 5.5 m estimated water depth. For a yacht drawing 1.8 m with a chosen 0.7 m under-keel allowance, the minimum planned requirement is 2.5 m, leaving 3.0 m against that simplified requirement before any extra allowance for squat, waves or forecast uncertainty.
Worked secondary port: standard-port HW is 1140 at 4.8 m and the applicable secondary HW differences are +0020 and -0.3 m. Corrected secondary HW is 1200 at 4.5 m. This one HW example does not supply the secondary LW or the intermediate height; calculate those separately from their proper rows before using a curve.
Worked example
Decide whether the arithmetic alone is enough to approve a shallow-water transit.
- 1Confirm source, port, date, time basis and datum for every input.
- 2Calculate corrected port heights, then the intermediate height and estimated water depth.
- 3Subtract draught and the chosen under-keel allowance, then consider squat, waves, pressure, wind and survey or prediction uncertainty.
Sense check: The predicted height must stay between HW and LW, and every signed correction must move the result in the direction printed by the source.
| Stage | Visible working | Result |
|---|---|---|
| Range | 4.9 m - 1.3 m | 3.6 m |
| Rise at factor 0.50 | 3.6 m × 0.50 | 1.8 m |
| Height at 1500 | 1.3 m + 1.8 m | 3.1 m |
| Water over sounding | 2.4 m + 3.1 m | 5.5 m before margin |
| Simplified vessel need | 1.8 m draught + 0.7 m allowance | 2.5 m |
| Secondary HW | 1140 + 20 min; 4.8 m - 0.3 m | 1200 at 4.5 m |
Convert the answer into a timed limit
The arithmetic gives comfortable depth at 1500, but the tide is falling and the harbour entrance is exposed to swell.
1. Set the minimum
Combine draught, under-keel allowance, squat and sea-state allowance for this passage.
2. Find the limiting time
Use the correct curve to identify when predicted depth approaches that minimum, not just the height at 1500.
3. Create an earlier abort point
Allow decision and manoeuvring time before the calculated limit, with a safe alternative if observations disagree.
Sense check: A computed depth at one instant is not a passage window until the trend and the time needed to turn away are included.
Common mistake or limitation
- A correct arithmetic result can still be operationally unsafe if draught, uncertainty and a turn-away margin are omitted.
- Secondary-port corrections belong before the curve; applying them to the final intermediate height can use the wrong method or sign.
Recap
- Keep the complete source-to-decision calculation visible.
- Use bounds and direction-of-change checks to catch errors.
- Separate predicted water level from the skipper's safety decision.
Optional quick check
Section 8 of 8
A calculated water depth is 5.5 m and the yacht draws 1.8 m. What is still needed before treating the transit as safe?
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.