Module 11 - Tides & Tidal Streams
Worked Height and Secondary-Port Examples
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.
- Range = HW height - LW height
- Rise = range × supplied curve factor
- Predicted height = LW height + rise
- Estimated depth = sounding + predicted tidal height
- A depth estimate needs draught and operating margins before it supports a decision
| 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 |
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.