Module 11 - Tides & Tidal Streams
Worked Secondary-Port and Tidal-Gate Example
Standard-port HW is 0535 at 5.2 m and LW is 1148 at 1.1 m. The applicable secondary differences are HW +0015 and -0.4 m; LW +0025 and +0.2 m. Corrected secondary HW is 0550 at 4.8 m and LW is 1213 at 1.3 m.
Corrected range = 4.8 - 1.3 = 3.5 m. The supplied curve says that two hours after HW, 0.72 of the fall has occurred. Predicted height = 4.8 - (3.5 × 0.72) = 2.28 m, rounded to 2.3 m before operational allowances.
The foul stream begins at HW +0400. From secondary HW 0550, that is 0950. To be clear 30 minutes earlier, the gate time is 0920. Work backwards from 0920 with realistic speed over ground and an earlier abort if progress is slow.
- Apply separate secondary HW and LW corrections first
- Read whether the supplied factor is rise or fall
- Check the height remains between corrected HW and LW
- Convert tidal phase to clock time before route timing
- Add vessel and decision margins after the arithmetic
| Stage | Visible working | Result |
|---|---|---|
| Secondary HW | 0535 + 15 min; 5.2 - 0.4 | 0550 at 4.8 m |
| Secondary LW | 1148 + 25 min; 1.1 + 0.2 | 1213 at 1.3 m |
| Range | 4.8 - 1.3 | 3.5 m |
| Height at HW +2 | 4.8 - (3.5 × 0.72) | 2.28 m ≈ 2.3 m |
| Foul stream starts | 0550 + 4 h | 0950 |
| Clear-by gate | 0950 - 30 min | 0920 |
Optional quick check
Section 11 of 11
Corrected HW is 0550. Foul stream starts at HW +0400 and the gate must be clear 30 minutes earlier. What is the clear-by time?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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