Module 11 - Tides & Tidal Streams
Worked Secondary-Port and Tidal-Gate Example
What you will learn
Complete a linked secondary-port, falling-tide and tidal-gate calculation while preserving signs, time basis, bounds and an operational abort margin.
A defensible answer leaves a visible chain from standard-port source values through corrected secondary waters, curve result and gate time to the skipper's decision.
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.
Worked example
Audit the calculation before it becomes a shallow-water and gate decision.
- 1Confirm publication, standard/secondary relationship, date, time basis and datum.
- 2Check each signed correction moves the time or height in the printed direction.
- 3Check 2.28 m lies between 4.8 m HW and 1.3 m LW and is falling.
- 4Compare predicted depth with draught and all operating allowances.
- 5Work backwards from 0920 and set an earlier progress-based abort.
Sense check: A falling-tide answer above HW or below LW exposes a sign, factor or baseline error before the yacht commits.
| 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 |
Make the answer bridge-ready
The arithmetic is complete, but forecast pressure is high and the yacht is slower than planned against a residual sea.
1. Reduce confidence
Treat lower-than-predicted water and slower progress as adverse rather than averaging them away.
2. Recalculate both limits
Update the depth window and time required to clear the gate.
3. Use the alternative
Hold, delay or divert when the earlier abort criterion is met.
Sense check: Passing the arithmetic check does not override adverse observations or remove the need for a safe fallback.
Common mistake or limitation
- Applying secondary-port corrections after the curve can use the wrong height band or sign.
- The 0.72 factor is specific to the supplied curve and means fall from HW in this example, not a universal rise from LW.
Recap
- Keep the complete source-to-decision chain visible.
- Use bounds and direction checks to catch arithmetic errors.
- Separate the prediction from the skipper's operational margin and abort.
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.
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.