Module 11 - Tides & Tidal Streams
Worked Secondary-Port and Tidal-Gate Example
Worked secondary-port example: the standard port HW is 0535 at 5.2 m and LW is 1148 at 1.1 m. The secondary port differences are HW +0015 and height -0.4 m; LW +0025 and height +0.2 m. Use secondary HW 0550 at 4.8 m and secondary LW 1213 at 1.3 m before using the curve.
The corrected range is 4.8 - 1.3 = 3.5 m. If the tidal curve factor two hours after HW is 0.72 of the fall from HW toward LW, the height is HW - (range x 0.72) = 4.8 - (3.5 x 0.72) = 2.28 m, rounded to about 2.3 m before applying safety margin.
Worked tidal-gate example: a headland has a foul tide beginning at HW +0400. If the corrected secondary-port HW is 0550 and you need to clear the gate at least 30 minutes before the foul stream, plan to be past the headland by 0920. Then work backwards using distance and realistic speed over ground.
0535 + 15 min; 5.2 m - 0.4 m
Result: 0550 at 4.8 m
1148 + 25 min; 1.1 m + 0.2 m
Result: 1213 at 1.3 m
4.8 m - 1.3 m
Result: 3.5 m
4.8 - (3.5 x 0.72)
Result: About 2.3 m
0550 + 4 h - 30 min
Result: 0920
| Step | Calculation | Result |
|---|---|---|
| Secondary HW | 0535 + 15 min; 5.2 m - 0.4 m | 0550 at 4.8 m |
| Secondary LW | 1148 + 25 min; 1.1 m + 0.2 m | 1213 at 1.3 m |
| Range | 4.8 m - 1.3 m | 3.5 m |
| Height two hours after HW | 4.8 - (3.5 x 0.72) | About 2.3 m |
| Gate latest time | 0550 + 4 h - 30 min | 0920 |
Key points
- Apply secondary-port time and height differences before using the tidal curve.
- For a falling tide, subtract the relevant fraction of the range from HW height.
- Round sensibly and keep a safety margin for pressure, wind, and under-keel clearance.
- For a tidal gate, convert the tidal phase into a clock time, then work backwards from the gate.
Continue studying Tides & Tidal Streams
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