Module 11 - Tides & Tidal Streams
Worked Height and Secondary-Port Examples
Active recall
Section 8 of 8
Explain the main idea from worked Height and Secondary-Port Examples in your own words.
Worked height example: HW is 4.9 m at 1200, LW is 1.3 m at 1800, and you need the height at 1500. The range is 4.9 - 1.3 = 3.6 m. 1500 is three hours after HW, so if the curve factor is 0.50, the height is LW + half the range = 1.3 + (3.6 x 0.50) = 3.1 m.
Depth example: if the charted depth is 2.4 m and the predicted height of tide is 3.1 m, the estimated depth is 2.4 + 3.1 = 5.5 m before applying any skipper's safety margin for pressure, wind, swell, sounding error, and under-keel clearance.
Secondary-port example: if the standard port HW is 1140 at 4.8 m, and the secondary port difference is HW +0020 and height -0.3 m, the secondary port HW is 1200 at 4.5 m. Use those corrected values before using the curve.
- Problem
- Range
- Calculation
- 4.9 m - 1.3 m
- Answer
- 3.6 m
- Problem
- Height at 1500
- Calculation
- 1.3 m + (3.6 m x 0.50)
- Answer
- 3.1 m
- Problem
- Depth over 2.4 m sounding
- Calculation
- 2.4 m + 3.1 m
- Answer
- 5.5 m before margin
- Problem
- Secondary HW
- Calculation
- 1140 + 20 min; 4.8 m - 0.3 m
- Answer
- 1200 at 4.5 m
| Problem | Calculation | Answer |
|---|---|---|
| Range | 4.9 m - 1.3 m | 3.6 m |
| Height at 1500 | 1.3 m + (3.6 m x 0.50) | 3.1 m |
| Depth over 2.4 m sounding | 2.4 m + 3.1 m | 5.5 m before margin |
| Secondary HW | 1140 + 20 min; 4.8 m - 0.3 m | 1200 at 4.5 m |
Tip: Keep units visible in every line. Most beginner tide mistakes are a skipped datum, a missing safety margin, or applying the secondary-port correction after the curve instead of before it.
Key points
- Range = HW height minus LW height.
- Height at time = LW height + (range x curve factor).
- Actual depth over a sounding = charted depth + height of tide.
- Secondary-port corrections are applied before the tidal-curve step.
Continue studying Tides & Tidal Streams
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