Tides and Weather Awareness
Tide questions in Part A are calculation questions dressed as seamanship: will the boat float over the sill, when is slack water, how much water is under the keel at high water. Answers come from the Hong Kong Observatory tide tables, not from guesswork.
The basics
- Hong Kong tides are mainly semidiurnal, with two high and two low waters a day, but the range varies: springs near new and full moon, neaps near the quadratures.
- Predicted heights are above chart datum, so the depth available at any time is the charted depth plus the predicted height.
- Secondary ports are worked from a standard port using time and height differences.
- Under-keel clearance equals charted depth plus height of tide minus draught.
Rule of twelfths
Over the six hours from low to high water the rise is roughly 1/12, 2/12, 3/12, 3/12, 2/12 and 1/12 of the range. It assumes a sine curve and is good enough for pilotage decisions.
Weather effects
- One hectopascal below standard pressure raises water about one centimetre.
- Sustained onshore wind and low pressure add to predicted heights; a typhoon can drive a surge far above prediction.
- Tidal streams in Hong Kong are complicated by Pearl River discharge; use the tidal stream atlas and local observation rather than theory.
Worked example
Charted depth over a sill is 1.5 m. Your draught is 2.2 m and you want 0.5 m under the keel. High water is 1200 at 3.0 m; low water is 1800 at 0.6 m.
Reasoning: required depth is 2.7 m, so required height of tide is 2.7 minus 1.5, which is 1.2 m. The range is 2.4 m, so you need a fall of 1.8 m from high water - nine twelfths of the range. Cumulative twelfths after four hours are 1+2+3+3, which is 9. Height at 1600 is 3.0 minus 1.8, which is 1.2 m. Earliest crossing with the required clearance is 1600.
Traps
- Using the range as the height of tide.
- Adding a height when the chart shows a drying height.
- Assuming slack water coincides with high or low water; locally it often does not.
- Ignoring barometric and wind setup when crossing a shallow bar.