Tides, Currents and Tidal Calculations
The tide cycle
South African coasts experience mainly semidiurnal tides - two high and two low waters in roughly 24 hours 50 minutes, so each tide is about 6 hours 12 minutes apart. Spring tides occur near new and full moon (larger range); neap tides near the quarters (smaller range). Know the terms height of tide, range, duration, stand of tide and chart datum.
The rule of twelfths
When a full tidal curve is not available, the rule of twelfths estimates the rise or fall in each hour of a six-hour tide: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 of the range. It is a rough approximation and is least accurate near high and low water.
Worked example
High water 10:00, low water 16:00, range 1.8 m. What is the height of tide at 13:00 (three hours after HW)? Three hours is halfway through the fall, so half the range has gone: 1.8 ÷ 2 = 0.9 m fallen. If chart datum is the reference and HW height was 2.4 m above datum, height at 13:00 ≈ 2.4 − 0.9 = 1.5 m. Using the twelfths: after three hours, 1+2+3 = 6/12 = half the range - same answer.
Traps
Questions frequently give the time of high water at a standard port and require a secondary port correction (time and height differences from a tide table). Do not confuse tidal stream (horizontal movement of water caused by the tide) with current (a broader term including ocean and wind-driven flow). Set is the direction the stream flows towards; drift is its speed. Always check whether a question asks for height above chart datum or depth of water - add the charted sounding to the height of tide to get actual depth.