How to use
- Choose metres or feet and whether the tide is rising or falling.
- Enter the low and high water heights from the tide table.
- Enter the hours from low to high water and the hours since the turn of the tide.
How it works
- range = high water − low water;
- tidal hour = duration ÷ 6; in each tidal hour the water moves 1, 2, 3, 3, 2, 1 twelfths of the range, spread evenly within the hour;
- rising tide: height = low + range × share; falling: height = high − range × share;
- rate = twelfths of the current hour × range ÷ 12 ÷ length of a tidal hour.
Example
Low water 0.5 m, high water 4.1 m six hours later. How high is the water 2½ hours after low water?
- range 4.1 − 0.5 = 3.6 m;
- after 2 hours: 1 + 2 = 3 twelfths; half of the third hour adds 1.5 more: 4.5/12 = 37.5 %;
- 3.6 × 0.375 = 1.35 m, so the height is 0.5 + 1.35 = 1.85 m;
- in the third hour the water rises 3/12 × 3.6 = 0.9 m per hour; the cosine curve gives 1.83 m.
Tips and common mistakes
- Heights are above chart datum; add the charted depth to get the depth of water.
- Use times corrected for summer time and for your port (secondary port corrections) before you start.
- Leave a margin under the keel: weather and pressure change the real tide.