The Rule of Twelfths

When we arrive in an anchorage and the tide is in mid-cycle, we need to estimate how much more (or less) water there will be at high or low tide. We use the Rule of Twelfths. It goes like this:

• In the first hour, the tide will rise 1/12 of the difference between low and high tide
• In the second hour it will rise 2/12 of the difference (for a total of 3/12 = 1/4)
• In the third hour it will rise 3/12 of the difference (for a total of 6/12 = 1/2)
  
• In the forth hour it will rise 3/12 of the difference (for a total of 9/12 = 3/4)
• In the fifth hour it will rise 2/12 of the difference
• In the sixth hour it will rise 1/12 of the difference

For falling tides, use the same rule, but it will fall 1/12, 2/12, etc.

The Rule of Twelfths