Rules of Thumb for Tides: 50/90, Thirds, Twelfths

Rule of 50/90 is used to estimate current speed at the end of each hour of the six-hour tidal period

50/90 Rule gives you “… the SPEED OF THE CURRENT at the end of each hour.” Counting from slack, the current will flow at 50% of its maximum speed at the end of the first hour, 90% at the end of the second hour and full 100% or maximum speed at the end of the third hour and then back to zero with the same steps.  The full rule should be stated as 0/50/90/100/90/50/0.

Rule of Thirds is used to calculate drift or distances that the current travels in one-hour increments

The Rule of Thirds: “… over the period of the first hour the CURRENT WILL FLOW at one third of its maximum rate …, for the second hour it will flow at two-thirds and the third hour at … three-thirds.” Hence the ubiquitous 1:2:3:3:2:1 abbreviation of the rule which stands for 1/3 in the first hour, 2/3 in the second hour, 3/3 during hours three and four, and then down to 2/3 in the fifth hour and 1/3 during the last hour of the cycle.

The Rule of Twelfths is about the rise and fall of the water levels at various stages of the tide:

The Rule of Twelfths  is a method of determining the TIDE HEIGHT BETWEEN HIGH AND LOW WATERS.  In the six-hour period that separates the low and the high waters, the overall level will rise/fall 1/12^th in the first hour, 2/12^ths in the second hour, 3/12^ths in the third hour, 3/12^ths in the fourth hour, 2/12^ths in the fifth and 1/12^th in the last sixth hour. The Rule of Twelfths is also frequently presented as 1:2:3:3:2:1