The Rule of Twelfths is a rule of thumb for estimating the height of the tide at any time, given only the time and height of high and low water. This is important when navigating a boat or a ship in shallow water and when launching and retrieving boats on slipways on a tidal shore.

The rule assumes that the rate of flow of a tide increases smoothly to a maximum halfway between high and low tide before smoothly decreasing to zero again and that the interval between low and high tides is approximately six hours. The rule states that in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence – 1:2:3:3:2:1

Find more here

 

Calculator

Below you can find calculator made by our club member . To use it simply enter starting hour of tide and ending time of tide , it will divide this period in to 6 segments . If sea levels will be specified it will also calculate see state in given segment.

example:

Low Water is at 12:00 and High Water Is at 18:00  state of see is for low  0.5m and 4.1 for high tide.

We should enter :

1. start time : 12:00
2. end time : 18:00
3. start level :0.5
4. end level: 4.1
5. click calculate

result columns  will be :

1. start – start of time segment
2. end – end of time segment
3. rule of 12 – this is how many parts of 12 will be changing in this period
4. change m/h – this show you what will be the change in state during this hour
5. start level – this shows sea level at the beginning of this segment (above chart datum)
6. end level - this shows sea level at the end of this segment (above chart datum)

For any problems with this tool please contact mk@firegnom.com