The rule of twelfths is based on the time between low and high tide being approximately 6 hours and that the tide doesn’t rise and fall at a constant rate. It increases in power to the 3rd and 4th hour before decreasing again. It works best in areas that have regular tidal curves such as Dartmouth and Plymouth. It is not reliable in areas with irregular tidal curves such as Poole and Southampton.
The rule of twelfths is a good approximation for small boats, surfing, or mooring, but it’s not precise enough for precise navigation or large vessels.
It works by first working out the tidal range for the tidal period you are interested in. This is done by taking the low tide value from the high tide value. For example: HW 5.1m and LW 0.9m the tidal rane would be 4.2m. Now you have the range you can divide it by 12. (4.2/12 = 0.35) Each 1/12 is therefore 0.35m
1st hour – Add 1/12 – 0.35 So 1 hour after HW = 5.1-0.35 = 4.75m
2nd hour – Add 2/12 – 0.7 So 2 hours after HW = 4.75-0.7 = 4.05m
3rd hour – Add 3/12 – 1.05 So 3 hours after HW = 4.05-1.05 = 3m
4th hour – Add 3/12 – 1.05 So 4 hours after HW = 3-1.05 = 1.95m
5th hour – Add 2/12 – 0.7 So 5 hours after HW = 2.95-0.7 = 1.25m
6th hour – Add 1/12 – 0.35 6 hours after high water = LW = 0.9
If you are working from low to high water then you would use the above but replace the subtraction of twelfths with addition.
REMEMBER this is a rough way to quickly calculate tide. It works well for the majority of harbours but not all. For most accurate calculations use local tidal graphs.
