Rule of Twelfths

In the Bristol Channel we experience a mean Spring Tide range of 12.2 m.  For this reason the echosounder  indicates readings which are much greater than the depth shown on the chart for our actual position. This spreadsheet makes use of the rule of twelfths to estimate the height of tide over a six hour period.

One eye on the charted depth and a knowledge of the height of tide is of use in confirming your position!

In the first hour there is an increase of height of 1/12 of the tidal range; 2/12, 3/12 in each of the succeeding hours and so on as shown above

I used the rule of twelfths to get a height of tide value in order to make a position check whilst on passage between Ilfracombe and Cardiff on August 8th 2006: 16:07 BST, echosounder reading 21.7 metres. I took three hand compass bearings and obtained the cocked hat as shown. At 16:12 the rule of twelfths predicts a height of tide of 6.9m; Actual depth - height of tide = 21.7 - 6.9 = 14.8m. Thus estimate of charted depth14.8m and the actual charted depth was 15.2m.

Falling Tide

Rising Tide

It is also useful to predict the change of depth of water whilst at anchor:

Here we are entering Cardiff Bay on 2nd June 2007 for the 18:15 lock; the gauge height 8.5m.

The gps/plotter has a useful tidal height function. However, as a rule I print and sail with a paper copy of the twelfths spreadsheet and use it to relate echo sounder readings to charted depths whilst on passage.