Hey Budgy,
Wals right. It's called the Cardinal Rule of twelfths. You divide the tide range by 12 which gives a measure to work with ie if the tidal movement between dead low and high tide is say 1.2m then dividing 120cm by 12 gives 10cm.
Given that the tide runs for 6 hours and has a half hour of slack at each end of the tide. When the water starts to move in the first hour it moves 1/12 (10cm), then in the second hour it moves 2/12 (20cm), in the third hour it moves 3/12 (30cm) in the fourth hour also 3/12 (30cm), fifth hour back to 2/12 (20cm) and final hour 1/12 (10cm) again until slack again.
This is used to give you an idea of the speed of the tide changes depending upon what hour into the flood or ebb it is. It is useful at places that are tide dependent like Dolls Point, Sydney, Caloundra Sunny Coast, and also Noosa river mouth. When the tide goes the same way as the wind you can calculate the times when the slowest water movement is. You can probably kite the first and last hour of the run in tide as well as the slack and also run out which gives you a maximum of about 9 hours of kiting from a possible 13, with the last remaining 4hours having too much water flowing with the wind to be fun.
Thats the best I can do at explaining it, hope it helps.
Good winds,
IKO Examiner
