As a newbie to BC I have spent some time looking at what I need to know and 'Wotayatties' comments about the curve are definitely worth considering. In the upper reaches of the Severn and the Avon the curve is distinctly bigger on the right than the left meaning the ebb is 'larger' than the flood which means the rule of 12ths is not accurate in those situations.
Intrigued to understand what is happening in Ramsay sound. Can someone explain?