I have a question that has been puzling me. I am a scuba diver and dive along the Dorset coast. When planing for a dive it is essential to have slack water or know the rate.When I look at the Admiratly Charts for the Potrland area the rates refer to H/W Plymouth.When I cross check this with the Tidal streams they refer to H/W Portland.There is a considerable varience in the H/W times. For example 17/4/01 there is 23 min difference 25/4/01 there is 64 minuets.Is there a difference table I know there both std ports ? I have asked this question with divers and had no responce .Many thanks for any help Paul.