I think it follows the 12ths rule fairly closely with the big rush in the middle hours. Others may correct me but when we were sat aground in Cuan, I did have some time to ponder and from my own research concluded that at low water springs there is absolutely no slack water. It was just like a switch had been thrown! Of course the confusing thing about Dorus Mhor, Corryvreckan, Firth of Lorne and Cuan is that they can be shot at times that are completely unrelated to HW Dover, Sydney, Liverpool 8, Ecclefechan or even Oban as my tide guide will demonstrate.
Again - a brighter spark than I will need to help you access my definitive tide guide - a spark as bright as Silkie or even the Webmeister himself.
Aha! That's what I was after. Thanks.
Ramsey and Jack Sound are odd, in that slacks occur when tide should be at it's strongest, and vice versa. But I've seen this 'switch' effect that you speak of on springs. It just switches. I've actually seen a 20" buoy with hardly any tide on it when the tide is going one way, be dragged under with the tide going the other. In 7 minutes. I timed it.
Add that to a strong wind against the tide when it switches, and you can get nice, becoming awful conditions in a very short space of time.
As a diver, it didn't bother me, as we could get out of there fast. As a sailor now, it terrifies me, as I know it could take far longer to get out.
When we sailed through Dorus mor, the instructor gave me the distinct impression that it could be dodgy. That day, it was a pussycat!