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gordmac said:
Can't be many places with a 16kn tide?
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You can get that in the Pentland Firth on a good day, mostly it hardly exceeds 13kn

Pentland Firth - Wikipedia
The Firth is well known for the strength of its tides, which are among the fastest in the world, a speed of 30 kilometres per hour (16 kn) being reported close west of Pentland Skerries. The force of the tides gives rise to overfalls and tidal races which can occur at different stages of the tide. Combined with gale force winds, they often give rise to extremely violent sea conditions, which have caused accidents such as the 2015 sinking of the cargo ship MV Cemfjord that resulted in the death of eight crew members.[8]

Some of the principal tidal races [9]are:
  • ‘The Merry Men of Mey’. Forms off St John’s point in the west-going stream and extends as the tide increases NNW across the firth to Tor Ness. The worst part is over a sand wave field about 5.5 kilometres (3.4 mi) west of Stroma. The waves formed by this race form a natural breakwater with relatively calm water to the east of it, particularly noticeable when a westerly swell is running. Tides in this area can exceed 19 km/h (10 kn).
  • ‘The Swelkie’. The race at the north end of Stroma, off Swelkie Point is known as "The Swelkie". It extends from the point in an easterly or westerly direction depending on the tide and can be particularly violent. The whirlpool of the same name was, according to a Viking legend, caused by a sea-witch turning the mill wheels which ground the salt to keep the seas salty. The name derives from an Old Norse term, Svalga meaning "the Swallower".
  • The ‘Duncansby Race’ forms off Ness of Duncansby at the start of the SE-going tidal stream (flood). Initially extending ENE but wheeling gradually anti-clockwise until it extends about 1.5 kilometres (0.93 mi) NW some 2 1⁄2 h later at which point it is known as ‘The Boars of Duncansby’. During the time of the SE stream there is additional turbulence off Duncansby Head, particularly to the East. The race temporarily ceases at the turn of the tide before forming in an ENE direction in the NW-going tidal stream (ebb) before ceasing again at the next turn of the tide. The race is particularly violent and dangerous when the tidal stream is opposed by gales in the opposite direction. During the east-going stream a race forms off Ness of Huna. This race can be particularly violent in an easterly or southeasterly gale.
  • The ‘Liddel Eddy’ forms between South Ronaldsay and Muckle Skerry in the East-going stream (flood). A race also forms for part of the time off Old Head at the SE part of South Ronaldsay.
In addition to ‘The Swelkie’, races form at both the north and south ends of Stroma and Swona. Between the races there is a calm eddy which extends down tide as the tide strengthens. The races are highly visible with overfalls and whirlpools. Large swell waves can also be present, especially in bad weather conditions. When entering or leaving the eddies, crossing the races, even large powerful vessels can be pushed off course, such is the demarcation between the relatively calm eddy and the fast-moving tide in the races. There are other races in the firth particularly off Brough Head.
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gordmac said:
Thought that would be where you were talking about. Best avoided unless you know what you are doing!
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Couldn't avoid it at all, I lived up there boating and diving.
My first real excursion into the full force of the stream was on an Offshore 105, 450hp and cruising at 22kn, Garry who owned the boat and diving company let me see just how fast the islands came towards you and seeing the wreck of an old tug boat on the Skerry really brought it home just how dangerous it was there.
We spent a few days going there twice a day at slack water looking for a wreck that sank just before WW2.
Waiting for a buoy to pop up that marked the last position that we scanned the seabed at, the DGPS got us fairly close then wait for the tide to slacken off for the 24" buoy to break the surface, get close and you can see it slowly ascend out the depths and the buoy was attached to 4 45gallon oil drums that were filled with concrete. We had about 30 minutes or so where we could put the camera down and scan the sea bed before the tide got too strong and we had to bring it back up again.
I've dived in the Firth a few times but always at slack water and not for very long as the tide picks up speed very quickly, even at Wick you can feel the tide, though by the time it gets there it's only running at about 3kn
 
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