Editing Pentland Firth (section)

==Tidal races==
The Firth is well known for the strength of its tidal currents, which are among the fastest in the world, a speed of {{convert|30|km/h|knots}} being reported close west of Pentland Skerries. The force of the tides gives rise to overfalls and [[tidal race]]s 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.<ref name=":0">{{Cite web|url=https://www.iims.org.uk/poor-decision-making-led-cemfjords-capsizing-reveals-maib-investigation-report/|title=Poor decision making led to the Cemfjord's capsizing|date=2016-04-21|website=The International Institute of Marine Surveying (IIMS)|language=en-US|access-date=2019-05-27}}</ref>

Some of the principal tidal races are:<ref>van Rijn, Leo C. 2011. Principles of fluid flow and surface waves in rivers, estuaries, seas and oceans. Aqua Publications. {{ISBN|978-90-79755-02-8}}</ref>

[[Image:PentlandFirthMap.png|thumb|300px|Map of the Pentland Firth and associated lands]]
*‘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 {{convert|5.5|km|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 {{convert|19|km/h|kn|abbr=on}}.
*‘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]], according to a Nordic tradition recorded in the [[Eddas]], is caused by two [[gýgjar]] named [[Grottasongr|Fenja and Menja]] turning the millstone Grótti, which grinds out the ocean's salt.<ref name="Simek">{{cite book |last1=Simek |first1=Rudolf |title=Dictionary of northern mythology |date=1993 |publisher=D.S. Brewer |location=Cambridge [England] |isbn=9780859915137 |page=120}}</ref> The witches 'Grotti Finnie' and 'Grotti Minnie' feature in later [[Orcadians|Orcadian]] and [[Shetland|Shetlandic]] folklore.<ref name="Marwick">{{cite book |last1=Marwick |first1=Ernest W. |title=The folklore of Orkney and Shetland |date=1975 |publisher=B.T. Batsford |location=London |isbn=0713429992 |page=32}}</ref> The name of the race is derived from {{langx|non|Svelgr}}, 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 {{convert|1.5|km|mi}} NW some {{frac|2|1|2}}&nbsp;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.{{Cn|date=February 2025}}