The strait between the Orkney Islands and mainland Scotland known for strong tidal streams is the Pentland Firth.
The Pentland Firth separates Orkney from Caithness in northern Scotland, with the Atlantic Ocean to the west and the North Sea to the east. Despite its name, it is not a firth in the usual sense of a broad estuary or fjord. Its Scottish Gaelic name, An Caol Arcach, means “the Orcadian Strait.”
The channel is famous for tidal currents among the fastest in the world. Speeds approaching 30 kilometres per hour, or 16 knots, have been reported near the Pentland Skerries. These currents create turbulent tidal races and overfalls, including the Swelkie, the Merry Men of Mey, and the Duncansby Race. Gale-force winds opposing the current can produce especially dangerous seas.
The Pentland Firth is also important for transport and renewable energy. Ferries connect the Scottish mainland with Orkney, while the predictable currents have attracted tidal-power projects. It should not be confused with the Minch, the broader sea passage west of Scotland and the Hebrides, or the North Channel between Scotland and Northern Ireland.