Between Holborn and Dunnet Heads  
Tidal streams in the area set ENE/WSW and both streams begin earlier and are probably stronger off Dunnet Head rather than off Holborn Head.

Between Rough Head and Dunnet Head there is an almost continuous N and NE-going eddy which increases the size of the waves off Briga Head (1 mile SW of Dunnet Head). When a NW swell persists the sea up to 5 cables off Dunnet head becomes confused and strong offshore NW sets can be experienced.

Dunnet Bay itself can experience very heavy seas rolling into the bay during W and NW gales.

South-west coast of Hoy Off Rora Head  
The E-going tidal stream has a spring rate of 1 � to 2 kts in both directions.

The E-going stream W of Hoy divides off Rora Head and sets SE towards Tor Ness and quickly increases in strength as the ness is approached, and NE towards Kame of Hoy. The W-going stream W of Hoy is, similarly, the combination, W of Rora Head, of the SW-going coastal stream from the vicinity of Kame of Hoy and the NW-going coastal stream, emanating from Pentland Firth, which rapidly loses strength with the distance from Tor Ness.

PENTLAND FIRTH
Pentland Firth separates the Scottish mainland from Orkney and is entered from the W between Dunnet Head and Tor ness (6 � miles NNE) and from the E between Duncansby Head and Old Head (6 � miles NE).

The E approach to the firth is divided into 2 channels by Pentland Skerries, an extensive group of islets and rocks lying nearly midway between Duncansby Head (mainland) and Old Head (Orkney).

Stroma and Swona, 
Two islands of appreciable size that lie in the firth, respectively 3 miles NW and 5 � miles N of Duncansby Head.

Routes through Pentland Firth

Outer Sound  
The outer sound is the passage between the isles of Stroma and Swona. It is 2 � miles wide, deep, and is the recommended and usual route through the firth by day and night.  Tidal streams are strong attaining a spring rate of about 9kts.

Inner Sound  
The inner sound is about 1 � miles wide and is the passage between isle of Stroma and the coast of mainland Scotland.  Tidal streams attain a spring rate of only 5kts. Under normal conditions using the Inner Sound, with local knowledge, mainly by slow or smaller vessels can save time; it may be preferred in bad weather or in order to proceed against a strong contrary stream in the Outer Sound.  Large vessels are not recommended to use the Inner Sound at any time.

Tidal Information for Pentland Firth

General Information:  
Tidal streams are highly significant in or through Pentland Firth and should not be overlooked or under-estimated at any time. They meet a number of obstacles, which give rise to eddies and races, which, in several areas of the firth, can be very strong and extremely violent.

Tidal streams run with great strength and rates of up to 16kts have been reported close W of Pentland Skerries.  

Eddies:  
In some parts of the firth the transition between the main stream and an eddy; and vice versa, can occur so suddenly as to cause even a large vessel to sheer violently.  All the islands, headlands and other dangers in the firth should be given a wide berth, especially in poor conditions/visibility.

Races:  
Even in calm conditions there can be heavy turbulence in the races: in disturbed conditions, particularly when the tidal streams are opposed by strong winds or a swell, the sea in the races can be extraordinarily violent and confused, and extremely dangerous to small vessels which may become unmanageable.

Pentland Firth West
On the S side close off Dunnet Head the E and W-going tidal streams travel at up to 3kts which increases E towards St John's Point where they reach 5 � kts E-going and 7 Kts W-going.  Brough Bay lies to the SE of Dunnet Head and experiences an eddy which runs continuously for about 12 hours along the E side of Dunnet Head.

In mid-channel about 3 � miles N of Dunnet Head the E-going stream begins at (-0450 HW Dover).  The initial direction is SE but changes anti-clockwise, spring rate increasing rapidly from � kt; this stream has a  mean direction of 105� and is at full strength with a spring rate 3 � kts, between (-0350 HW Dover) and (+0010 HW Dover).  At the end of this period the stream begins to change direction anti-clockwise and rapidly loses strength.  The W-going stream begins at (+0210 HW Dover) with a mean direction 280�  and a spring rate of 3 � kts.  Further E, about 3 miles N of St John's Point, the E-going stream begins at (-0450 HW Dover) and the W-going stream at (+0120 HW Dover) with a spring rate of 7 kts in both directions.

Towards the N side of the firth, off Tor Ness, the E-going tidal stream begins at (-0530 HW Dover) and the W-going stream begins at (+0030 HW Dover) with rates of 6 to 7 kts. The E-going stream is diffused E of Tor Ness and runs towards the N and S of Swona.

Tide race: Merry men of Mey
The most extensive and dangerous race in the Pentland Firth is called the Merry men of Mey, which forms off St John's Point during the W-going tidal stream, and when fully established, extends the entire distance across the firth to Tor Ness. The most violent part of the Merry Men of Mey lies over a large sandwave field which lies approximately 3 � miles W of Stroma.  In a W Sea or swell the entire race becomes very violent; large waves will form very suddenly and from come from varying directions, making it difficult to counter or anticipate. The race forms a natural breakwater across the firth, and even when the race is most violent the firth can be crossed E of the breakers in smooth conditions. Small vessels are advised to remember that the W-going tidal stream emerging from the Outer Sound can be very strong, rates in excess of 10 kts can be expected and the danger of being swept into the race is very real.

The race forms off the Men of Mey rocks and initially extends  W towards Dunnet Head, but as the W-going stream increases in strength the race begins to extend from the bank 1� miles NW off St John's Point in a NNW direction.  When the W-going tidal stream has attained its full strength heavy breaking seas extend the whole way across the firth between St John's Point and Tor ness even in fine weather.

At (+0535 HW Dover) the SE end of the race becomes detached from the Men of Mey Rocks leaving a clear passage, which gradually widens N, between the rocks and the breaking seas.  In other areas' the race persists.

At (-0530 HW Dover) the NW end of the race off Tor Ness begins to subside.

The race subsides in mid-channel at the onset of the E-going tidal stream.

Outer Sound
Tidal streams are very strong through the Outer Sound, the rate of the E-going stream probably greater than the W-going.  Off the Tails of the Tarf in mid-channel, the E-going stream begins at a rate of about 9 kts, and the W-going stream at about 8 kts.

Eddies and races adjacent to Stroma 
The Swilkie persists almost continuously off Swilkie Point where the strong E and W-going main streams through the Outer Sound meet the N-going eddies, which flow on the E and W sides of Stroma.  The Swilkie can be dangerous and should be avoided even in fine weather. It is most violent when the W-going stream through the Outer Sound is opposed by a W gale.

During the E-going streams through the Outer and Inner Sounds an eddy forms off the E side of Stroma and gradually extends about 1 mile E of the island, this eddy causes a N-going stream along the E side of Stroma.

A corresponding eddy forms off the W side of Stroma during the W-going streams through the sounds and gradually extends about 2 � miles WNW from the island and causes a N-going stream along the W side of Stroma.

Eddies and races adjacent to Swona
The E-going stream passing between Swona and Orkney forms an eddy E of the island and extends about 1 � miles SE.

During the E-going and W-going tidal streams races will form, respectively SE and SW of the Tails of The Tarf on the edge of eddies E and W of Swona; where they may become violent when wind is opposed to the tidal stream.

In the Inner Sound the main tidal streams run in mid-channel, the E-going stream begins at (-0530 HW Dover) and the W-going stream at (+0030 HW Dover) at a rate of about 5 kts in both directions.  Eddies in general are weak and reach a maximum rate of 1 kt.

During the E-going tidal stream a race which can be particularly violent during E and SE gales, forms off Ness of Huna.

Channel between Duncansby and Pentland Skerries
The SE-going tidal stream begins at (-0500 HW Dover) and reaches a rate close to 9 kts.  Off the Pentland Skerries a rate of 12 kts is quite likely.  The NW-going stream begins (+0115 HW Dover) at a rate of 8 kts.

Duncansby Race
At (-0500 HW Dover) the race forms WNW off Duncansby Head coincident with the beginning of the SE-going tidal stream, and extends initially ENE but wheels gradually anti-clockwise.  Turbulence is experienced off Duncansby Head throughout the period of the SE-going stream.

At (-0220 HW Dover) the race extends NW from the Ness of Duncansby and gives rise to the Boars of Duncansby, 1 mile NW of the ness.  3 hours later the race begins to subside.

At (+0115 HW Dover)  the race ceases temporarily, coincident with commencement of the NW-going stream and re-forms at (+0505 HW Dover) off the Ness of Duncansby extending ENE.  At (-0535 HW Dover) the race ceases again temporarily.

Duncansby Race is extremely violent and dangerous when a SE-going stream is opposed by E and SE gales and when the NW-going stream is opposed to W gales.

Channel between South Ronaldsay and Muckle Skerry
In the passage between the S coast of South Ronaldsay and Muckle Skerry tidal streams begin and the Liddel Eddy occurs.

At (-0600 HW Dover) the E-going stream begins with a spring rate of about 8 kts. As the stream gains strength the Liddel Eddy forms, initially SE of Old head.

At (-0350 HW Dover) the Liddel eddy gives rise to an S-going stream along the SE side of South Ronaldsay and as it's strength increases and the strength of the E-going stream S of Old Head starts to decrease, it forces its way S and W past Old Head towards Muckle Skerry and along the S coast of South Ronaldsay.  A race starts to form off Old Head and persists until about (-0220 HW Dover).

At (-0320 HW Dover) the Liddel eddy is W-going and becomes firmly established along the S coast of South Ronaldsay gradually extending S from the coast. Farther offshore the E-going stream persists but with less strength.

About (-0220 HW Dover) the Liddel Eddy is W-going and occupies the N half of the passage, the E-going stream still persists in the S half.  Heavy overfalls and breakers will occur along the boundary between the eddy and the stream.

At about (-0020 HW Dover) the Liddel Eddy is W-going and extends the whole way across the passage except for what remains of the E-going stream close N of Muckle Skerry.

At (-0220 HW Dover) the true W-going stream begins throughout the passage and the race off Old Head subsides.

During the E-going tidal stream, the SE-going main tidal stream between Swona and South Ronaldsay joins the main tidal stream S of Lothar Rock.

Lothar Eddy and Race 
A violent race of great strength occurs during the W-going main stream SW of Lothar Rock where the S-going part of Lothar Eddy meets the NW-going tidal stream at the SE end of the passage between Swona and South Ronaldsay.

South-Eastern Approach
At (-0500 to about HW Dover) when the SE and E-going tidal streams are respectively SW and N of Muckle Skerry and are well established and eddy forms of the SE side of Muckle Skerry and eventually extends about 3 miles ESE from Pentland Skerries. 

At, (+0100 to -0600 HW Dover) during the NW and W-going tidal streams, respectively SW and N of Muckle Skerry, an eddy forms along the S side of the channel between Muckle Skerry. 

Tidal streams 4 miles ESE of the skerries are affected by the eddies described above;  they rotate clockwise and are relatively weak.