Hourly tidal streams around the Pentland Firth

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PENTLAND FIRTH GENERAL REMARKS

Pentland firth, the strait separating the Orkney islands from the mainland of Scotland, is bounded westward by a line joining Dunnet head to Tor ness (Lat. 58° 47’ N., Long. 3° 17’ W.), the south-western extremity of Hoy, and eastward by the Pentland skerries; the main passage between Stroma and Swona is 2¾ miles wide, with depths of from 30 to 40 fathoms (54m to 73m).  Its navigation is attended with special difficulties, arising from the rapidity of the tidal streams, and the high sea at times resulting therefrom.

The following remarks apply chiefly to sailing vessels, or steam vessels of low power:-
With smooth water and a commanding breeze, the firth is divested of its dangers, but when a swell is opposed to the tidal stream, a sea is raised which can scarcely be imagined by those who have never experienced it; and, if, at the same time, the wind is light and with the stream, a sailing vessel becomes unmanageable.

It is affirmed by some of the most experienced pilots, that in a calm, if a vessel be allowed to drift with the stream, she will clear all the dangers of the firth, but this is not to be depended upon, especially when there is a swell.  Most of the casualties have occurred from trying to tow or steer in light winds across the stream, or obliquely to the direction in which the stream is setting; and it is here where local knowledge is so valuable in placing the vessel’s head in the right direction, before approaching the islands and dangers too closely.

Before entering the Pentland firth, all vessels should be prepared to batten down, and the hatches of small vessels ought to be secured even in the finest weather, as the transition from smooth water to a broken sea is so sudden.

The change from light winds to a strong breeze which occurs when passing from an eddy into the stream, and vice versa, is rapid, so that too much sail must not be carried.  So distinct is the line of demarcation between the stream and the eddy that, in passing from one into the other, the largest vessel may be twisted round with considerable velocity.

The bottom of the northern side of the firth is generally composed of sharp uneven rocks, among which the lead gets entangled, rendering it nearly impossible to obtain soundings by that method when the stream is strong.

It has been noted on several occasions whilst navigating in strong tidal streams in and near Pentland firth that fog signals were less audible when the ship was proceeding towards a fog signal with the stream than against the stream.

Approaching Muckle skerry from the eastward the likelihood of hearing the fog signal has been found to be considerably less on the west-going than on the east-going stream.
 
 
 
TIDAL STREAMS PENTLAND FIRTH

Tidal streams – Although the actual times of high and low water in different parts of the Pentland firth vary considerably, ranging over a period of upwards of 2 hours, the turning of the tidal stream is practically simultaneous throughout, at about half an hour after, and 5½ hours before high water at Dover, that is, the stream in the main body of the firth will always be found running from the Atlantic towards the North sea from 5½ hours before to half an hour after high water at Dover; and from the North sea to the Atlantic from half an hour after to 5½ hours before high water at Dover.

The change of stream in the main body of the firth coincides practically with these times, for, although the actual moment of change may be a little earlier or later, or may be influenced in a small degree, especially at neaps, by the wind, yet slack water will be found at half an hour after and 5½ hours before high water at Dover in nearly all parts of the firth, except close inshore; there the stream is accelerated or retarded. 

Thus, off Dunnet Head the stream changes at one hour before and 5 hours after high water at Dover; off Duncansby head the south-east-going stream continues running for nearly 1½ hours after high water at Dover; between Pentland skerries and South Ronaldsay, although the east-going stream begins at 5½ hours before high water at Dover, the west-going stream begins at high water at that place, and continues running off Brims ness until 4¾ hours before the following high water at Dover though, off Tor ness, it changes at half an hour after and 5½ hours before high water at Dover. 

In the Outer sound, the change of stream is one hour after and 5 hours before high water at Dover, but midway between Dunnet head and Tor ness, the west-going stream continues running until 4 or 4½ hours before high water at Dover.
 
 
 
 
PENTLAND FIRTH SAILING DIRECTIONS

Sailing vessels.– The following directions apply chiefly to sailing vessels, but steam vessels of low power navigating against the stream may find them useful.  A pilot should be employed by those unacquainted with the navigation of the firth.

Approaching from westward with the east-going stream and a fair wind into Pentland firth no special directions are necessary until well up with St. John’s point, when the Inner sound should be used if at the early part of the stream, and the Outer sound if at the latter part, taking care in this latter case to avoid the eddy on the eastern side of Stroma, and to keep midway between Duncansby head and the Pentland skerries, so as to retain the last of the stream.

(The Outer sound is always to be preferred for a large vessel on account of its width and the rate of the favourable stream, but not with an adverse stream).

With a fair wind during the west-going stream, if the sea is not too heavy, approach Dunnet head to the distance of half a mile, and steer straight for the centre of Stroma, under which there will be less sea and stream; but if there is not sufficient wind to stem the stream, keep near the eastern side of Dunnet head, haul into Brough bay, and keep within half a mile of the coast thence to St. John’s point.  When thus far, if the stream continues strong, stand right across the race named the Merry Men of Mey (page 58) into Stroma west-going stream eddy; then, from close under Mell head, steer across into Gills bay, and along the coast within 3½ cables of it until Duncansby head has been rounded.

To work from westward with the east-going stream, vessels at anchor in Scrabster road should get under way about 3 hours after high water at Dover, and turn towards Clardon head, tacking inshore when Brims ness, to the westward, on the southern shore, appears open at Spear head.  At 5 hours after high water at Dover, the east-going stream begins at half a mile outside Dunnet head, and from about half an hour before to half an hour after high water at Dover, will be slack entirely across the firth, where long or short boards must be made according to circumstances; taking care, if the wind be light, not to stand more than half-way across to the Orkney coast, especially near spring tides, to avoid being set northward of Swona.  It is on account of this risk that the southern side of the firth is generally preferred.

When eastward of Dunnet head, and standing towards Brough bay, Duncansby head must be kept bearing more than 101°, and open northward of St. John’s point, for, southward of this line, an eddy sets towards Hoy, about 5¾ hours before high water at Dover.

When within 1½ miles of Stroma, and intending to pass through the Inner sound, keep Duncansby head bearing less than 110°, and open southward of Mell head, to avoid getting out of the fair stream.  There is an advantage in using this passage at the latter part of the west-going stream, on account of the general slack water on either side already alluded to.  Standing towards the Merry Men of Mey, with the west-going steam, do not lose sight of the chimneys of Barrogill castle westward of St. John’s point, and, on the outward tack, take advantage of Stroma west-going stream eddy, if necessary.  As at this time of stream, Stroma skerries will be partly covered, they will be avoided by bringing Old head in South Ronaldsay to bear 057° and open to the south-eastward of Scarton point, before the north-western extreme of Stroma bears 025°, and is closed in by Mell head.  Having passed Stroma skerries, work right across from side to side, for though a narrow stream in mid-channel may still be going westward the slack water on either side will make up for it.

Should the west-going stream make while a vessel is working through the Inner sound, anchorage may be taken up either eastward of Stroma skerries, in Gills bay, or opposite John o’Groat’s hotel, or she may stand off and on in Stroma west-going stream eddy until the west-going stream ceases.

At night, approaching from westward in thick weather, it is advisable to heave-to abreast Strathy point; or, if Holborn head (Lat. 58° 37’ N., Long. 3° 32’ W.) has been made out before dark, to anchor in Scrabster road, but on no account incur the danger of shooting the firth in thick weather.  With a fair wind and a favourable stream, the directions for steam vessels at night will apply.

Approaching from south-eastward with a fair wind during the east-going stream, sailing vessels should keep close to the southern side northward of Freswick bay, for along this coast to Duncansby head there are 10 hours of slack water.  By doing this a vessel will be ready to round Duncansby head at the turn of the stream, whereas, by keeping mid-channel over towards the Pentland skerries, where the south-east-going stream continues for about one hour after high water at Dover, the risk is run of being carried south-eastward.  In rounding Duncansby head, be careful of Baxter rock (page 71), and also of Hell rock (page 59).

The advantage gained by rounding Duncansby head before the west-going stream begins will be trifling, if unable to evade the strength of the Bore of Duncansby (page 60), which continues until one hour before high water at Dover.  This, however, may be done by keeping so close to Duncansby ness as to have Dunnet head a little open of, or touching St. John’s point; in this track the least depth is 6 fathoms (11m0), and the only danger is that with a vessel steering badly she may sheer in towards the rocks lying off the ness.  Afterwards keep within 3½ or 5 cables of the southern side until up to St. John’s point, but, without a commanding breeze, do not attempt to pass it until slack water, as the last of the east-going stream sets directly from the Merry Men of Mey onto Stroma skerries, or onto the south-western coast of Stroma.

With the west-going stream, and strong winds from between west and north-west, a heavy breaking sea will be found westward of Stroma and Swona, which few, having once experienced, would be rash enough to encounter a second time.

Having stood into the Inner sound to ascertain the state of the sea on the Merry Men of Mey, and finding it as above, the judicious seaman will bear up, and steering north-eastward athwart the stream, will pass eastward of Swona and retain smooth water, whence he may work up to Long Hope, see North Sea Pilot, Part I, or, if the east-going stream should have made, he may bear up for Widewall harbour.

Approaching from eastward during the east-going stream in an easterly or south-easterly gale, it is absolutely necessary to keep an offing of from 6 to 8 miles abreast the Pentland skerries until this stream has ceased.

A seaman, well acquainted with the coast, might evade the heaviest sea by keeping close to Duncansby head and inside the Bore, but it is a critical undertaking, and the safest plan is to maintain an offing.

Approaching from north-eastward during the west-going stream, if the wind be southerly, sailing vessels should steer southward until Dunnet head (Lat. 58° 40’ N., Long. 3° 22’ W.) is well shut in with Swilkie point, Stroma, the latter bearing about 264°, and the west-going stream will then set the vessel well southward of Lother rock, and between Stroma and Swona; but should Dunnet head be in line with Swilkie point, the direction of the stream would be towards Swona, upon which she would run a risk of striking.

If the firth be entered within half a mile of Old head the vessel will pass very close to Lother rock beacon, then be set towards the centre of Swona, and afterwards round North head of that island.

During the east-going stream, a vessel may, if the wind be light, remain in Liddel eddy, westward of Old Head, until the stream ceases at about high water at Dover; a course can then be shaped between Stroma and Swona.  If a vessel leaves the eddy too soon, she will be hurried off south-eastward and will incur a risk of being set upon one of the Pentland skerries.
 
 
 
 
TIDAL STREAMS – Stroma

Tidal streams – During the east-going stream an eddy extends from the eastern side of the island, and when the stream is at its greatest strength, is about 1½ miles broad.

The east-going stream from one mile westward of the island sets right down upon that part of the island where the cliffs are highest, and a sailing vessel becalmed thereabouts would be in a critical position.  On approaching Stroma closely, the stream divides and passes very near the rocks at either end of the island.  Having passed Stroma skerries, keep Dunnet head open southward of Mell head, and the eddy will be avoided.

The west-going stream causes an eddy which at springs is felt at a distance of 2½ miles from the western side of the island, and is an excellent space for sailing vessels to heave-to in while waiting for the turn of the stream.  The south-western boundary strikes off from Stroma skerries, and bending round northward, almost touches the race named the Merry Men of Mey.  The northern boundary lies with Pentland skerries just open northward of the island, but it gradually works farther northward as the west-going stream grows older.

The northern limit of both eddies, for a quarter of a mile from the island, have whirlpools and overfalls, which, with an opposing wind, cause a dangerous breaking sea.  They are collectively named the Swilkie, and must be avoided by boats even in the calmest weather.  With east-north-easterly winds the Swilkie is always dangerous, but the heaviest breaking sea is produced by the west-going stream against a north-westerly wind.

In the Inner sound the stream in mid-channel is very narrow, and has less velocity, about 5 knots, than that through the Outer sound.  In the centre, the east-going stream begins 5½ hours before, and the west-going stream half an hour after high water at Dover.  Both shores of Inner sound have slack water at about 2½ hours before, and 3½ hours after high water at Dover.

When proceeding westward, during the strength of the east-going stream, vessels after passing Huna ness should keep somewhat northward, to avoid the eddy setting towards Quoys ness and into Gills bay.
 
 
 
SWONA CAUTIONS TIDAL STREAMS


Swona – This island lies on the northern side of the fairway of the Pentland firth with North head, its northern extremity, 3¼ miles south-eastward of Cantick head light-tower; Tarf Tail, the south-western extremity lies 2¾ miles north-eastward of Swilkie point, Stroma, and between these two points is the Outer sound, the narrowest part of the main channel of the Pentland firth.  Warbister hill, the summit of the island, a rounded green summit 134 feet (40m8) high, is situated on the eastern side of the island, which is composed of cliffs.  The western side is low and is bordered by ledges of rocks extending half a cable offshore.  North Clett lies close westward of North head.  The West Bow, a rock awash, lies 1½ cables offshore, in a bay named the Brook, 5 cables south-westward of North head; and Selki skerry lies 1½ cables northward of Tarf Tail.  South Clett, 65 feet (19m8) high, and the East and West Windi skerries lie about a cable offshore off the southern side of the island.  The best landing place is in a bight, called the Haven, on the north-eastern side, but it is frequently impracticable to approach the island from any direction.  The only spring of fresh water is among the rocks at the south-western end, and is often covered by the sea.  In 1931 there were 6 inhabitants.

Light.– A light is exhibited, at an elevation of 57 feet (17m4), from a white tower, 22 feet (6m7) in height, situated near the south-western extremity of Swona.

Off-lying bank.-Triton bank has general depths of from 20 to 30 fathoms (36m6 to 54m9); the least depth, 20 fathoms (36m6), rock, lies in the centre of the bank 1¼ miles north-westward of North head (Lat. 58° 45’ N., Long. 3° 03’ W.).

Caution.-Numerous wrecks have occurred on Swona from sailing vessels driving down upon it with the tidal stream; vessels should in such cases, drop an anchor, for, as the bottom is composed of shells, it is seldom lost.
Tidal streams.-In the Outer sound, the south-east-going stream begins 5 hours before, and the north-west-going stream one hour after high water at Dover.

The eddy during the south-east-going stream extends from the south-eastern side of the island to within a mile of Lother rock (Lat. 58° 44’ N., Long. 2° 59’ W.), and the northern edge of it in east-south-easterly gales causes whirlpools and overfalls, almost as dangerous as that of Stroma.  On the eastern side of the island, there are great depths and a rocky bottom, and few vessels anchor there.  If drifting through the firth with the south-east-going stream, and, when within 2 miles of Swona, Pentland skerries are open southward of that island, the vessel will be set southward of Swona, at a rate of from 6 to 8 knots in springs, but should she pass within 3½ cables of the island, she will probably be set into the eddy on the eastern side, out of which it is difficult for a sailing vessel to get, in light winds.

During the north-west-going stream the eddy on the north-western side of the island has its southern limit from Tarf Tail in the direction of Tor ness, and is caused by the north-west-going stream, divided by the convexity of the eastern side, rushing past each end of the island.  See the approximate boundary on the chart.  The rate of the streams past North head and Tarf Tail is greater than those past any other point in the firth, from 7 to 8 knots.  The eddy is extensive, and the cross sets in it are very variable, so that even those most experienced cannot predict the direction they will take at any given time.

If drifting towards the island with the north-west-going stream during a calm, it requires an experienced pilot to determine which end of the island a sailing vessel will be set round, nor is it easy to give a stranger a guiding mark; for the appearances, nearly to the last, are as if she were drifting upon the centre of the island.  It may, however, be generally stated that if Cantick head is open of North head she will drift northward, and if Brims ness is open of Tarf Tail she will be set southward of Swona.
 
 
 
LOTHER ROCK TIDAL STREAMS

Tidal streams.– The worst period of the tidal stream near Lother rock is about 2½ hours before high water at Dover, for then the stream sets directly upon the reef, at a rate of 9 knots, leaving an eddy on the eastern side, marked by a margin of broken water.  A narrow belt of the stream also rushes through inside the reef, at the first turn of the stream.

During the west-going stream, an eddy is formed on the northern side of Lother rock, and the stream coming round Brough ness sets right onto the rock.



TIDAL STREAMS – BRIMS NESS

Tidal streams – Off Brims ness the tidal streams attain a considerable rate, causing heavy overfalls, which are dangerous to small vessels.  The east-going stream sets fairly past the point; it begins about 5½ hours before high water at Dover, and runs rapidly along the southern coast of South Walls towards Herston head (Lat. 58° 48’ N., Long. 3° 01’W.); at about half-tide this stream slackens along the coast of South Walls, and an eddy is formed which extends about half a mile offshore but off Brims ness the stream continues to run east-south-eastward towards Swona, until a little after high water at Dover.

The west-going stream begins about half an hour after high water at Dover, and, joining the eddy along the southern coast of South Walls, run past Brims ness at a greater rate than the east-going stream, causing heavy overfalls.
 
 
 
 
TIDAL STREAMS – From the Men of Mey

Tidal streams – From the Men of Mey rocks, extends the race known as the Merry Men of Mey.  It begins with the west-going stream, about half an hour after high water at Dover, and for the first hour trends in the direction of Dunnet head; it then gradually works outward until about 3½ hours after high water at Dover, by which time it runs in the direction of the south-eastern point of Hoy, and has nearly joined the south-western edge of Stroma eddy, and so it continues until the west-going stream slacks, about 5¾ hours before high water at Dover.  During the last 2 hours of the west-going stream the race gradually becomes detached from the Men of Mey rocks, leaving a passage between the latter and the breaking sea.

About mid-channel this race is strengthened by the main body of the west-going stream which passes out through Outer sound; while, nearer the Orkney coast, it is further augmented by the stream that sweeps westward past Cantick head.  The whole together cause a heavy turmoil off Tor ness when there is a westerly swell, and also in fine weather during the strength of the stream at spring tides.  The times of the turn of the stream are affected by prevailing winds.

The west-going stream sets at a considerable rate along the rocks, which are steep-to, but it does not set over them, except at the turn of the stream; nor does the east-going stream, except very close inshore from out of Mey bay; during the latter stream there is no race off St John’s point.

 
 
Adapted from admiralty Sailing Directions, 1935

Article Info

Author:
VisitMyHarbour
Category:
Tidal Streams
Added:
02/11/2012
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