Hourly tidal streams , approaches to Portland and the Portland Race (NP257)
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Tidal streams. – The streams run very strongly off The Bill of Portland and eddies of great extent form in both East bay and West bay ; off both sides of Portland peninsula the streams run nearly continuously southward and there is violent turbulence off the Bill where these streams meet the east- and west-going coastal streams.
The following general account of the streams of the locality, and also the more detailed descriptions of the streams in West bay, off the Bill, and in East bay, refer to conditions at springs. The eddies are much smaller at neaps, at which time, though the general features of the streams are probably about as described, the streams may run in directions differing appreciably from those given, and the durations of the south-going streams on both sides of Portland peninsula may be considerably shorter than stated. Information sufficient for the compilation of a description of the neap streams is not available and the streams at times other than springs should be predicted in the usual manner but used with caution.
About 5 miles southward of the Bill the east-going stream begins -0100 Devonport (+0545 Dover), and the west-going stream begins +0520 Devonport (-0020 Dover).
Soon after the east-going stream begins an eddy forms eastward of the Bill, and about 2 hours after the stream begins its centre is situated about 2 miles eastward of the Bill.
From this time till the end of the east-going stream the eddy increases in size and towards the end of the stream it fills the whole of East bay with its centre situated about 5½ miles eastward of the Bill ; from about 5 miles southward of the Bill the stream runs east-north-eastward southward of the centre of the eddy, changing gradually to north-eastward and northward on its eastern side, north-westward and westward on its northern side, and southward in the direction of the coast on its western side.
At its greatest the actual eddy extends 5 to 6 miles eastward from its centre, or about 11 miles from the Bill, but the stream for several miles farther eastward runs north-eastward towards the coast ; the whole space between the centre of the eddy and the land northward is filled by west-going stream, and between the centre and the land westward by stream running southward in the direction of the coast.
When the east-going stream ends the west-going stream northward of the eddy, and the south-going stream westward of it, continue as the west-going stream. The spring rate of the east-going stream is about 3-5 knots 5 miles southward of the Bill, and about 3 knots 10 miles eastward of the Bill, where its direction is east-north-eastward.
As the direction of the stream changes northward and westward its rate decreases, and is not more than about 2 knots in the west-going stream northward of the eddy, but increases again to about 3 knots in the south-going stream westward of the eddy.
During the whole of the east-going stream the stream in West bay runs, near the land, about south-eastward in the direction of the coast and southward off the western side of Portland peninsula, changing gradually to about east-south-eastward as the land is left ; its spring rate clear of the land in the bay is not more than about 2-5 knots, increasing to 4 knots or more off the western side of Portland peninsula.
Charts 2255, 3315, 2615
Soon after the west-going stream begins an eddy forms westward of the Bill, and, though smaller and less symmetrical than the East bay eddy, resembles that eddy in its general features. At its greatest, shortly before the end of the west-going stream, the centre of the eddy is about 3 miles, and its western edge about 8 miles, westward of the Bill, but the stream runs north-westward towards the coast for several miles farther westward. The streams are generally rather weaker than in the East bay eddy.
During the whole of the west-going stream the stream in East bay, near the land, runs in the direction of the coast and southward off the eastern side of Portland peninsula, changing gradually to about west as the land is left ; its spring rate clear of the land in the bay is about 2.5 to 3 knots, increasing to 5 knots or even more off the eastern side of Portland peninsula.
At a position 1-3 miles 160o from the Bill of Portland lighthouse the stream is weak, spring rate about one knot, and changes direction from about south to about east-south-east, between -0145 and -0100 Devonport (+0500 and +0545 Dover) ; at the latter time the east-going stream begins, increases rapidly, and attains its greatest rate, 6 knots at springs in a direction 105o, + 0040 Devonport (-0500 Dover) ; the direction then changes slowly southward as the rate decreases and is about 130o by +0340 Devonport (-0200 Dover), when the spring rate is 3 knots.
After this time the direction changes quickly and the stream is weakest, spring rate 2 knots in a southerly direction, +0440 Devonport (-0100 Dover). During the next hour the direction changes quickly westward as the rate increases, by +0540 Devonport (H.W. Dover), the spring rate is about 4-5 knots in a west-south-westerly direction, and the west-going stream attains its greatest rate, 7.2 knots at springs in a direction 245o, - 0545 Devonport (+0100 Dover).
After this time there is no appreciable change in direction, and the rate decreases very slowly, till -0400 Devonport (+0245 Dover) when the spring rate is 6.5 knots, but later as the stream slackens the direction changes southward. For streams at a position 4-7 miles 171o from the Bill of Portland lighthouse (Lat 50o 31’ N., Long, 2o 27’ W.) see below.
Through the greatest observed spring rate of the stream off the Bill is 7.2 knots, at a position 1-3 miles 160o from the lighthouse, yet stronger streams, possibly up to as much as 10 knots, may be found in the immediate vicinity of, but not necessarily in, the race (see below).
There is very considerable excess south-going stream at this position, and appreciable excess south-going stream at a position 4-7 miles 171o from the lighthouse.
Portland race is caused by the very strong south-going streams from both sides of the Bill meeting the east- and west-going streams off the bill, and its violence is increased by the sudden decrease in depth off the bill and inequalities of the bottom on Portland ledge.
The race is very variable in both position and extent but usually extends, from off the Bill, south-eastward during the east-going stream and south-westward during the west-going stream ; it extends farthest from the Bill, up to about 2 miles, with strong northerly winds.
There is usually a passage, from a half to three-quarters of a mile wide, of relatively smooth water between the Bill and the race.
Though the stream run very strongly in the immediate vicinity of the race, they are not specifically strong in the race itself, which, in fact, consists of an area of overfalls and steep heavy breaking seas in which he streams are subject to great and sudden changes in both direction and rate, it is most violent, and dangerous to boats and even to small ships.
Iin heavy weather at the times when the streams are strongest, especially with a gale blowing against the direction of the stream ; with an easterly gale and a strong east-going stream the whole area from the Shambles shoal to about 1½ miles southward of the Bill, and from off the Bill as far northward as off Grove point (Lat. 50o 33’ N., Long 2o 25’ W.), may be covered by overfalls and heavy breaking seas.
During the east-going stream the whole of East bay is filled by an eddy ; the west-going stream near the land runs in about the direction of the coast and southward off the eastern side of Portland peninsula, changing gradually to about west clear of the land.
At a position 9.7 miles 096o from the Bill of Portland lighthouse the east-going stream, spring rate 3 knots mean direction 062o, begins –0035 Devonport (+0610 Dover) ; the west-going stream, spring rate 3.5 knots mean direction 277o, begins +0520 Devonport (-0020 Dover).
At a position 9.8 miles 072o from the lighthouse, the east-going stream, spring rate 2.2 knots mean direction 075o, begins -0135 Devonport (+0510 Dover) ; the west-going stream, spring rate 2.3 knots mean direction 275o, begins +0440 Devonport (-0100 Dover).
At a position 6.2 miles 060o from the lighthouse the east-going stream, spring rate 1.4 knots mean direction 045o, begins – 0120 Devonport (+0525 Dover) ; the west-going stream, spring rate 2.1 knots mean direction 260o, begins +0240 Devonport (-0300 Dover).
At a position 4.7 miles 089o from the lighthouse the east-going stream, spring rate 2.5 knots mean direction 043o, begins – 0115 Devonport (+0530 Dover) ; the west-going stream, spring rate 2.4 knots mean direction 246o, begins +0420 Devonport (-0120 Dover).
On the eastern side of Portland peninsula, about half a mile offshore and 1½ miles north-eastward of the lighthouse, the south-going stream, spring rate 4.5 knots, begins +0030 Devonport (-0510 Dover) and runs for about 10¼ hours ; the north-going stream, spring rate 1.8 knots, begins -0140 Devonport (+0505 Dover), and runs for about 2¼ hours ; farther southward the south-going stream is stronger, and the north-going stream weaker.
All the above streams are affected by the East Bay eddy. At positions 9.7 miles 096o and 9.8 miles 072o from the lighthouse there is no excess north-going stream, at a position 6.2 miles 060o from the lighthouse excess west-going stream, and at a position 4.7 miles 089o from the lighthouse, which is on the northern side of the eddy, also excess west-going stream ; off the eastern side of Portland peninsula there is very much excess south-going stream.
Owing to the strength of the tidal streams and to the strong onshore sets, vessels passing the Bill of Portland, and not bound to Portland or Weymouth or intending to anchor in East or West bays are advised to keep southward of the parallel of lat. 50o 27’ N.
Chart 2255.