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Detailed information about tidal streams round the Scilly Isles –
The streams are rotary in a clockwise direction.
Very generally, northward, westward, and southward of the isles the streams are weakest, direction north-westward, spring rate 0.7 knot, neap rate 0.3 knot, -0500 Devonport (+0145 Dover); strongest, direction north-eastward to east-north-eastward, spring rate 1.4 knots, neap rate 0.7 knot, -0200 and -0100 Devonport (+0445 and +0545 Dover); weakest, direction south-eastward, spring rate 0.6 knot, neap rate 0.3 knot, +0100 Devonport (-0440 Dover), and strongest, direction south-westward, spring rate 1.5 knots, neap rate 0.7 knot, +0400 and +0500 Devonport (-0140 and -0040 Dover).
Eastward of the isles, and in the channel between the isles and the coast of Cornwall, the streams are weakest, direction east-north-eastward, spring rate 1.1 knots, neap rate 0.5 knot, -0100 Devonport (+0545 Dover), strongest, direction south-south-eastward, spring rate 1.8 knots, neap rate 0.9 knot, +0200 Devonport (-0340 Dover), weakest, direction west-north-westward, spring rate one knot, neap rate 0.5 knot, +0600 Devonport (+0020 Dover), and strongest, direction northward, spring rate 1.6 knots, neap rate 0.8 knot, -0400 Devonport (+0245 Dover). Thus, eastward of the isles the streams are strongest in about the directions, and at about the times, of the weakest streams elsewhere round the isles.
If the water is smooth, an eddy extends 6 or 7 cables from Wolf rock, in the direction of the stream, at about the time of local low water, or +0540 Devonport (H.W. Dover), at which time the stream is running westward; in bad weather the eddy is less noticeable. As the tide rises, and the effect of the rock decreases, the eddy also decreases and probably disappears near local high water, or -0050 Devonport (+0555 Dover).
The rotary offshore streams run towards the isles from a different direction at each hour of the day, and, in passing round and between them, are much affected by the trend of the land and channels, and by shallow water. These effects vary with the direction of the approaching stream, so that the streams near and between the isles are subject to great irregularities and apparent inconsistencies, and may differ greatly at positions not far apart.
Though the streams round the isles are not of any great strength, their rates increase off salient points and over and near rocks and shoals, in which localities overfalls or races may occur.
Off the south-eastern side of the isles the north-east-going stream, spring rate 1.8 knots, begins off Pednathise head -0430 Devonport (+0215 Dover); the south-west-going stream, spring rate 2.5 knots, begins +0130 Devonport (-0410 Dover). A race extends from Gilstone ledges, westward of Pednathise head. There are overfalls southward of St. Agnes between about -0330 and -0030 Devonport (+0315 and -0610, Dover); these sometimes extend as much as 3 miles from the coast.
There is a race off Menawethan, and another off Hanjague; the latter is said to be south-going -0300 Devonport (+0354 Dover), at which time the offshore stream is north-going.
Off the northern side of the isles, about 2 cables northward of Round island, the east-going stream begins -0530 Devonport (+0115 Dover); the west-going stream begins +0030 Devonport (-0510 Dover); spring rate in both directions 2.5 knots. There is a race over Kettle Bottom spit, between the entrances to New and Old Grimsby harbours.
There are overfalls on the offshore bank north-westward of the Islands westward of Bryher; also over and near the rocks between North channel and Broad channel and over the rocks round Bishop rock (Lat. 49° 52’ N., Long. 6° 27’ W.).
In about mid-channel between the rocks westward of Annet and great Crebawethan, the stream is rotatory in a clockwise direction; it begins to run eastward, changing to southward and south-westward, -0200 Devonport (+0445 Dover), and begins to run westward, changing to northward and north-eastward, +0300 Devonport (-0240 Dover); spring rate south-south-eastward and north-north-westward 1.5 knots. In Gorregan neck and Muncoy neck the spring rates are about 2.5 knots.
Between Bryher and Samson the east-going stream begins +0500 Devonport (-0040 Dover) and runs for 8½ hours; the west-going stream begins +0100 Devonport (-0440 Dover) and runs for 4 hours.
Adapted from Admiralty Sailing Directions English Channel 1947