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Tresco Channel crossing the Tresco Flats (Scilly)

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What is the route?
The Tresco Channel is a narrow stretch of shallow water that lies between the islands of Tresco and Bryher. Large sections of the route dry and it is best made during the rise 2 hrs after LW for vessels carrying up to 1.3 metres and at half flood for vessels of 1.8 metres.

The Tresco Channel between Bryher and Tresco
Image: Michael Day via CC BY-NC 2.0

The route is sequenced from St Mary’s to the deep water in the south end of New Grimsby Sound Click to view haven cutting across Tresco Flats but is available both ways. If there is any concern that there is not sufficient depth to cross its shallowest point of Tresco Flats a slight detour is also available to gain better water by circumventing the flats.

Why sail this route?
This interior cut saves hours of sailing for vessels planning to pass between two of the group’s principal berthing locations of St Mary’s Harbour and New Grimsby Sound not to mention Green Bay on Bryher that is a perennial favourite location for vessels that can take to the bottom. It also may provide an easier exit out of New Grimsby during fresh winds from northwest, round through north, to northeast when it can be difficult to depart. It can provide an enjoyable piece of pilotage in suitable conditions.

Tidal overview
Today's summary tidal overview for this route as of Tuesday, October 20th at 04:06. With large sections of this route drying height of tide is essential. Vessels carrying up to 1.3 metres can as a general rule proceed 2 hrs after LW, which the timer is set for, with vessels of 1.8 metres waiting for half flood. For more detailed work.

MHWS 5.7m MHWN 4.3m MLWN 2.0m MLWS 0.7m
Highest Astronomical Tide (HAT) 6.4m, Mean Sea Level (MSL) 2.91m
HW St Mary’s is HW Dover – 0630, Devonport -00.55, Duration 0600
Tides are considerably affected by wind conditions and barometric pressure.

In New Grimsby Sound the streams change direction four times in a half-day controlled by the uncovering of Tresco Flats:

  • • The southeast-going stream begins + 0530 Devonport (-0010 Dover)

  • • The northwest-going stream begins -0520 Devonport (+0125 Dover)

  • • Southeast-going stream begins -0230 Devonport (+0415 Dover)

  • • Northwest-going stream begins +0200 Devonport (-0340 Dover).

The spring rate in each direction is one knot.

The south-going stream across Tresco flats begins -0400 Devonport (+0245 Dover). The north-going stream begins +0200 Devonport (-0340 Dover).

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