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Old Grimsby Sound from St Mary’s Road (Scilly)

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What is the route?
This is a passage across the broad stretch of shallow water that lies to the east of Tresco to allow vessels in Old Grimsby Sound to connect to St Mary’s Road. Large sections of the route are shallow and parts dry to 0.4 metres LAT so a sufficient rise of tide is required and it is ideally addressed on the flood. The route is sequenced from to St Mary’s Road to Old Grimsby Sound but it may is available both ways.

The east side of Tresco
Image: Tom Corser 2009 via CC BY-SA 2.0

Why sail this route?
Using this internal cut saves hours of sailing for those planning to pass around the north side of the island group in order to address Old Grimsby Click to view haven.

Old Grimsby Quay
Image: Michael Harpur

This route can be used in conjunction with Crow Sound to Tean Sound Route location to connect St Mary's Road with Tean Sound and all of the principal anchorages in the northeast and east end of the group as follows:

  • • Breaking off eastward for ½ a mile, between the Pentle and Lizard Point waypoints to the 'Central' waypoint of Crow Sound to Tean Sound Route location provides access to the routes destination of Tean Sound Click to view haven.

  • • The 'Central' waypoint is on the Men-a-vaur alignment which safely leads northwestward to St Helen's Pool Click to view haven (St Helen's Island).

  • • Likewise, to the southeast, the same alignment has the initial fix for Higher Town Bay Click to view haven (St Martin's).

  • • And finally close south of the 'Hats' commencement waypoint is the Watermill Cove Click to view haven (St Mary's) anchorage.

In a group where no anchorage provides complete protection, all of the time, and vessels have to prepared to shift in order to make the best of the weather, this ability to quickly cut between all of these anchorages is more than useful. Likewise, with a least depth of about 0.5 metres, this should not prove over limiting and particularly so during Neaps when, with a tidal range of 2.3 – 5 metres, it is a reasonably available option for most vessels most of the time. Using this internal cut saves hours of sailing for those planning to pass around the north side of St Martin's. In suitable conditions, it can also provide an enjoyable piece of pilotage.

Tidal overview
Today's summary tidal overview for this route as of Sunday, November 29th at 01:41. With large a small section of this route drying to 0.4 metres, a rise of tide will be required for vessels carrying any draft. Vessels carrying up to 1.3 metres can as a general rule proceed 1 hour after LW and freely pass during Neaps. Vessels of 1.8 metres should wait for 2 hrs after LW, which the timer set for 4 hours before and, conservatively, 2 after. 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
Please note

The current tidal event is springs so expect streams to be at their strongest.

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