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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 Thursday, July 18th at 09:18. 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

Shallow (HW +2 to -4 Hours)

(HW ST. MARY'S +0200 to -0400)


(Tidal flow )

Ends in 01:53:07

(Thu 04:47 to 11:12)

Tidal Window (HW -4 to +2 Hours)

(HW ST. MARY'S -0400 to +0200)

Starts in 02:00:07

(Thu 11:19 to 17:19)

What are the navigational notes?
Please use our integrated Navionics chart to appraise the route. Navionics charts feature in premier plotters from B&G, Raymarine, Magellan and are also available on tablets. Clicking the 'Expand to Fullscreen' icon opens a larger viewing area in a new tab.

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The above plots are not precise and are indicative only.


The cut through the flats requires moderately good visibility and an adequate rise of tide to pass over its shallowest point of 0.4 LAT located west of Little Pentle Rock. As it is shallow with parts that dry at LAT, with close-lying rocks and ledges, good charts/plotters are key.

Yacht approaching Old Grimsby Harbour via this route
Image: Michael Harpur

It is essential to understand that this route relies primarily upon eyeball navigation with clear visibility and moderate conditions. The route waypoints are a great addition but are not to be relied upon in isolation. They are only intended to assist in identifying the key turning points in the passage and the ledges, rocks and shallows are all best negotiated by keeping a watchful eye. Do not let this be in any way discouraging as the water here is crystal clear over white sand and the ledges have a large amount of weed that cling to then. So everything is highly visible.

Be prepared to take this route under power, in a steady fashion identifying the rocks and marks through the passage along the way. Conservatively planning for a maximum drying height of 0.5 meters LAT or a little more will provide adequate water for most leisure craft at half-tide with the prudent making the approach during the flood. However, this is not a passage a newcomer should attempt at night nor on the ebb.


With sufficient tide run into the north end of St Mary’s Road to the St Mary’s Road North waypoint located on about 400 metres westward of Creeb. This is on the alignment of Nut Rock on a bearing of 260° T in line with the summit of the South Hill on Sampson that clears The Pots. From here steer for the Crow Rock North waypoint.

Crow Rock isolated danger mark
Image: Michael Harpur

This is located about 400 metres north by northwest of the Crow Rock isolated danger mark Fl(2)10s and on the 160°T transit of the danger mark and the TV tower on St Mary's. This is the traditional alignment used for this part of the voyage, but in this track, slightly stepping out eastward of the transit to the Cones Waypoint as it passes unnecessarily close to the small Cones ledge.

Skirt Island and its ledges on the southwestern extremity of Tresco
Image: Photo by Annie Spratt on Unsplash

Cones is a small outlier of the Diamond Ledge, drying to 5.4 metres, that extends a ¼ of a mile eastward of Skirt Island, the southwestern extremity of Tresco.

Pentle Bay with Great Pentle Rock visible
Image: © Tom Corser 2009 via CC BY-SA 2.0

After Cones, the transit is re-joined to the east of Little Pentle Rock. Here the track passes through the 300-metre wide gap between Little Pentle Rock and the West and East Craggyellis reefs.

Lizard Point as seen from the centre of Pentle Bay
Image: Visit Isles of Scilly

After this, it continues on-transit until 200 metres east of Lizard Point. Thereafter it passes between the shore and Tea Ledge, drying to 3.7 metres and situated 200 metres north by northeast of Lizard Point.

Rushy Point and Tea Ledge as seen from the old battery above Old Grimsby
I6mage: Michael Harpur

It continues 200 metres off of Rushy Point, which may be readily identified by its yellow-diamond-shaped post (marking an underwater cable), and Great Cheese, drying to 3.1 metres, a ¼ of a mile eastward, to a position 80 metres west of Lump of Clay Ledge that lies close east of Little Cheese.

Principal rocks and ledges at the south end of Old Grimsby Harbour
Image: Michael Harpur

It then turns west by northwest to pass north of Cook's Rock, that dries to 4.3 metres, off Block House Point, into the southwest end of Old Grimsby Harbour about 60 metres south of Tide Rock. Tide Rock, that dries to 1.4 metres LAT, is the primary covered hazard at the southern end of the harbour. It lies about 100 metres inshore of the 3.8-metre rock on the western extremity of Long Ledge on a line between Block House Point and Peashopper.

The dark shadow of the covered Tide Rock just visible
Image: Michael Harpur

From here simply steer for the mooring buoys off Old Grimsby Harbour and pick up a Tresco Estate buoy or anchor off.

Old Grimsby Harbour
Image: Michael Harpur


The complete course is 1.97 miles from the waypoint 'Saint Mary's Road North' to 'Tide Rock' tending in a north north westerly direction (reciprocal south south easterly).

Saint Mary's Road North, 49° 55.963' N, 006° 19.054' W
The north end of Saint Mary's Road about 400 metres westward of Creeb and on the alignment of Nut Rock on a bearing of 260° T in line with the summit of the South Hill on Sampson.

       Next waypoint: 0.57 miles, course 30.59°T (reciprocal 210.59°T)

Crow Rock North, 49° 56.455' N, 006° 18.602' W
400 metres north by northwest of the Crow Rock isolated danger mark Fl(2)10s and on the 160°T transit of the danger mark and the TV tower on St Mary's.

       Next waypoint: 0.15 miles, course 348.13°T (reciprocal 168.13°T)

Cones, 49° 56.602' N, 006° 18.650' W
40 metres east of the cones ledge, a small outlier of the Diamond Ledge, drying to 5.4 metres, off of the southeastern extremity of Tresco.

       Next waypoint: 0.31 miles, course 336.82°T (reciprocal 156.82°T)

Little Pentle, 49° 56.883' N, 006° 18.837' W
On the 160°T transit of the danger mark and the TV tower on St Mary's and 160 metres east of Little Pentle.

       Next waypoint: 0.33 miles, course 341.31°T (reciprocal 161.31°T)

Lizard Point, 49° 57.195' N, 006° 19.001' W
200 metres east of Lizard Point.

       Next waypoint: 0.42 miles, course 329.07°T (reciprocal 149.07°T)

Lump of Clay Ledge, 49° 57.557' N, 006° 19.338' W
80 metres west of Lump of Clay Ledge.

       Next waypoint: 0.19 miles, course 291.46°T (reciprocal 111.46°T)

Tide Rock, 49° 57.627' N, 006° 19.615' W
40 metres south of Tide Rock that dries to 1.4 metres LAT.

What is the best sailing time?
May to September is the traditional UK Sailing season with June-July offering the best weather. The British Isles weather is highly variable, and the amount of bad weather varies quite widely from year to year. This is because they are islands positioned between the Atlantic Ocean and the large landmass of continental Europe. As a result, the entire area lays under an area where five main air masses meet and alternate:

  • • Tropical Maritime Air Mass - from the Atlantic

  • • Polar Maritime Air Mass - from Greenland

  • • Arctic Maritime Air Mass

  • • Polar Continental Air Mass - from central Europe

  • • Tropical Continental Air Mass - from North Africa

Depending on the movements of the jet stream, any and all of these air masses can come in over the isles, creating weather fronts where they meet and bringing with them all types of weather.

The prevailing winds for the British Isles as a whole are from the western quarter which generally blows for two-thirds of the year predominantly from the southwest. Gales from the westward are felt in all seasons, but from November to March, inclusive, they are most frequent and generally last three or four days. Of these, a southwest gale is considered to be the most powerful system. The winter period is largely characterised by wind and rain.

The fine summer weather of the sailing season is typically punctuated by the passage of an Atlantic depression that bringing periods of strong wind and rain, and sometimes poor visibility. These gales rarely cause surprises as they are usually forecasted well in advance. Good weather windows of 48 hours are easy to predict but any longer than that there's an increasing chance of change.

Fogs are frequent in all parts of the Channel and are formed both on the English and French coasts. In summer they only obscure the land in the morning and are readily dispersed by heat or a light breeze. But the moist haze, driven in by westerly winds from the sea, tends to linger and is only dispersed by strong winds. In the eastern part of the Channel, it is rare for the land to be completely free from mists. The only exception is when the wind is from the northeast which makes the mist free coastline highly distinctive from a great distance.

With thanks to:
Michael Harpur, eOceanic.

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