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England Ireland Find Routes


Crow Sound to Tean Sound (Scilly)

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What is the route?
This is a passage across the broad stretch of shallow water that lies between the islands of Tresco and St Martin’s connecting Crow Sound with Tean Sound. 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 Crow Sound to Tean Sound but it may is available both ways.

Crow Sound to Tean Sound
Image: Visit Isles of Scilly

Why sail this route?
This interior cut joins the west and east seaward approaches to St Martin’s so a vessel may effectively circumnavigate the island with a sufficient rise of the tide. This route can be used in conjunction with Old Grimsby Sound from St Mary’s Road Route location to connect St Mary's Road and Crow Sound with all of the principal anchorages in the northeast and east end of the group as follows:

The route may also be very easily adapted to pass Tean Sound and St Mary’s Road connects two of the group’s principal deepwater berthing locations and makes the facilities of St Mary’s Harbour and Hugh town more readily accessible for vessels in Tean Sound.

The northern section of the route approaching Tean Sound
Image: © Tom Corser 2009 via CC BY-SA 2.0

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 09:22. 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 01:49:46

(Thu 04:47 to 11:12)

Tidal Window (HW -4 to +2 Hours)

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

Starts in 01:56:46

(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 to the northwest of Gunter’s Island. As it is shallow with parts that dry at LAT, with close-lying rocks and ledges, good charts/plotters are key.

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 rise. However, this is not a passage a newcomer should attempt at night nor on the ebb.


Hats South Cardinal as seen over Innisidgen
Image: Michael Harpur

With sufficient tide run into Crow Sound’s Route location 'Hats' waypoint, about 200 metres northwest of Hats south cardinal, VQ(6)+LF1.10s. Then steer the on the 322° (reciprocal 142°) transit of the gap between the two highest summits of Men-a-vaur in line with St Helen's Landing Carn which is a prominent lump of rock Landing Carn at the southwestern end of St Helen's.

Men-a-vaur and Round Island
Image: Andrew via CC BY 2.0

Just over ¾ of a mile further along, the transit passes 300 metres to the southwest of Gunter's Island. Guther's Island is easily recognised by its twin flat-topped rocks, 12 metres, and its surrounding rocky ledges. The alignment keeps sufficiently clear to avoid the Lower Ledge, drying to 3.3 metres, and then the Higher Ledge, drying to 4 metres, that lie midway between the track and the island. The shallowest and only drying section for this route lies beyond Higher Ledge where it dries to 0.4 LAT. It is passed over once the vessel arrives at the 'Central' waypoint located ¼ of a mile further along when St Mary's TV Tower should be due south.

Guther's Island with its distinctive twin flat-topped rocks
Image: Michael Harpur

Arriving at the 'Central' waypoint provides the aforementioned option of staying on the 322° (reciprocal 142°) transit between Men-a-vaur and St Helen's Landing Carn for St Helen's Pool Click to view haven (St Helen's Island). The alignment will take a vessel safely into the anchorage, 1 mile from this waypoint, passing its shallowest part, drying to 0.7 metres, east of the awash Scattered Rocks that lie close southeast of the always visible Great Cheese Rock, 3.2 metres. The alignment is at its narrowest part between the Chinks Rocks, awash LAT, and Hunters Lump, dries 0.9 metres which are close, either side of the transit. The south end of the Chinks Rocks are ½ a mile due east of the ruin on Block House Point (Tresco).

Or, alternatively, it is also possible at the 'Central' waypoint to turn west for Lizard Point, less than ½ a mile, to connect with Old Grimsby Sound from St Mary’s Road Route location for Old Grimsby Click to view haven (Tresco).

Gunter's Island to Tean Sound
Image: Visit Isles of Scilly

To continue on to Tean Sound Click to view haven break off the transit at the 'Central' waypoint and steer for the 'Broad Ledge' waypoint located 200 metres west of Broad Ledge north-western end.

With Broad Ledge abeam to the east, proceed northward to 'West Broad Ledge' waypoint. This is located 200 metres east of Broad Ledge West and about midway between it and Stephen’s Ledge and Dog Ledge to the east.

Passing John Martin's Ledge to starboard
Image: Andrew Abbott via CC BY-SA 2.0

Then continue to the northwest of West Broad Ledge steering to pass between it and John Martin’s Ledge to the south end of 'Tean Sound South' waypoint.

The southern end of of Tean Sound
Image: Michael Harpur

The water is deep before John Martin's Ledge and when the ledge passes abeam the southern end of Tean Sound will have 5 metres or more.


The complete course is 1.78 miles from the waypoint 'Hats' to 'Tean Sound South' tending in a north north westerly direction (reciprocal south south easterly).

Hats, 49° 56.246' N, 006° 17.226' W
200 metres northwest of Hats south cardinal, VQ(6)+LF1.10s.

       Next waypoint: 1.20 miles, course 322.13°T (reciprocal 142.13°T)

Central, 49° 57.196' N, 006° 18.374' W
On the Men-a-vaur in line with the landing cairn on the southwest corner of St Helen's on 322° T, 0.4 miles northwest of Gunther' Island and due north of St Mary's TV Tower.

       Next waypoint: 0.21 miles, course 38.75°T (reciprocal 218.75°T)

Broad Ledge, 49° 57.357' N, 006° 18.173' W
160 metres westward of the northwestern end of Broad Ledge.

       Next waypoint: 0.23 miles, course 352.79°T (reciprocal 172.79°T)

West Broad Ledge, 49° 57.581' N, 006° 18.217' W
Midway between West Broad Ledge on the west and Stephen’s Ledge and Dog Ledge to the east.

       Next waypoint: 0.14 miles, course 329.92°T (reciprocal 149.92°T)

Tean Sound South, 49° 57.702' N, 006° 18.326' W
The southern end of Tean Sound in 5 metres.

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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