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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 Friday, April 12th at 19:27. 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).

Shallow (HW +2 to -4 Hours)

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

Starts in 02:43:06

(Fri 22:11 to 04:36)

Tidal Window (HW -4 to +2 Hours)

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


(Tidal flow )

Ends in 02:43:05

(Fri 16:11 to 22:11)

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. Conservatively planning for a maximum drying height of 1.7 metres over Tresco Flats provides adequate water for most leisure craft 2 hours either side of high water, with the prudent making the approach during the rise.

The view southward from Merrick Island to St Mary's
Image: Michael Harpur

The waypoints provided are only intended to assist in identifying the key turning points in the passage and should not be used in isolation. It is essential to understand that this route relies primarily upon eyeball navigation with clear visibility, moderate conditions and as much of this area dries at LAT with close-lying rocks, good charts/plotters. Be prepared to take this route under power, in a steady fashion identifying the rocks and marks through the channels along the way. Although the principal marks are lit this is not a passage a newcomer should attempt night nor on the ebb.

The north end of the Tresco Channel
Image: Michael Harpur

Above all do not be tempted to follow the local St Mary’s Boatmen’s Association ferries and trip boats. More often than not they will be seen cutting many of the corners listed on this route. But the boats are surprisingly shallow most notably the Bryher ferry. Despite the LOA and passenger carrying capability, it draws a little over a half metre. The helm also avails of deep and intimate knowledge of these waters ad cut through tight channels within meters of covered rocks. Follow these boats at your peril.

South bound boat passing Hulman
Image: Michael Harpur

Finally, although it looks complicated and contains large drying sections with numerous rocks, it really is very straightforward for vessels of average draught. The vast majority of it is in protected crystal clear waters with the dangers are covered in weed so they stand out clearly amidst the clean white sands. Outside of the rocks and ledges, the bottom is all sand so should the boat touch on a rising tide it will come to no harm. Should there be any concern that there is not be enough water over the Tresco Flats they may be circumvented by a brief dogleg, detailed below, or by avoiding the whole southern section of this route by using the North West Passage Route location and the Western cut into the Tesco Channel Route location.


Leave St Mary's taking care to avoid Bacon Ledge that dries to 0.3 metres LAT. Bacon Ledge is easily avoided by passing through the harbour's lateral marks. The port 'Bacon Ledge' buoy, Fl(4)R.5s, is moored 100 metres southwest of the shoal.

Exiting via St Mary's lateral marks avoids the Bacon Ledge
Image: Michael Harpur

The path through the lateral marks is also supported by a charted entry transit that leads south of Beacon Ledge. This is set on Mount Flagon on the line of bearing 097.3° T of white beacons, with a triangle top mark pointing upward, and an 'X' topmark on the skyline. The two white beacons may be difficult to distinguish behind the rigs of visiting yachts.

The Mount Flagon 097.3° leading marks in St Mary's Harbour
Image: Michael Harpur

The best approach is to first look for the lifeboat, then to the skyline for the 'X' topmark close right of a prominent bungalow, and then finally triangle top mark pointing upward. The transit leads out between the lateral marks and to the north of Woodcock Ledge. This is are not the easiest transit to pick out during the day, particularly in poor light or with the morning sun behind.

The Tresco Channel as seen from the south
Image: Michael Harpur

Once in Saint Mary's Road shape a course on around 315° T towards the isolated 1.5 metres high Nut Rock sitting east of Samson steering to pass it about 200 meters east of it. Be mindful of a cross tide on the approach.

Nut Rock on a bearing of 260° T in line with the summit of the South Hill on

Image: Michael Harpur

1. Nut Rock Waypoint: Nut Rock is steep to at low water with only a small edge extending about 10 metres southeastward. This waypoint is on the alignment of Nut Rock on a bearing of 260° T in line with the summit of the South Hill on Sampson.

The alignment is useful for vessels approaching from Crow Sound as it clears The Pots, that dry to 1.8 metres, and Round Rock, that dries to 1.5 metres, which both lie close together nearly a ½ mile off the southern shore of Tresco.

Hulman ledge exposed
Image: Michael Harpur

2. Hulman Waypoint: This is positioned 60 metres westward of the Hulman beacon, green triangle top mark Fl.G.4s, in order to clear its rock ledge that dries 3.7 metres and extends well beyond the perch.

Little Rag Ledge Beacon
Image: Michael Harpur

3. Little Rag Ledge Waypoint: This is 70 metres east of the Little Rag beacon, a small with a red square top mark Fl(2)R.5s. It is midway between the Little Rag Ledge and the Chinks on the east side that dry 1.3m.

North and southbound boats approaching the marks
Image: Michael Harpur

Similar to the Hulman it is important not to steer directly for the perch so as to avoid north-eastward extending reef, that dries 1. 3 metres, that lies south-eastward of it. But keep a shape eye eastward to avoid the isolated patch Chinks rocks that also dry 1.3 metres and wait there for the unwary.

Merrick Island just open of Hangman Island (for identification)
Image: Michael Harpur

Once Little Rag Ledge Beacon is abeam, and there is sufficient rise of tide, it is safe to proceed directly across Tresco Flats keeping the unmistakeable pyramid-shaped profile of Hangman Island, 16 metres in New Grimsby Sound, over Merrick Island, on 340° T. This alignment clears all the isolated rocks but passes a tad unnecessarily close to rocks that extend from Appletree Point that drying 1.1 metres. Hence the additional conservative Appletree Point waypoint.

Southbound yacht passing Appletree Point as seen from Bryher
Image: Michael Harpur

4. Appletree Point Waypoint: This slightly westward alignment of the Hangman Island over Merrick Island to comfortably clear the weed-covered rocks that extend from Appletree Point. It also marks the commencement of the Tresco Flats, the large sandy spit which extends nearly mile westwards from Appletree Point at the northern end of Appletree Bay.

Tresco Flats drying as seen from Tresco Island
Image: Michael Harpur

The large sandy spit dries to as much as 1.7 metres in places and is the shallowest part of the journey – this may be circumvented with some additional pilotage set out below. The Tresco harbourmaster has placed two white pole alignment marks on the south of Plumb Hill.

The two white poles aligned on the south of Plumb Hill
Image: Michael Harpur

Once in line, the vessel is clear of the north side of the sandbank and rocks running southwestward from Appletree Point - the marks also providing a leading line to clear Lubber's Rock as set out below. After crossing Tresco Flats keep an eye out for the Plump Rocks, that dry to 2.2 metres, on the east side of the route between Plump Island and Appletree Point.

South bound yacht passing between Merrick and Plump Island
Image: Michael Harpur

5. Merrick Island Waypoint: This is the deeper water scoured between Merrick Island, 2.6 metres high, and Plump Island, 7 metres high. Tend towards Merrick Island to avoid two isolated rocks, awash, that lie close to the eastern side of the course.

Passing Merrick Island with Great Crabs Ledge in the backdrop
Image: Michael Harpur

Vessels intending upon drying in Green Bay Click to view haven should simply round Merrick Island’s northern side at a sensible distance and proceed into the north end of the bay.

Continuing northward towards the lower end of the moorings the final danger to keeping an eye out for Queen’s Ledge, that dries 2.5 metres, on the Bryher side of the channel.

Anneka’s Quay
Image: Michael Harpur

7. Anneka’s Quay Waypoint: This is about 120 metres northeastward of Anneka’s Quay that has an east beacon on its head, Q(3)10s.

The commencement of the moorings in New Grimsby Sound
Image: Michael Harpur

It is in 2 metres of water close south of the yacht moorings and the water continues to deepen northward into New Grimsby Sound.


With the use of the Admiralty indicated course, available on our eOceanic Go chart pack and other good plotters, it is possible to make a small dog-leg a ½ mile north-westward after Little Rag. Steer towards Samson Hill on a bearing of 302°T (astern 122°T) to turn around the head of the Tresco Flats bank. This should provide at least one additional metre of water.

The front of the harbour master's posts, with yellow rock coming into alignment
with the Bishop in the backdrop

Image: Michael Harpur

Once the distant Bishop Rock lighthouse is over the centre of Yellow Rock, a small rocky islet midway between Samson and Bryher, it is time to turn northeastward. The key hazard for this added leg is the isolated Lubbers Rock that dries 1.7 metres on the Bryher side. Mincarlo open of Works Point, the southern extremity of Bryher, clears Lubbers Rock.

Tresco Harbourmaster's alignment marks
Image: Michael Harpur

The harbour master has also placed two white posts in Cliff Field, beneath Plum Hill on the Tresco shore, which provide a transit to clear Lubbers Rock. It will also keep Little Crab Ledge, that dries 2.4 metres and Great Crab Ledge, dries 5.3m and just covers at HW, as does Hangman Island over Merrick Island, on 340° T once it is rejoined.


The complete course is 1.62 miles from the waypoint 'Nut Rock' to 'Anneka’s Quay' tending in a north north westerly direction (reciprocal south south easterly).

Nut Rock, 49° 55.899' N, 006° 20.000' W
On the alignment of Nut Rock on a bearing of 260° T in line with the summit of the South Hill on Sampson. This alignment is useful for vessels approaching from Crow Sound as the alignment clears The Pots and Round Rock situated nearly a ½ mile off the southern shore of Tresco.

       Next waypoint: 0.45 miles, course 326.43°T (reciprocal 146.43°T)

Hulman, 49° 56.276' N, 006° 20.389' W
60 metres westward of the Hulman beacon, Fl.G.4s green triangle top mark, in order to clear its rock ledge that dries 3.7 metres and extend well beyond the perch.

       Next waypoint: 0.16 miles, course 16.86°T (reciprocal 196.86°T)

Little Rag Ledge, 49° 56.429' N, 006° 20.317' W
70 metres east of the Little Rag beacon, a small with a red square top mark Fl(2)R.5s. It is midway between the Little Rag Ledge and the Chinks on the east side.

       Next waypoint: 0.35 miles, course 331.91°T (reciprocal 151.91°T)

Appletree Point, 49° 56.740' N, 006° 20.575' W
80 metres clear of the weed-covered rocks that extend from Appletree Point. It also marks the commencement of the Tresco Flats, the large sandy spit which extends nearly mile westwards from Appletree Point at the northern end of Appletree Bay.

       Next waypoint: 0.39 miles, course 356.07°T (reciprocal 176.07°T)

Merrick Island, 49° 57.124' N, 006° 20.616' W
Between Merrick Island and Plump Island.

       Next waypoint: 0.27 miles, course 340.09°T (reciprocal 160.09°T)

Anneka’s Quay, 49° 57.380' N, 006° 20.760' W
Circa 120 metres northeastward of Anneka’s Quay that has an east beacon on its head, Q(3)10s and close south of the yacht moorings.

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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