South coast Isles of Scilly Flood
St Mary's to Tresco: a six mile channel through the Isles of Scilly
The Isles of Scilly are 28 miles off Land's End and the water between them is shallower, clearer and more turbulent than anything on the mainland coast. The 6.4 nm from St Mary's to Tresco crosses the main channel of the archipelago, past two of its most important seal colonies, and lands at a harbour with a subtropical garden above it.
- Distance
- 6.4 nm
- Tide window
- 3.8-5.6 m springs, 2.0-3.0 m neaps
- Wind limit
- 12 kt
- Grade
- Intermediate
- Published
- 8 Oct 2026
A shallow archipelago with deep-water currents
The islands sit on a granite plateau that is 20 to 40 m deep across most of its area, with narrow channels between the islands scoured to over 40 m. That combination — a shallow plateau and deep narrow channels — accelerates the tide between the islands far beyond what the tidal range suggests.
Between St Mary's and Tresco the stream reaches 1.5 to 2.5 knots on springs, and at the northern end, past the Norrard Rocks, there is a north-going stream that runs for most of the flood and is much stronger than the chart's general arrow implies. This is the single most important local fact for a kayak crossing here.
The seals and where they are
The Scillies hold one of the largest grey seal populations in England, with around 300 pups born in a good year. The main pupping islands are the Eastern Isles and the Western Rocks, both of which are closed to landing between August and December. There is a haul-out on the Norrard Rocks which is used all year and is not a landing site at any time.
On the water, the rule is 100 m from any haul-out and no approach between a seal and the water. In the Scillies the seals are habituated to boats and will often follow a kayak for a mile; this is fine as long as you do not stop, turn towards them, or feed anything.
Tresco and the channel crossing
The crossing itself is straightforward in settled weather: leave Porthmellon on the flood, aim north of the Norrard Rocks, and hold a line for the New Grimsby channel between Tresco and Bryher. If the stream is stronger than expected, the bail-out is to turn back on the last of the flood, which is easy, rather than to try to reach Tresco against it.
New Grimsby harbour on Tresco is sheltered in all but a strong northerly, and there is a quay, a pub, and the Tresco Abbey Garden a mile's walk inland. The garden is genuinely subtropical — 20,000 species, including proteas and tree ferns that grow nowhere else in Britain — and it is the reason most people make this crossing.
Weather, visibility and the Scilly microclimate
The islands have the mildest climate in Britain and one of the foggiest. Sea fog forms when warm moist air from the southwest meets the cooler water over the plateau, and it can reduce visibility from 10 miles to 200 m in twenty minutes. A compass and a known bearing are not optional here; GPS helps but the fog can arrive faster than you can enter a waypoint.
The other local feature is that the wind is typically 5 knots stronger between the islands than the inshore forecast suggests, because the channels funnel it. A forecast of force 4 from the southwest means force 5 through the Norrard passage, which is above the sensible limit for this crossing.
Plan table
Before departure
- 01
Check the Scilly sea area forecast, not the Land's End one
The islands have their own forecast area and their own wind speed, usually higher.
- 02
Work out the flood and ebb direction, not just the times
The channel floods northeast and ebbs southwest; the Norrard stream runs north for most of the flood.
- 03
Confirm the seal colony closures
Eastern Isles and Western Rocks are closed August to December for pupping.
- 04
Carry a compass and a bearing to New Grimsby
Sea fog here can cut visibility from 10 miles to 200 m in twenty minutes.
En route
- 01
Leave Porthmellon on the flood and aim north of the Norrard Rocks
This uses the north-going stream instead of fighting it.
- 02
Keep 100 m from every haul-out and never between a seal and the water
The Scilly colony is habituated to boats but the pupping sites are protected.
- 03
Hold a tight group through the Norrard passage
This is where the wind funnels and the stream is strongest.
- 04
Take the falling-tide exit from New Grimsby through the eastern channel
The western channel past Bryher dries at low water springs.
Key numbers
- 6.4 nm one way; 13 nm if you return the same day.
- Spring stream 1.5 to 2.5 kt in the channel, stronger over the Norrard plateau.
- Wind limit 12 kt; the channels funnel the wind, so the local figure is usually higher than the forecast.
- 100 m from haul-outs; the Eastern Isles and Western Rocks are closed August to December.
- Sea fog is a real hazard here and arrives faster than most groups expect.
Bail-outs
- B1 Porth Cressa beach on St Mary's, 1 nm south, usable in most conditions
- B2 New Grimsby harbour on Tresco, sheltered inside the channel
Return leg：Return southwest on the ebb with the Norrard Rocks well to the north. The ebb sets you back towards Hugh Town and the entrance to St Mary's harbour.