West coast Anglesey and Gwynedd Flood
The Menai Strait and the Swellies: a five-knot channel with a Telford bridge over it
The Menai Strait is 14 miles of water separating Anglesey from Wales, and at its narrowest point, under Telford's suspension bridge, it squeezes through a rock-strewn passage called the Swellies where the tide reaches five knots and the direction reverses in a quarter of an hour. It is the best place in Britain outside Scotland to learn to read tidal water.
- Distance
- 8.6 nm
- Tide window
- 4.2-6.4 m springs, 2.2-3.3 m neaps
- Wind limit
- 14 kt
- Grade
- Intermediate
- Published
- 7 Sept 2026
Why a strait works like this
The strait has two open ends, one to the Irish Sea at Caernarfon and one to the northeast at Puffin Island, and the tide arrives at each end at a different time. That means for part of every tide cycle the water is flowing in at one end and out at the other, and at the narrow points between the two the stream can reach 5 knots on a 6.4 m spring.
The Swellies is the worst of it: a 1.5 nm stretch between the two bridges where the navigable channel is 30 to 50 m wide, the bottom is rock and boulder, and a set of named rocks including the Swelly Rock and the Platters sit right in the fairway. The result is a set of standing waves and whirlpools that appear and disappear within a single tide.
Slack water and the local rule
The Swellies do not go slack at the same time as the rest of the strait. Slack water at the Swellies is roughly 1 hour 45 minutes before Liverpool high water, and again about 1 hour 30 minutes before Liverpool low water, and the stream reverses almost immediately after.
This is the single number you need. Local paddlers work to it exclusively: be at the Swellies at slack, and you will pass through in flat water with 15 minutes either side to spare. Be there 30 minutes early or late and you are in 3 to 4 knots of stream with eddies, boils and rock on both sides.
What it feels like and what to do
At full flow the Swellies produce boils, which are upwellings of water that appear without warning, stand a foot above the surface, and can spin a kayak 90 degrees before disappearing. There are also strong eddies behind every rock, and a well-known effect at the Swelly Rock where two streams meet and produce a confused zone 20 m across.
The technique is to keep the boat moving, use the eddies deliberately rather than fighting them, and never stop. If you are caught in a boil, edge into it and let it pass rather than bracing hard against it. Groups should be strung out, not rafted up, so that one boat's mistake does not involve three others.
The rest of the strait
Outside the Swellies the strait is a much gentler proposition: 3 to 4 nm of sheltered water with wooded shores on the Gwynedd side, the grounds of Plas Newydd, the Britannia Bridge, and a resident population of seals on the sandbanks. The current still runs at 2 to 3 knots on springs, but the water is flat and the land is never more than half a mile away.
The classic day is to launch at Menai Bridge, run southwest through the Swellies to Felinheli or Caernarfon, and return on the opposite stream. Total distance is 8.6 nm if you turn at Felinheli, and the whole thing can be done in four hours with an hour at the turn.
Plan table
Before departure
- 01
Calculate slack at the Swellies from the Liverpool tide
Slack at the Swellies is 1 hour 45 minutes before Liverpool high water.
- 02
Confirm the sea state at both ends of the strait
The strait is sheltered but a strong southwest wind over the Caernarfon end makes the exit unpleasant.
- 03
Check the swell and the state at the Swellies rocks on the day
The standing waves are height-dependent; a 4.2 m neap is a very different place from a 6.4 m spring.
- 04
Agree the group order and the eddy-hopping plan
The Swellies are paddled one boat at a time in places and the group needs to know where to regroup.
En route
- 01
Enter the Swellies only within 15 minutes of slack
The stream reverses almost immediately after and reaches 3 to 4 kt within half an hour.
- 02
Keep the boat moving and use the eddies deliberately
A stationary kayak in a boil is a kayak being rotated at random.
- 03
Stay off the Swelly Rock and the Platters
These sit in the fairway and produce the strongest boils in the passage.
- 04
Watch for the Caernarfon and Beaumaris ferries
The strait is a working waterway with seasonal passenger traffic on fixed routes.
Key numbers
- 8.6 nm round trip from Menai Bridge to Felinheli and back, 3.5 to 4.5 hours.
- Spring stream in the Swellies reaches 5 kt and reverses within 15 minutes of slack.
- Wind limit 14 kt inside the strait; the Swellies are essentially wind-independent but not boat-traffic-independent.
- Slack at the Swellies is 1 hour 45 minutes before Liverpool high water.
- The technique is eddy-hopping, one boat at a time, and never stopping in moving water.
Bail-outs
- B1 Felinheli (Port Dinorwic) marina, 2 nm southwest, all states of tide behind the gate
- B2 Beaumaris, 4 nm northeast, a harbour with a pier usable near high water
Return leg：Return on the opposite stream with the turn taken at the Swellies at slack. The whole strait reverses twice a day, so an out-and-back on one tide is a natural round trip.