Is the Portsmouth to St Malo crossing rough?
Sometimes, but you will mostly be asleep for it. Portsmouth to St Malo is the longest of the Channel ferry routes at about 11 hours, run overnight by Brittany Ferries with compulsory cabins. It passes west of the Cotentin peninsula into more Atlantic-influenced water in the small hours, which is exactly when you want to be horizontal anyway.
Central and western English Channel · about 11 hours, overnight · Reputation among passengers: mixed: commonly reported as uncomfortable in the wrong conditions.
- Low
- Moderate
- High
- Very high
Roughness summarises typical reported conditions on this route, not a forecast for any given sailing.
The longest Channel crossing, slept through
Key facts
- The Portsmouth to St Malo crossing takes about 11 hours, overnight, operated by Brittany Ferries.
- It crosses the Central and western English Channel, and passenger reports place the route in the moderate roughness band.
- What drives the rough days: The longest Channel crossing, slept through.
- Conditions are reported roughest in January, February, November and December.
- The calmest months are usually May, June, July and August.
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Why the Portsmouth to St Malo crossing gets rough
The route runs the length of the central Channel and then swings south-west around the Cotentin into the Gulf of St Malo. That western leg picks up Atlantic swell that the eastern Channel filters out, and it arrives in the middle of the night when the ship is furthest from shelter. The compensation is the format: motion sickness is mildest lying down with eyes closed, and this crossing is built around a bed.
This route crosses the English Channel. For the full picture of that water, every crossing on it and what drives its reputation, see is the English Channel rough?
When it's worst, month by month
Summer nights are commonly calm enough to sleep straight through. The October to February depression season brings rolling nights, and the west-of-Cotentin leg is where a winter sailing will wake light sleepers.
| Roughest water in Jan | Roughest water in Feb | Roughest water in Mar | Roughest water in Apr | Roughest water in May | Roughest water in Jun | Roughest water in Jul | Roughest water in Aug | Roughest water in Sep | Roughest water in Oct | Roughest water in Nov | Roughest water in Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Rough | Rough | Mixed | Mixed | Calm | Calm | Calm | Calm | Mixed | Mixed | Rough | Rough |
Typical seasonal pattern, not a forecast. Any month can deliver a calm crossing or a bad one. Check the marine forecast for your sailing date. Select a month to see every sea and ferry crossing ranked for it.
Month bands: Dizzout sea-state data for the Portsmouth to St Malo crossing, built from the reports and sources listed on this page. Free to cite with a link to this page.
Where to be on board
Front to back
- Bow (front)Avoid
- MidshipBest
- Stern (aft)OK
The crossing: about 11 hours overnight
Roughest stretch: West of the Cotentin, usually in the small hours
Cabins are compulsory overnight, so choose well: midship, low deck, and do not pay extra for the high forward cabins with the view if you are motion sensitive.
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What helps on this crossing
- Pick a midship cabin on a low deck at booking time; cabin position is the biggest lever on this route.
- Eat dinner early and moderately; the open-water leg comes after midnight.
- Get horizontal when the motion starts rather than riding it out in a lounge chair.
- Pack anything you might want in the night in a bag you keep with you; the car decks are closed at sea.
- If you sail in winter, treat a rolling night as normal for the season, not a sign something is wrong.
- Dizzout, a drug-free sound therapy app for passengers: it works with any headphones once nausea starts, with relief in about 90 seconds.
Buying a tablet in the UK before you sail? UK pharmacies sell Kwells, Stugeron and Avomine for travel sickness, and their labels differ on when to take them. Kwells vs Stugeron and Stugeron vs Avomine set out the label facts side by side. Which one suits you is a question for the pharmacist.
For the full toolkit see the seasickness guide, or what to do once it has already started. Choosing between a fast catamaran and a conventional ship elsewhere too? The physics is the same everywhere: see the catamaran guide. If the rocking follows you ashore afterwards, that is land sickness, which is normal, and it passes.
Frequently asked questions
How long is the Portsmouth to St Malo ferry?
About 11 hours, sailing overnight with Brittany Ferries. Cabins are compulsory on overnight sailings, which suits motion-sensitive travellers well: you cross the most exposed water asleep in the most tolerable position, lying flat.
Which part of the crossing is roughest?
The leg west of the Cotentin peninsula, roughly the second half of the night. The eastern Channel gives some shelter early on; once the route turns towards the Gulf of St Malo it meets water with more Atlantic character. In settled summer weather the difference is barely detectable.
Is this crossing worse than Portsmouth to Caen?
It is longer and its western leg is more exposed, but it is also entirely overnight, so most passengers experience less of the motion than they would on a 6 hour daytime Caen crossing in the same weather. If you sleep well at sea, St Malo is arguably the gentler experience; if you cannot sleep on ships, Caen's daytime option wins.
What about seasickness medicine for the overnight crossing?
A pharmacist or doctor is the right person to advise you, particularly for anything taken at night. When you ask, mention it is an overnight sailing: most motion sickness medicines are labelled for use before travel, and some list drowsiness effects that matter differently when your plan is to sleep anyway.
Other ferry routes
Sources & further reading
Your next question, probably
Where on the ship should you be?
Which cabins move least, and what to do if you already booked one
Not your question? What actually works at sea?Sick right now, mid-crossing?Still rocking after you land?
Conditions described here are typical patterns drawn from published marine and operator information as of September 2026, not a forecast: any month can deliver a calm crossing or a bad one, and schedules, vessels and operators change. Route, vessel and operator names are trademarks of their respective owners; Dizzout is not affiliated with or endorsed by them. Seasickness sensitivity varies from person to person; this page is informational and not medical advice.