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Sea conditions

Is the sea rough today? How to check before you sail

Two minutes with the right forecast tells you what the water is doing on your route. Here is how to read it, and what the numbers feel like on a ferry.

To know if the sea is rough today, check the marine forecast for your stretch of water: waves under 1.25 m are smooth to slight, 1.25 to 2.5 m is moderate, and 2.5 m or more is rough on the sea state scale. Then check the wind, because wind blowing against the tide makes short, steep waves, and check your ferry operator on the day, because operators decide whether a sailing runs.

What the wave heights mean

Marine forecasts describe the sea with a standard scale. These are its steps for the heights a ferry usually meets, and what each tends to feel like on board.

The sea state scale and what each step feels like on a ferry
Sea stateWave heightOn a ferry
Smoothunder 0.5 mLittle motion on any ferry. Most passengers never notice the sea.
Slight0.5 to 1.25 mA gentle roll on a large ferry. Fast craft and small boats start to bounce.
Moderate1.25 to 2.5 mPitching and rolling you can feel on most ships, and a bumpy ride on fast craft.
Rough2.5 to 4 mHeavy motion on every size of ship. Walking needs a handhold, and fast craft feel it most.
Very roughover 4 mSevere motion on any vessel. Check your operator's status page before you leave home.

Heights from the sea state scale the Met Office publishes. Above 4 m the scale continues through high, very high and phenomenal seas.

How to check the sea before you sail

  1. Find the marine forecast for your water

    Use your national weather service's coastal or inshore waters forecast for the stretch of sea you are crossing, not a city weather app. City forecasts give wind on land, which says little about the waves.

  2. Read the wave height for your sailing time

    Forecasts give the significant wave height: the average of the highest third of the waves. Individual waves can be taller, so treat the number as typical, not the maximum. Place it on the sea state scale above.

  3. Check the wind against the tide

    Wind blowing against the tidal stream stands the waves up short and steep, which feels far rougher than the height suggests. Narrow straits and shallow channels do the same.

  4. Look at the swell period if it is listed

    A long period, say ten seconds or more, means long rolling swell that a big ferry rides slowly. A short period means choppy, sharper motion at the same height.

  5. Check your ferry operator on the day

    Operators decide whether a sailing runs and publish it on their website or app. Check it before you leave for the port.

Official marine forecasts

What makes a crossing feel rough

  • The size of the boat. A large ferry rides through waves that throw a fast catamaran or a small boat around. The same 2 m sea is two different trips.
  • The shape of the waves. Short, steep chop is harder on the stomach than long swell of the same height.
  • How long you are out. Half an hour of moderate sea is easier to sit out than four.
  • Where you sit. Low and in the middle of the ship moves least. Face forward and look at the horizon, not your phone.

Each ferry route page explains why that water gets rough and where to sit on the boats that run it. The seasickness guide covers what helps on board.

Rough water on your route?

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Typically rough and calm in October

This is what each water usually does in October, from its long-term pattern. It is not a forecast for today.

Usually rough in October

Usually calm in October

Every sea and ferry route ranked for the month is in the October ranking.

Frequently asked questions

What wave height is rough for a ferry?

On the sea state scale, waves of 2.5 to 4 m are rough and anything above 4 m is very rough. How it feels depends on the boat as much as the number: a large conventional ferry moves slowly through seas that make a fast catamaran jolt.

Is a 1 metre sea rough?

No. On the sea state scale, 0.5 to 1.25 m is slight. On a large ferry most passengers barely notice it; on a fast catamaran or a small boat it is a noticeable bounce.

Are 2 metre waves rough?

Two metres is moderate on the scale, which runs from 1.25 to 2.5 m. Expect pitching and rolling you can feel on most ferries, more on fast craft. Rough starts at 2.5 m.

How far ahead can I trust a wave forecast?

About a week. Beyond that no wave forecast is worth planning on, so use what the water typically does in your month instead. Every ferry route page on this site has the full year, month by month.

Will my ferry run if the sea is rough?

Only the operator decides that, and they publish it on their website or app. The forecast tells you what the water is likely to be doing, which is worth knowing whether or not the sailing runs.

Related

This page explains how to read a marine forecast; it holds no live data and never says whether a sailing runs: operators decide that. Route 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.