Waves, swell and wind sea: what wave height means for sailing

The wave height in forecasts is the significant wave height: the average of the highest third of the waves, roughly what an observer would estimate. It combines wind sea and swell. Individual waves can be up to about twice as high. How uncomfortable the sea is also depends on period, steepness and direction relative to your course.

What does significant wave height mean?

Significant wave height (Hs) is the average of the highest third of all waves, measured from trough to crest. It is close to what experienced observers report as the wave height.

Statistically, about one wave in seven is higher than Hs, about one in a hundred is one and a half times as high and about one in three thousand twice as high. With a 6 s period, 3,000 waves pass in five hours. So on a day sail with 2 m Hs you should expect individual waves of around 4 m.

What is the difference between wind sea and swell?

Wind sea is produced by the local wind, is short and steep and travels with the wind. Swell is sea state that formed elsewhere or has outlasted the wind: long, regular waves with a long period, often from a different direction than the wind.

How high the wind sea gets depends on wind strength, duration and fetch (the distance over water on which the wind acts). On the Baltic and on inland lakes it stays lower than on the Atlantic, but it is short and steep instead. Wind sea and swell do not simply add up: the total height is roughly the square root of the sum of their squares, so 1 m of wind sea and 1 m of swell give about 1.4 m.

Why are period and steepness so important?

The period is the time between two wave crests. The shorter it is for the same height, the shorter and steeper the waves and the harder the boat has to work.

In deep water the wavelength in metres is about 1.56 times the square of the period in seconds. Steepness is height divided by length: from about 1:20 the sea counts as steep, from about 1:14 the crests break frequently, and at about 1:7 at the latest every wave breaks. A 1.5 m wave with a 4 s period is about 25 m long and, at 1:17, steep; the same height with 10 s is 156 m long and lifts the boat only gently.

At what wave height does sailing become uncomfortable?

As a guideline for a cruising yacht of 30 to 36 ft: up to 1 m is usually unproblematic, 1 to 2 m of wind sea is noticeable and tiring, especially when it is short and on the nose, and over 2 to 2.5 m is too much for many crews. A long 2 m swell with a 10 s period, on the other hand, is often harmless in deep water.

Marine forecasts also describe the sea state in words. The WMO sea state code assigns these terms to the significant wave height:

How does wave direction affect course and comfort?

Waves from ahead make the boat pitch and bury its bow, waves from the side make it roll, and waves from astern make it yaw, up to broaching. Often a course change of just 10 to 20 degrees makes the motion noticeably calmer.

• From ahead: pitching and hard slamming in a short sea, a lot of lost speed, especially with wavelengths of around one to two boat lengths.
• At an angle from ahead: pitching and rolling at the same time, the typical corkscrew motion.
• From the side: rolling; breaking crests on the beam are the most dangerous situation.
• At an angle from astern and from astern: the boat surfs and yaws, wants to broach, and the risk of an accidental gybe rises.

What is a cross sea?

A cross sea develops when two wave systems from different directions meet, for example swell and a new wind sea after a wind shift, or waves reflected back off a cliff. The sea becomes irregular, and individual waves pile up.

It is typical after a front has passed, around headlands and off steep coasts. It tires crew and rig, even when the wave height is moderate.

What happens in shallow water and with wind against tide?

When the water becomes shallower than about half the wavelength, waves slow down and become shorter and steeper; at a depth of about 1.3 times the wave height they break. When a current runs against the waves, it compresses them: they become shorter and higher and break sooner.

That is why bars off river mouths and tidal inlets, banks in onshore wind and narrows with wind against tide are dangerous. Plan them for slack water or a favourable stream; see Tides and tidal streams.

How does Sailseer show waves and sea state?

Sailseer shows waves as a map layer from the wave models of the DWD, Germany's national weather service (EWAM for Europe, hourly up to 78 hours, otherwise GWAM): wave height, swell, wind sea, period and direction of travel. For each leg Sailseer rates the comfort on your course from the angle of the waves, wavelength relative to boat length, steepness and cross sea, in four levels from calm to very rough, and checks the waves against your boat's limit (default for 30 to 36 ft: 2 m, adjustable). Open the wave map, and find suitable days with the guide Weather window. No model resolves narrow bays; the marine forecast and your own look at the water take precedence.