CWO Zeezeilschool | RYA Training Centre | Jachtverhuur | Est. 2000

Lesson 16 Wave prediction

Let's start this lesson with the following definitions:

: created by the current wind.
Swell : by the wind that was blowing earlier.
Waves = sea + swell.
Below is a table with the translation in English, French and German.

Dutch Zeegang Deining
English Sea Swell
German Windsee Dunung
French Mer Houle

Douglas sea scale

  Hoogte (m) Omschrijving
0 vlakke zee Calm (Glassy)
1 0–0.10 Calm (rippled)
2 0.10–0.50 Smooth
3 0.50–1.25 Slight
4 1.25–2.50 Moderate
5 2.50–4.00 Rough
6 4.00–6.00 Very rough
7 6.00–9.00 High
8 9.00–14.00 Very high
9 14.00+ Phenomenal

Forecasts for wave heights

For example, at you can view the forecast for the waves.

Wave characteristics

Waves have the following features:
• Wave length
• Period
• Wave speed
• Steepness
• Wave height

Significant wave height is average of the highest 1 / 3rd part.
Maximum wave height = H max = 1.8 x H signi.

Breaking waves, ground seas, surf

High long waves are not necessarily dangerous. Breaking waves, on the other hand, are more dangerous because they can turn the ship upside down. The origin of breaking waves can be explained by means of the following formula: Wavelength = speed x period. A shallow, eg beach, sandbar, slows down the wave. The wave period will remain the same, so the wavelength becomes shorter. The mass remains the same, so the wave becomes higher and steeper, breaking waves occure. This happens when the water becomes shallow (ground sea) and in front of the beach (surf). When there are ground seas in front of the harbor, the waves also contain a lot of sand. The waves are therefore sand-colored.

Tidal current against wind direction

Current through the tide or in a river has major consequences for the waves. Current against wind shortens wavelength and increases wave height. If the current and wind come from the same direction, we have longer wavelength and lower wave height.

Cross seas

When a depression passes quickly, we are faced with different wind directions and therefore also with different wave patterns. These wave patterns will then start to mix up and cross seas will arise. Sailing is very difficult and unpleasant. A cruising sea can also be caused by waves bouncing off a steep deep coast.

Calculate wave height

The wave height is determined by the following factors:
• Duration certain wind
• Wind speed
• Water depth
• fetch

The height depends on wind speed, the duration in which the wind blows, the wind path (fetch). You can find the wave height from the graph below. The y-axis shows the wind speed in knots and the x-axis shows the wind path (Fetch). The curved lines show the wave height in feet.


Source: NOAA

More extensive graphs also include the wave period and duration.


The graph above shows:

  • On the left axis the wave height in meters.
  • On the right axis, the wind speed in meters per second.
  • On the horizontal axis the duration of the wind in hours.
  • On the transverse lines the fetch is in kilometers.

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