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Sea State

The Douglas Scale

The state of the sea is as important as the strength of the wind; often the two are closely related but sometimes you can have a very large sea with light winds. Apart from the obvious dangers of being out in a sea too large for your boat and skill, the state of the sea can also have a dramatic effect on the consumption of fuel.

The state of the sea is described using the Douglas scale, the lesser known brother of the Beaufort scale. The scale was devised by the British Navy Captain HP Douglas in 1921.

There are actually two scales, both ranging from 0 to 9; one to measure swell (longer flatter waves that travel over oceans) and one to measure the wind sea (waves caused by the wind in the local area). The wind sea is the scale more commonly used in coastal areas.

Wind Sea Scale

Scale Symbol Description Wave Height (m)
0 single flat line Calm (glassy) 0
1 single flat line Calm (rippled) 0-0.10
2 one smooth cuve Smooth 0.10-0.50
3 two smooth curves Slight 0.50-1.25
4 three smooth curves Moderate 1.25-2.50
5 one rough line Rough 2.50-4
6 one rough line Very rough 4-6
7 on very rough line High 6-9
8 on very rough line Very high 9-14
9 lines curling over Phenomenal over 14

Inshore forecast example:

From Cape Wrath to Rattray Head including Orkney.
24-hour forecast:
Wind: east to southeast 6 to gale 8 gradually decreasing

4 or 5.
Weather: cloudy, periods of rain.
Visibility: moderate or good.
Sea state: rough to very rough decreasing moderate.

Outlook for the following 24 hours:
Wind: east to southeast 4 or 5 gradually backing northeast to east 3 or 4.
Weather: cloudy, rain or showers.
Visibility: moderate or good.
Sea state: moderate decreasing slight.