Wind and Waves

Waves (often referred to as “sea” in maritime weather reports)
-are created by the wind passing over the sea.
Wave height is the vertical distance between the top (crest) of the wave and the bottom or trough.
As wind increases, the crests of waves become more pointed while the troughs become rounded.

As wind strengthens further the waves steepen until they become unstable and break producing “whitecaps.”

The wave height stated in weather reports is the AVERAGE OF THE HIGHEST THIRD OF THE WAVES. Some waves will be higher and some lower. This figure can also be modified by the swell.

(See below.)

 

Swell
-is the regular wave motion caused by large meteorological disturbances operating over a distance. Swell persists after the disturbance has disappeared and maintains a constant direction as long as it keeps in deep water and can travel for considerable distances.

 

Waves and swell
Waves caused by local wind are often superimposed on swell moving in from a distance. This often causes unpredictably high waves, known as freak or rogue waves, and endangers all water users.  To calculate the average wave height from combined sea and swell figures given in maritime weather reports a Pythagorean calculation is used, eg.

(sea height)2 + (swell height)2 = (average wave height)2


Therefore

average wave height = √(sea height)2 + (swell height)2

Beaufort Wind Scale

 

BEAUFORT NUMBER

MEAN WAVE HEIGHT IN METRES

0

Calm 0-1 knots
Sea like a mirror

1

Light Air  1-3 knots  0.1 waves
Ripples with appearance of scales
No foam crests

2

Light Breeze 4-6 knots 0.2 waves
Small wavelets; crests of glassy appearance; not breaking.

3

Gentle Breeze 7-10 knots 0.6 waves
Large wavelets; crests begin to break; scattered whitecaps.

4

Moderate Breeze 11-16 knots 1.0 waves
Small waves, becoming longer; numerous whitecaps.

5

Fresh Breeze 17-21 knots 2.0 waves
Moderate waves, taking longer form, many whitecaps; some spray

6

Strong Breeze 22-27 knots 3.0 waves
Larger waves forming, whitecaps everywhere, more spray

7

Near Gale 28-33 knots 4.0 waves
Sea heaps up, white foam from breaking waves, begins to be blown in streaks

8

Gale 34-40 knots 5.5 waves
Moderately high waves of greater length, edges of crests begin to break into spindrift, foam is blown into well-marked streaks

9

Strong Gale 41-47 knots 7.0 waves
High waves, sea begins to roll, dense streaks of foam, spray may reduce visibility

10

Storm 48-55 knots 9.0 waves
Very high waves with overhanging crests, sea takes white appearance as foam is blown in very dense streaks, rolling is heavy and visibility is reduced.

11

Violent Storm 53-64 knots 11.5 waves
Exceptionally high waves, sea covered with white foam patches, visibility still more reduced.

12

Hurricane 64-71 knots 14.0 waves
Air filled with foam, sea completely white with driving spray, visibility greatly reduced.

 

Wave size/Boat size

 

Boat Length

Wave height

3.5 metres

4.3 metres

5 metres

6 metres

0.2 metres

gentle patter of water on hull, no spray, comfortable

no spray, gentle patter of water on hull, comfortable ride

no spray, armchair ride

great ride

0.5 metres

speed reduced, some pounding on hull at speed

max cruising throttle, slight bumps, spray thrown clear

set cruising throttle, slight bumps, spray thrown clear

set cruising throttle, some light pounding

0.8 metres

speed reduced, water comes aboard, care required

speed reduced, some pounding, spray taken aboard

slight speed reduction desirable, some spray, moderate pounding

reduced speed, pounding increases

1.0 metres

heavy going, limit of safety for this size vessel

speed reduced further, green water taken on decks

half speed, spray and green water come aboard

speed reduced further, spray coming aboard aft end of boat

1.2 metres

throttle backed right off, lots of spray, getting scary

dead slow, waves break on deck, lots of spray

half speed, lots of spray, water on foredeck

1.5 metres

for the experienced only, lots of heavy water thrown aboard

steering through seas individually and using throttle at times

windscreen sprayed, green water on foredeck

1.8 metres

limit of family comfort in this sized vessel

dead slow, steering individually for the seas

2.0 metres

dead slow, occasional big seas break aboard, for the experienced skipper only

 

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