Beaufort scale & Sea state

Marine environment

There are several effects of marine environment such as wind, wave, tidal and temperature. These effects will affect the wooden boat. In the open water, unlimited space for the boats to maneuver, thus the boats will be exposed to the marine environment. The effects of marine environment are as follows:

Wave

Wind-generated waves also known as wind waves in fluid dynamics is a surface waves that occur on the free surface of lakes, rivers, oceans and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Wind waves range in size from small ripples to huge rogue waves. Wind sea is a wind wave system when directly being generated and affected by the local winds. After the wind ceases to blow, wind waves are called swell. Or, more generally, a swell consists of wind generated waves that are not or hardly affected by the local wind at that time. Wind waves in the ocean are called ocean surface waves. Some waves in the oceans can travel thousands of miles before reaching land. This phenomenon has been occurring naturally and they have been generated elsewhere, or some time ago.

Wind

Basically, the main influence of the wind is felt indirectly through the waves it generates on the surface of the sea. As stated above, the severity of these waves will depend on the three main factor that occurs; the time it acts (it duration), upon the strength (velocity) of the wind, and the distance over which it acts (fetch).

Beaufort scale has broadly classified the strength of the wind. Originally, there was no specific wind speeds associated with these numbers in the world. But the values showed in Table #: Beaufort scale has been adopted internationally and has been use widely in many countries in the world.

The wind velocity varies with height. At the surface of the water, the relative velocity is zero due to a boundary layer effect. Stability calculations often adopt a nominal wind at 10 meter above the waterline, and the variation in wind velocity with height is assumed to be in accord with figure #. It will noted that these nominal velocities will be about 6 per cent less than those defined by the Beaufort scale at a height of 6 meter above the surface. Also given in figure #a is a curve based on this nominal height. Because of this variation of wind speed with height, it is important to define both in any calculation. [19]

Figure 1: Variation of wind speed with height above sea surface

Figure 2: Variation of wind speed with height above sea surface (nominal wind speed 100 knots)

Beaufort scale and Sea State

Beaufort scale and Sea State is used to describe wind speed by observing the sea conditions. The description includes the approximate wind speed, wind conditions, sea conditions and wave heights. This commonly used by mariner and navigator since it is very important because there are many considerations take into account.

Beaufort scale

Beaufort's scale of wind force was revised in 1874 to reflect changes in the rig of warships, and expanded two decades later to include particulars of the sail required by fishing smacks. A scale of equivalent wind speeds was introduced in 1903, its basis being the formula: [20]

V = 0.836 B^3/2 m/s

Where:

B is the Beaufort number, and

V is the equivalent wind speed at 10 meters above the sea surface.

For all normal purposes, the Beaufort scale extends from Force 0 (calm) to Force 12 (Hurricane), with Force 12 defined as a sustained wind of 64 knots (32.7 metres per second) or more. [20]

Beaufort's scale of wind force assumed its present form around 1960, when probable wave heights and probable maximum wave heights were added. The latter is the height of the highest wave expected in a period of 10 minutes, and wave heights refer to the open sea, well away from land. Strictly, it applies only when the sea is fully developed; that is, when waves have reached their maximum height for a particular wind speed. Care must be exercised when the fetch and duration of the wind are limited (the fetch is the distance over which the wind has blown, and the duration the time it has been blowing). It is also worth remembering that the appearance of the sea's surface is influenced not only by wind but also by swell (waves from far away), precipitation, tidal streams and other currents. [20]

Table1: Beaufort scale

Sea state

Figure 3: Beaufort number:  0

Wind speed: Less than 1 knot

Sea condition: Flat.

 

Figure 4: Beaufort number:  01

Wind speed: 1-3 knots

Sea condition: Ripples without crests.

Figure 5: Beaufort number:  02

Wind speed: 4-6 knots

Sea condition: Small wavelets. Crests of glassy appearance, not breaking.

Figure 6: Beaufort number:  03

Wind speed: 7-10 knots

Sea condition: Large wavelets. Crests begin to break; scattered whitecaps.

Figure 7: Beaufort number:  04

Wind speed: 11-16 knots

Sea condition: Small waves with breaking crests. Fairly frequent white horses.

Figure 8: Beaufort number:  05

Wind speed: 17-21 knots

Sea condition: Moderate waves of some length. Many white horses. Small amounts of spray.

Figure 9: Beaufort number:  06

Wind speed: 22-27 knots

Sea condition: Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.

Figure 10: Beaufort number:  07

Wind speed: 28-33 knots

Sea condition: Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.

Figure 11: Beaufort number:  08

Wind speed: 34-40 knots

Sea condition: Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.

Figure 12: Beaufort number:  09

Wind speed: 41-47 knots

Sea condition: High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.

Figure 13: Beaufort number:  10

Wind speed: 48-55 knots

Sea condition: Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility.

Figure 14: Beaufort number:  11

Wind speed: 56-63 knots

Sea condition: Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility

Figure 15: Beaufort number:  12

Wind speed: 64 knot

Sea condition: Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.

Other extreme environment

In addition to the conditions of wind and waves to which all boats are subject, there are other extreme conditions the boats and equipment may need to allow for. These include driving rain, dust and sand which can abrade exposed surfaces, chemical deposits (including salt from spray) and fungi which can harm surfaces and eat away certain materials. Sea-spray and snow can cause icing up in cold climates. Ice impedes the operation of moving items and can pose a serious stability problem. The conditions upon which designs of boats and equipment should be based are laid down in various specifications. These also define suitable tests and should be consulted by the designer.

References

[19]      E.C. Tupper and K.J. Rawson, Basic Ship Theory, Fifth Edition, Volume 1, Copyright 2001, Butterworth – Heinemann, Jordan hill, Oxford.

[20]      National Meteorological library and Archive, PDF, 15 OCTOBER 2010

[21]      Wind wave, October 2010, “Wind wave”,

http://en.wikipedia.org/wiki/Wind_wave