The Influence of the Wind on the Sea – The Wave

Its Crest and its Trough represent the sea wave.

The Wave Frequency is the number of wave crests passing the same point each second – The Wave Period is the time required for the crest to reach the second position.

A wave’s characteristics are released by:

  • the length (L), which is the distance between two ridges;
  • the height (H) between the crest and the hollow;
  • the steepness (S), which is the ratio between height and length;
  • the speed (C-Celerity), which is the space travelled by the wave in a given time;
  • the period (T), which is the time it takes the wave to travel a distance equal to its length and
  • the propagation (D), the direction in which the ripples go.

There is a direct relationship between a wind blowing with a constant intensity and direction, for a certain period, on a particular sea surface. The wave formation process is slow and takes time and space to develop. The stretch of sea concerned is the ‘Fetch’.

Due to friction, the wind begins to drag the upper layers, generating capillary waves a few centimetres long. If the wind pushes constant over time, the wave grows in height. Different pressures are formed, minor on the crests and significant on the cables. The increased pressure differences due to the wind’s action exponentially increase the waves’ height.

To power the wave motion, the wind must move faster than the crests of the waves and transfer energy. Otherwise, the waves tend to shrink in size, opposed only by the force of gravity, while maintaining the stored energy for a long time. On the oceans, waves can travel hundreds of kilometres and reach heights of over 30 meters (100ft.).

The waves’ movement effect is superficial, and due to the wind, in reality, the sea waves do not move, but the water oscillates vertically. If we observe a buoy floating at the mercy of the waves, we notice that it rises & falls; the dragging in the direction of the waves’ motion is very slow. The surface movement in the direction of propagation is the orbital one of the water molecules, which follows the vertical movement of the wave reducing in amplitude from the surface to the seabed, where the sea is calm.

The basic principle for establishing the wave’s height is simple; the stronger the wind – with the exact fetch – the higher the wave. However, there is a physical limit to which the wave breaks beyond a particular steepness (breaking with whitecaps).

  • In the open sea – the limit identifies the relationship between the height and length of the wave H> 0.17 L;
  • In shallow water – where friction intervenes, the limit indicates the balance between waves height and depth of the wave H> 0.8 d.

This phenomenon is essential and can help identify dangerous shallows on the high waters. The breaking of the wave feeds the energy of the wind. It leads to a lengthening of the period and swelling of the sea.

From a practical point of view, sailors are interested in knowing a particular wave element. The nautical bulletins define the significant wave height H1/3 (top third of a wave) or the ‘highest wave’ average. It expresses the measure of the size and force of the wind sea.

Unlike the dead sea or swell, the wind sea is characterised by a background confusion for which there are waves of similar length but with different heights, sometimes significantly.

The period of the waves is critical. The period T remains almost constant even with a rough wind sea, which depends on the wavelength; it does tend to be stable.

Estimating how long a complete oscillation lasts is necessary to measure the period. The measurement should take place from a fixed position. When we proceed against the sea, the period decreases; as it grows with the sea aft. We must consider this aspect.

Finally, the speed is related to wavelength, seabed depth, and the earth’s gravity. The wavelength determines the size of the orbits of the moving water molecules; the seabed determines their shape, becoming flattened with depth. The force of gravity, like friction, is an opposing force that is always present. Generally speaking, the longer the wave, the faster the energy moves through the water.

The relationship between length, period and velocity are:

C (Celerity) is the speed, L is the wavelength, and T is the period in seconds.

The speed of the wave is influenced by the seabed and the force of gravity. We can calculate the C value in the open sea or offshore using the following formulas:

Where ‘d’ is depth – The bottom begins to affect the speed of the waves when the depth is less than half the wavelength.

From a sailor’s point of view, it is good practice to take to the sea at the port/starboard bow, cutting the wavefront diagonally as far as possible and appropriately adjusting the speed to avoid excessive ship movements.

What should be avoided is the oscillation period resonating with the oscillation period of the ship. It would amplify the amplitude of the ship’s oscillations, ‘the roll’, even without the waves being exceptionally high.

Observing the waves is essential for comfort onboard and maritime safety.

The Beaufort wind force and Douglas sea force scales are conventionally used to indicate the values ​​of these forces and the effects they produce in the surrounding environment.