The air-water interface

The air-water interface

Water and air are two different fluids that enter into contact at the ocean’s surface at what is called the air-water interface. This interface, also known as boundary surface or contact discontinuity, plays an important role in ship hydrostatics and in describing the flow of water around a vessel.

Air is a gas made up of a mixture of gasses. Like any other gas, it is compressible. However, because the airspeed around the hull, rig, and sails is just a small fraction of that of the speed of sound, it can be considered as being incompressible for all our purposes. The density of the air depends on the temperature, the atmospheric pressure, and the humidity.

Water is, on the other hand, a liquid. It is incompressible, and its density depends on the temperature and on the salinity. The density of the water is much higher than that of air. At sea level and 15°C the density of air is around 1.225 kg/m3 while, at the same conditions, that of the seawater is 1,026 kg/m3, around 838 times that of the air.

Air and water remain both at their side of the interface. They do not mix as it will be the case, for example, of two gases in contact. Besides, the pressure of the two fluids at the interface is the same (it is just the atmospheric pressure, which is the result of the weight of the column of air above it). These two points just mentioned are the necessary conditions for having what is known as a free surface.

At the free surface, the water is “free” to deform creating surface waves. Two common causes for the creation of surface waves are the wind and a vessel sailing through the water. The gravity force plays a significant role in the creation and propagation of surface waves.

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Surface waves

Surface waves are created by disturbance forces applied to a specific area. It can be either the wind, a surface vessel moving through the water, an earthquake, landslides, a splash, gravity (as it is the case for tides), or any combination of them. When studying ocean waves, we consider them to be the superimposition of single sinusoidal waves with different frequencies and amplitudes.

Wind-generated sea waves

Wind-generated waves occur when the wind blows over the surface of the sea. They are divided into gravity waves (wind sea, and swell) and capillary waves. Their period is usually equal to or less than 30 seconds, and their wavelength ranges from some millimeters up to 1500 meters.

Wave spectral density (calculation)

The performance of a sailing boat is affected by the presence of surface waves. To understand the boat’s behavior under waves, it is necessary first to model the irregular shape of the water surface or sea state. The most common method used for describing the sea state is using the “wave spectral density, “also known as “wave energy spectrum,“

Sea state and wave forecasting

The performance of a sailing boat is affected by the presence of surface waves. To understand how the waves influence the boat’s performance, we need first to model the irregular shape of the water surface.

What is the Froude number?

The Froude number is a dimensionless number used to quantify the influence of gravity on the motion of a fluid. It is the ratio of the inertia forces to the gravitational forces related to the mass of water displaced by a floating boat.

What the hydrodynamic resistance is and why it matters

The motion of a sailing yacht through water requires energy to overcome resistance. It is essential to know the mechanisms behind the generation of this force that works against the movement so that we can make the resistance of a new design match the project’s overall performance goals.


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