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.

Do you want to read more articles like this?

References:

Some of the links shown below are affiliate links and we may earn a commission at no additional cost to you:

Send us a Message

    Related Articles

    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.

    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.

    Introducing the hull

    The hull provides a volume to house accommodation, machinery, supplies, and cargo. It has to be seaworthy enough for the routes the yacht will sail and provide the lowest possible resistance to forward movement. It has to resist the heeling forces generated by the wind’s interaction with the sails and have a large resistance to sideways movement to reduce the sideways drift to leeward of the desired course. The hull determines most of the yachts’ main attributes: stability, resistance, seaworthiness, maneuverability, and load-carrying capacity.

    The evolution of hull and keel forms – Part 1

    Until the 19th century, the art of yacht design had been mainly influenced by tradition, economic and commercial motivations, understanding of materials and their availability, and genuine or semi-scientific developments in hydro and aerodynamics. Boats had been evolved through a long but reliable process of trial and error. But now, rating rules, which most of them had nothing to do with real scientific knowledge, started to play a significant role in hull design on both sides of the Atlantic.

    Responses