Konstantin Matveev – matveev@wsu.edu

Washington State University
Pullman, WA 99164

Popular version of 1pEA7 – Acoustic oscillations of drag-reducing air pocket under fast boat with stepped bottom
Presented Monday afternoon, May 23, 2022
182nd ASA Meeting
Click here to read the abstract

A lot of fuel is usually consumed by a fast boat to overcome water drag. Some of this resistance is caused by water friction which scales with the hull wetted area. By injecting air under the hull bottom with a special recess and maintaining a thin but large-area air pocket, total boat drag can be decreased by up to 30%.

boat hull

Boat with bottom air cavity.

However, generating and keeping the bottom air pocket in waves is rather tricky, as periodic wave pressure may excite an acoustic resonance in a compliant air cavity, resulting in large oscillations of the air cavity accompanied by significant loss of air to the surrounding water flow. The deterioration of the air pocket will drastically increase resistance of the hull, and the boat may be unable to reach sufficiently high speeds to operate in a planing regime.

Side view of air-cavity hull in waves.

Bottom view of air-cavity hull in waves, showing increased air leakage.

A simplified oscillator model, similar to a mass on a spring, is employed in this study to describe and simulate oscillations of the air cavity under the boat hull. The main inertia in this process is the so-called added water mass, which is a mass of an effective water volume under the air pocket, while the spring action comes from the compressibility of air inside the bottom recess.

boat hull

Oscillator model for air cavity under hull in waves.

An air-cavity boat accelerating through waves may hit the resonance condition, when a frequency of encounter with waves coincides with the natural or preferable oscillation frequency of the air pocket under the hull. Simulations using the developed model have demonstrated that acoustic oscillations may grow in magnitude and disintegrate the air cavity. However, if the boat accelerates sufficiently fast and the time spent near the resonance state is short, then oscillations will not have enough time to amplify, and the boat can successfully reach a high speed to glide on the water surface. Alternatively, if the damping is increased, for example by baffles, morphing surfaces or even sound from underwater loudspeakers, one can suppress the oscillation growth as well. The presented model can help boat designers develop higher performance boats.

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