Ultrasonic antifouling is a technology that helps reduce fouling on underwater structures, through using small-scale acoustic cavitation to destroy, denature and discourage attachment of algae and other single-celled organisms.
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History
The technology is believed to have been created by the US Navy in the 1950s: during sonar tests on submarines, it was discovered that the areas surrounding the sonar transducers were cleaner than the rest of the hull. Since then, research has been conducted into the effectiveness of different frequencies and intensities of ultrasonic waves on various marine life, such as barnacles, mussels and algae.
Commercial ultrasonic systems have been used to control algal blooms in ponds, harbours and reservoirs. In controlling the algae, the first stage in the fouling sequence is halted, acting as a prevention, rather than a cure as with traditional anti-fouling paint.
Theory
Acoustic cavitation can be predicted theoretically through the calculation of acoustic pressure and where this pressure is low enough, the liquid can reach its vaporisation pressure. This results in localised vaporisation of the liquid, forming small bubbles; these collapse quickly and with tremendous energy and turbulence, generating heat on the order of 5000K and pressures of the order of several atmospheres. At lower intensity levels, this rapid pressure change is enough to create small movements of the surrounding water, making it extremely difficult for marine life to firmly attach to a surface. Research has also shown that low-intensity ultrasound has an effect on behaviour of some species without inducing cavitation, such as inhibiting the settlement of barnacle cyprids.
Modern usage
Modern commercial systems are available in a wide range of powers and installation variants, however all use similar ceramic piezoelectric transducer as the ultrasound source. There are dedicated systems for:
However, most of these systems are controlled by fairly simple variable-frequency drive units, which run random frequencies in the ultrasonic spectrum of 20–45 kHz over an operational cycle. Intelligent systems will target specific frequencies, as well as manage power consumption, protect batteries & power supplies and come with various other features and options such as remote monitoring, alarm systems and daylight sensors. Several companies have patents on their intelligent systems.
Some systems will actually automatically calibrate after installation and target the ideal frequency range for the substrate to maximise effectiveness.
Limitations
Ultrasonic anti-fouling should not be considered a complete replacement for traditional anti-fouling paints; instead, it should be viewed as a preventative measure to deter marine growth from a surface. Most companies will suggest applying a coat of ‘hard’ antifouling paint.
Ultrasonic systems cannot work on wooden-hulled vessels, or vessels made from ferro-cement. Vessels with foam or wooden cored composite hulls will require modification to the hull in the specific locations that the transducers are to be installed. Ultrasonic systems will work with reduced effectiveness on vibration isolated fittings, such as sterndrives. This is because the hull must pass the ultrasound waves from the transducer located inside the hull through to the water, and these materials act to dampen the amplitude of ultrasonic waves.
Applications
Sites and structures using ultrasonic anti-fouling include: