Keeping growth off our boats is a constant, unsatisfying battle. Growth rates of barnacles, mussels, and plant life change with water temperatures and seasons, bottom paint recipes change with environmental regulation, and the frequency at which we use our boats all impact how quickly ugly beards grow below the waterline. We employ a variety of methods to deal with growth, from having divers clean the hull, to costly haul-outs to remove growth and apply new bottom paint. In this never ending battle there is a tool that is often under-utilized: ultrasonic antifouling systems.

Ultrasonic antifouling is not a new concept. The United States Navy first discovered it in the 1950s during sonar testing with submarines. It was found that hull growth was far less substantial around the sonar tubes than on the rest of the hull. Further scientific and technological research has developed a number of products that utilize ultrasonic waves to control algae blooms in ponds and cleaning swimming pools.

Growth on our hulls starts as microorganisms and bacteria attach to the surface and begin to form a biological film. Once a film starts to form, it creates an ideal ecosystem for the growth of algae, seaweed, and more complex organisms, like barnacles and mussels. The theory behind ultrasonic antifouling is to disrupt this growth cycle in its early stages by inhibiting the microorganisms and algae from bonding to the hull.

Sailboat in need of bottom paint with no ultrasonic protection

Sailboat in need of bottom paint with no ultrasonic protection

This disruption is created by the use of transducers that are attached to the interior of the hull. A control unit that creates specific low-powered ultrasonic waves operates the transducers. The hull transfers these vibrations to the water where it creates acoustic cavitation, which creates acoustic pressures low enough to cause the vaporization of the water right along the surface of the hull. These vapor bubbles then burst, creating an incredible hyper-localized pressure differential. This pressure differential generates strong water movement on the microscopic level, but it also creates a difficult environment for microorganisms to attach themselves to the hull due to the strong micro-currents created by acoustic cavitation.

Same sailboat after being protected with an ultrasonic system.

Same sailboat after being protected with an ultrasonic system.

While ultrasonic antifouling systems work great with many boats, they won’t work with wooden or cored hulls. This is because the foam or wood dampens the ultrasonic wavelength making it ineffective at vibrating the exterior hull. Steel and solid FRP hulls offer the best transfer of ultrasonic waves, amplifying them like a soundboard and allowing each transducer to protect a large area.

While these systems are unique and offer great protection against fouling, they are best utilized in conjunction with bottom paint, soft or hard. Most manufacturers, however, suggest using them in combination with a hard antifouling paint. What is the point of using a system like this if you still need bottom paint, you ask? Well, ultrasonic systems will extend the life of your bottom paint by two to three times. This means fewer haul-outs, fewer visits from your diver, and, when the time comes, easier prep for new bottom paint. These systems also can be set up to protect your running gear and rudders, places you can’t apply coats of traditional bottom paint.

There are various producers of ultrasonic antifouling systems, including Ultrasonic Antifouling LTD, Sonihull, Sonic Shield, and Ultra Soni-Tec. They utilize the same technology, but they do differ slightly in what frequencies they employ in their active cycles and how frequent and long their active cycles run. Most systems designed for yachts use a built-in computer to manage power consumption, protect battery banks, provide remote monitoring, and alarms. Some producers, such as Sonihull, also offer specific transducers for protecting running gear. Sonihull has a shaft production transducer as well as a transducer for stern drives.

Sonihull Duo

While these systems offer easy DIY installation, it is important to do your research and figure out the best locations for installation to provide optimal coverage. Since most systems can be protected with one to two ultrasonic transducers, it is important to find the right placement and create a strong bond with the hull free of voids or bubbles in the epoxy that will dampen the transfer of the ultrasonic waves.

Ultrasonic antifouling systems won’t ever be a replacement for bottom paint but they can help keep your hull cleaner, extend your paint life, and improve your fuel economy.