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Ultrasonic Anti-Fouling Unit For Boats

Marine growth on the hull is the bane of all boat owners. Left unchecked, marine growth slows the boat down considerably and if it's a power boat, leads to large increases in fuel consumption. If it's a yacht, marine growth will also slow it down and make it less manoeuvrable, to the point where it becomes very sluggish. The cure is to haul the boat out of the water every year and water-blast and scrape away all the growth and then coat the hull in toxic anti-fouling paint.

Pt.1: By Leo Simpson & John Clarke


Overall output frequency range: from 19.08kHz to 41.66kHz in 14 bands; frequency overlapping included between each band

Frequency sweep in each band: 12 frequencies ranging from approximately 80Hz steps at 20kHz to 344Hz steps at 40kHz

Signal burst period: 600ms at 20kHz, 300ms at 40kHz (1000 cycles/ burst)

Pause between each band: 500ms

Dead-time for push-pull driver: 5µs

Output drive: 250VAC (up to 800V peak-peak)

Low voltage threshold: 11.5V (switch-on voltage = 12V)

Supply Voltage: 11.5 - 16V maximum

Current drain: 220mA average at 12V driving a 3.6nF load

Peak current at transducer resonance: 3A

Quiescent current below 11.5V: 6.7mA

Everyone knows that owning and maintaining a boat is expensive; the bigger the boat, the more expensive it is.

Many readers will be familiar with trailer sail-boats and power boats. These are relatively cheap to run and since they are not left in the water, they should never have problems with marine growth. However, once you have a boat on a swing mooring or tied up to a berth in salt water, marine growth is endemic and the warmer the water, the more severe the problem.

Click for larger image
The driver circuit is housed in an IP65 ABS box with a clear lid. It produces the high-voltage pulsed waveform that's used to drive the ultrasonic transducer.

The vast majority of larger boats in Australia and New Zealand are moored in warm, salty waters and so marine growth is a big problem. In years past, the solution was to coat the hull in an arsenic-based anti-fouling compound but these were highly toxic to all marine life and have now been banned. This means that the anti-fouling compounds used now, while still toxic to marine growth, are far less effective.

The problem is even more severe for boats that are moored in canal developments. There, because the water is much warmer and there is little water movement, marine growth can be so rapid that anti-fouling needs to done as often as every six months.

If a boat is not being used, marine growth can still rapidly take hold and there can be significant growth after only a few months. This is because anti-fouling coatings are “ablative” which means that they depend for their operation on the boat moving through the water to literally wear off the surface and thereby expose fresh (and toxic) anti-fouling compound.

So anti-fouling needs to be done at least once a year and in some cases, more frequently if the boat is seldom used or moored in a canal. If you do this work on your own boat, it is tedious, dirty and expensive (even hauling the boat out of water is expensive). If you pay someone else to do it, it is much more expensive. All boat owners would love to avoid this cost.

Now there is ultrasonic anti-fouling for boats. This electronic method means the end of chemical anti-fouling and a big reduction in cost for boat owners. It involves installing a high power piezoelectric transducer inside the boat’s hull and then ultrasonic energy keeps marine growth at bay.

How it works

The way that this works is that the ultrasonic vibration of the hull disrupts the cell structure of algae and this stops algal growth adhering to the hull. And because there is no algal food source on the hull, larger marine organisms have no reason to attach themselves to the hull – no food, no lodgers.

Click for larger image
The large ultrasonic transducer is driven with high-frequency signal bursts ranging from 19.08kHz up to 41.66kHz.

The principles of ultrasonic anti-fouling have been known for a long time. The effect was discovered 80 years ago by French scientist Paul Langevin who was developing sonar for submarines. By accident, he found that ultrasonic energy killed algae. He was working with high power transducers and it was assumed that cavitation was causing algal death. In recent times though, it has been found that high ultrasonic power and cavitation is not required to kill algae.

Instead, it has been found that ultrasonic frequencies can cause resonance effects within algal cell structures and relatively low powers are still enough to cause cell death. So if the boat’s hull can be vibrated over a range of ultrasonic frequencies, algae will not be able to attach to it and other marine growth will similarly be discouraged.

Commercial ultrasonic anti-fouling systems have been available for the last few years but they are very expensive, costing thousands of dollars to install. There is still a cost benefit though and these systems are gradually becoming more popular as news of their effectiveness grows. However, we should state at the outset that the manufacturers do not make blanket guarantees that ultrasonic anti-fouling systems work in every situation. We can understand that.

It’s this lack of a blanket guarantee that’s probably holding back market acceptance. Most boat owners will be very cautious about investing several thousand dollars in a system that may not work in their case. That is where the SILICON CHIP design will be a game-changer. It will cost a fraction of the price of equivalent commercial systems yet should have the same effects.

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