Last Updated on 24 June 2021 by Eric Bretscher
I had hauled out the sloop Nordkyn for its less-than-annual bottom paint job. It was November, that year had been long, and so was getting the thin stringy weed clinging to the hull. In this condition, the boat is unusable. I was determined to make the effort last and so I generously sprayed the new antifouling in two thick coats. A tinge of irritation showed when, three weeks later, I noticed light green slime already forming on the rudder blade, and six weeks later this had turned into thick heavy slime over the entire hull – in the cold waters of Southern New Zealand. Nothing that the odd dash at 18 knots didn’t remediate when I sailed off in January, but this of course only postponed the disaster. By March, in warmer waters, I was diving underneath the hull and scraping off barnacles in the hundreds.
At this point, a short discussion was up with the paint company, and I politely suggested they had sold me paint from a faulty batch. “Certainly not”, responded their sales manager, an individual who has earned himself here a well-deserved reputation for always blaming the customer, “you simply didn’t apply enough product for it to be effective”. Not enough paint to last three weeks, as if more of a useless product was going to deliver an enhanced result. In fact, I had applied 50% more paint than the previous year. So I objected, only to be told this time that the water was “special” this year, “not our fault”, but primarily around my boat, because others seemed to cope with the special water quite normally. “Aluminium-compatible antifoulings are naturally less effective, there is nothing wrong with it”. As we were about to debate the concept of fitness for purpose in a venue I was going to kindly organise, they shipped me a very reasonable amount of replacement product. Fine.
Still, it took a while to get there, the new paint didn’t apply itself onto the bottom and I had the pleasure of hauling out again. During these lengthy proceedings, the idea of installing an ultrasonic antifouling system for preventing organisms from attaching too readily found a new incentive. These systems are priced like the rest of the marine electronic gadgetry and there seemed to be no clear verdict out about their effectiveness. They always seemed most effective from the manufacturer’s point of view basically. Aluminium hulls however appeared to show consistently positive results. In these circumstances, I looked at what I had to lose in relation with the cost of painting a 43-footer: certainly not much if I built my own system. The project had therefore literally “started in anger”.
Ultrasonic Antifouling: Principle and Limitations
The theory behind ultrasonic antifouling systems is that the vibration induced in the hull plating, and maybe the surrounding water, disrupts cellular growth in organisms trying to attach and develop against the hull. It creates an adverse environment. What these systems don’t do is produce cavitation and a “layer of micro-bubbles” against the hull as some inept claims suggest. That would require the kind of power found in an ultrasonic cleaning tank, 0.5-0.6W/cm2, or a mere 180kW for a vessel like mine, and it would probably strip the antifouling off in a matter of minutes! Interesting scientific research supporting the use of ultrasonic energy to keep marine hulls clean was published in 2016 .
The vibration is induced into the hull using a piezoelectric transducer, at the heart of which are ceramic elements sandwiched between electrodes. When a voltage is applied to the electrodes, an electric field forms through the ceramic, which either expands or contracts depending on the polarity. An ultrasonic transducer attached to a hull produces a longitudinal wave on the axis of the device, which passes through the thickness of the hull and dissipates into the water, as well as a guided shear wave that travels radially away from the transducer. This shear wave is the one that can propagate throughout the hull and installing the transducers on a section of unsupported plating maximises it.
The material and construction of the hull has a significant impact on the ability of the shear wave to travel away from its source, which is enough to explain the variability experienced in the results. Aluminium, being light and extremely rigid, is notoriously good at transmitting sound; steel is excellent too. In these materials, the propagation velocity of the shear wave exceeds 3000m/s . Solid fibreglass normally gives good results too. Timber, on the other hand, absorbs vibrations. Cored hulls are usually deemed unsuitable, unless maybe the core material is removed to install the transducer against the outer skin, but I have no experience with them.
Ultrasonic transducers are designed for a specific resonant frequency and they don’t operate well at all frequencies, but they can resonate to some extent at frequencies other than their design frequency . Research suggests that organisms also react differently with regard to frequency and the standard approach in this application appears to be sweeping over a relatively broad spectrum of frequencies to maximise the chances of success, both in terms of transducer performance and impact on growth.
A design for a single-transducer ultrasonic antifouling driver, engineered by Leo Simpson and John Clarke, had been published in the September 2010 issue of Silicon Chip Magazine, #264. Shortly after, it was commercialised in the form of a kit by Jaycar in Australia and New Zealand at an attractive price when compared to ready-made commercial systems. I had heard some good first-hand reports about it. At 13 metres, Nordkyn would require two transducers however and, rather than installing two kits, I decided to build my own modified version of the Silicon Chip design that would be able to drive two transducers instead of one. It appeared easy enough and much more interesting.
The piezoelectric transducers are capacitive loads driven at voltages of a few hundred volts through high-frequency step-up power transformers in a very conventional arrangement . The transformer winding, the cable and the capacitive transducer form a resonant circuit and driving two transducers required a whole new second power stage. Some of the components used in 2010 were no longer in production, I wished to improve some aspects and, altogether, this led to building similar, but different electronics. Still, I expected little trouble as the starting point was a working design, but the circuit handles quite high peak power levels and this assumption didn’t prove entirely correct. When the time came, I decided to test a single channel first: it is not every day that one can freely opt out of 50% of a potential disaster. The circuit failed promptly and the fuse protested with an orange flash, but this was nothing a few alterations couldn’t overcome.
It has now operated continuously for over 3 ½ years at the time of writing (mid-2018) without any intervention, so I have decided that it must be robust and reliable.
One aspect I was very unhappy with when I reviewed the Silicon Chip article was the presence of soft, rubbery potting compound between the ultrasonic transducer and the hull. Of course, some of the vibration will propagate through, but there is nothing like direct contact between hard incompressible materials to transmit sound. A large amount of efficiency would almost certainly be lost this way. The transducers used in this type of application are primarily manufactured for ultrasonic cleaning machines; in this kind of equipment, they are epoxy-bonded to the outside of the tank wall and literally vibrate with the tank. Gluing the transducers to the hull is perfectly possible and would even be desirable. At the time, I elected to install them in a less permanent way, as the whole venture was very much an experiment.
Some original Jaycar systems showed much improved performance when the potting compound was scraped off the face of the transducer and replaced with a piece of fiberglass board epoxy-glued into place.
As the voltage at the transducer terminals can exceed 800V at times, electrical insulation is essential for safety. On a boat, the transducers are also by definition in the bilge and they need to be completely sealed from moisture. I used synthetic plumbing fittings and polyurethane resin to encapsulate the transducers, similarly to what the article described, but I first bonded a piece of hard fibreglass/epoxy board to the face of the transducer to insulate it and made sure it would come into direct contact with the hull.
I located the transducers by first splitting the underwater hull surface into two equal halves and then determining the approximate geometric centre of each area. This gave roughly 15m2 of hull plating per transducer with the keel and rudder in addition to that. I made sure the transducers were installed in the middle of a hull panel, as far away as possible from the frames and other stiffeners. This prompted mounting each transducer off-centre: the front one is forward of the mast and to starboard and the aft one is behind the engine and to port.
Did It Work?
Yes. I built the prototype presented above in the second half of 2014 and commissioned it in early 2015. In New Zealand waters, we get little creatures we refer to as snapping shrimps because of the sharp crackling noises they make as they crawl under the hull. While they initially dislike the taste of fresh antifouling and always wash off with boat speed, it doesn’t take them long to get comfortable once the boat stops and the music starts. When I powered up the ultrasonic system, their noise started fading away and, after a while, the hull was much quieter, other than for the faint clicking sounds produced by the transducers. It suggested that the vibration was indeed propagating throughout the underwater hull and this appeared positive.
The system has now operated continuously for 3½ years and it essentially eliminated all of the hard growth on the hull: no more barnacles and coral-like formations. Barnacles migrate through the water as tiny larvae, attach to the hull and then start growing a shell. When the water is choppy, I can sometimes see some larvae beneath the waterline; a few days later they are gone. They simply can’t live and develop against the hull plating any more. The only place where I sometimes find a few grown-up barnacles is at the very aft tip of the keel bulb; there appears to be a dead spot there. The effectiveness on the rudder is slightly less, obviously because of the bushes. In particular, the very leading edge seems more vulnerable.
When it comes to algae fouling, there doesn’t appear to be any silver bullet there: the antifouling still has a role to play. Once it has completely worn away, the hull can get colonised by algae and even sponge-like growth, but this is always easy to peel off. I was never able to distinguish any difference in fouling nearby or away from the transducers; the effect over my hull appears uniform. This was not the case on a 66′ aluminium fishing vessel fitted with two Jaycar kits. In this case, the owner told me that growth was visibly reduced over a large radius around each transducer and their effectiveness then faded away with distance. He said his boat needed four transducers. Because the effect on my hull appears uniform, it is more problematic to evaluate performance with regard to algae growth. In the first 9 months or so after painting, the antifouling normally keeps the hull free of weed anyway. However, if I dive and clean the hull once the antifouling is essentially gone, slime won’t form again for at least a few weeks, but only over areas that were 100% clean. Even when neglected, my hull has never become as fouled as I had seen it before, so the system may somewhat hinder algae growth, but this assessment is somewhat subjective. If left long enough without any care, the bottom does eventually end up filthy and this has been the case with all ultrasonic antifouling “solutions” I have directly heard of.
Ultrasonic Antifouling and Antifouling Paint
All up, the system eradicated the barnacle problem and it is immensely valuable to me for this reason. In the absence of hard growth, the surface is easy to clean, stays smooth and it can be recoated with minimal effort each time. It extends the intervals between haul-outs for me provided I dive and scrape off the soft growth from time to time once the antifouling is a year old or so. I experimented with applying significantly more paint to the keel and rudder (it is a moderately ablative formulation) and I must say that it has allowed cleaning underwater for much longer without running out of paint. This old and thicker antifouling is not as effective as a new coat, but these surfaces still perform noticeably better than the areas of the hull left with nothing. Before I installed the ultrasonic system, barnacles would attach to the paint as soon as it lost some of its effectiveness and then the surface couldn’t be cleaned without effectively removing most of the paint in the process and my conclusion was that applying a thick coating primarily benefitted the paint company. This is no longer true. I would undoubtedly get better results again if I could use a cuprous oxide-based antifouling, which is both stronger and longer lasting than aluminium compatible products.
Risks or Harmful Effects
The awareness about the toxicity of antifouling paints for the marine ecosystems has been increasing over the years and – maybe for this reason – people often ask whether such a system is actually environmentally friendly, and safe for other marine life, for the hull or even people living on board!
Risks to Aquatic Life
Dolphins, which use a sonar-like system for echo-location, must almost certainly be able to hear the transducers at least at times, as they operate in the 20-40kHz spectrum, starting just above our range of perception. They still come, play, swim underneath the hull and stay with the boat for long periods when I am sailing, so they don’t appear to mind at all. My view is that the power level involved is too small and the power density in the water too low to be an issue. The peak power may reach 100W at times, distributed over an area of about 37m2 on my vessel, so less than 3W/m2; a small loudspeaker in a portable radio can operate at 350W/m2 or more (60mm diameter cone and 1W output power). I normally turn the system off before diving underneath the hull. I once forgot and realised it when I was already in the water. I decided to still get underneath and approach the hull cautiously on the basis of the above considerations. From inside the boat, I can hear the transducers ticking away if I listen intently, but I couldn’t while underwater and I cleaned the hull normally. I have read claims from commercials that their system would not only protect the boat, but also the surrounding area and even nearby vessels… sure. The power density is just far too small in my experience to have any effect beyond the hull itself.
During the development of the hardware, I sometimes handled active transducers without any effect, until one day when I firmly clamped one of them between the palms of my hands. All I could sometimes feel was the odd vibration accompanying the faint ticking noise as the ultrasonic bursts started and stopped. A day or two later, I noticed a dark circular spot in my palm in the exact area that had been in contact with the face of the transducer. The best way I can describe it is like a dark brown grease stain that hadn’t fully washed off. It wouldn’t wash off or even scrape off, because it wasn’t on the skin. I believe the ultrasonic energy broke the capillaries underneath the skin and caused superficial internal bleeding. It took weeks to fade away. Probably not a great idea. The transducers emit short bursts and a lot of them are at frequencies where the energy output is quite low, but they do punch out some power at times when they hit a resonance peak. Such a transducer continuously driven at high power would most likely be able to cause soft tissue injuries.
Risks to the Hull
A friend, retired engineer, got interested in such a system for his alloy yacht, so I gave him the Silicon Chip article to read as background material, but he eventually decided against it. His concern was that the vibration induced in the hull would cause metal fatigue and cracking. I disagree with his conclusion for reasons I will develop shortly, but the thought process he followed is correct. As there would be no more evidence to either support or rule out hull damage like cracking or delamination from ultrasonic energy, if such hull damage was found, its root cause could probably be debated with no end if someone decided to blame the system for it.
The amplitude of the vibration produced by the transducers is measured in nanometres: millionth of a millimetre. It appears insignificant when compared to the vibrations induced by engines and wave impacts and I know of aluminium passenger ferries that have logged in excess of 30 years of continuous commercial service. Their hulls haven’t fractured into little pieces, so I don’t share the concern that sound waves propagating through the plating can cause sufficient stress to induce cracking. Stainless steel ultrasonic cleaning tanks don’t appear to crack either in spite of comparatively massive ultrasonic energy levels. In ultrasonic antifouling applications, the power level is just far too low to damage materials in my opinion and there is no record of hull damage that I am aware of.
I am not aware of any scientific impact studies for hull antifouling applications. A few papers have been published about the effects of ultrasonic energy on algae in suspension in the water and they indicate that prolonged exposure indeed damages them, but the power levels used were high in comparison and the algae was contained in a tank, not drifting. It appears to be a rather clean and low-impact technology.
At some point after I built the prototype in 2014, Jaycar appeared to have discontinued their ultrasonic antifouling kit. People who had originally bought the Jaycar kit needed to build the transformer and encapsulate the transducer. Later, the kit apparently shipped with those items ready-made, albeit at a higher cost. Some buyers may have experienced difficulties with assembling it. Winding the transformer was not difficult, but it required care and attention, because mistakes almost invariably resulted in destroyed transistors on the circuit board. The construction of the transducer certainly was at the level of many hands-on boat owners and soldering the board wasn’t any different than building any other electronic kit.
In any case, Silicon Chip design revised their 2010 design in May 2017, with a number of small improvements, as well as the much needed ability to drive a second transducer as a separate option. This is to the benefit of Jaycar only, as the details published would no longer be sufficient to construct a functional unit. Both the transformers and the transducers are supplied ready to be installed. I was disappointed to see that the transducer encapsulation had not improved; it has obviously worked sufficiently well as it is. The driver board is not as powerful as the one I built, because they are still getting away with the very feature that had caused my original prototype to blow up near-instantly. A notable difference between the revised Silicon Chip design and my board is that they drive their transducers alternately, not simultaneously. This reduces the peak current draw, but also eliminates interference effects. These effects can be cumulative or cancelling, but the constant frequency shifts can be expected to move the positive interference zones over the hull. As a result, I tend to think that synchronous drive should increase the local peak power levels in the hull, especially in the region half-way between the transducers. The fact that I never observed any dead zones over the hull surface would support this thinking.
As I recently happened to come across a very rare commodity referred to as spare time, I used it to revisit this old project. I updated the design to use mostly surface-mounted components (SMD) this time, ordered professionally-made circuit boards and built a few very nice-looking new units. These have proved to be quite sought after and I am curious to get feedback from a broader user base.
As the new board is much more straightforward to reprogram in terms of operation, I have started thinking about trying to improve the performance of the system with regard to fighting algae growth.
 “An Acoustic Antifouling Study in Sea Environment for Ship Hulls using Ultrasonic Guided Waves”, Habibi, H., Gan, T.-H., Legg, M., Carellan, I., Kappatos, V., Tzitzilonis, V., & Selcuk, C., in International Journal of Engineering Technologies and Management Research 3 (4), 14-30, 2016.
 “Ultrasonic Transducer Technical Notes”, Olympus NDT, 2006.
 “Power Converters Design and Analysis for High Power Piezoelectric Ultrasonic Transducers”, Davari, P., Ghasemi, N. and Zare, F., in Power Engineering Conference (AUPEC), 2016 Australasian Universities (pp. 1-5). IEEE
 “Power amplifier for ultrasonic transducer excitation”, Svilainis, L. and Motiejūnas, G., in Ultragarsas, Nr.1(58), 2006.