Oct 072018
Water-blasting yields a clean and readily-paintable surface.

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In July, I wrote about my experience with a homebrew twin-transducer ultrasonic antifouling system over 3 1/2 years on the aluminium sloop Nordkyn. It didn’t keep the hull clean once the antifouling reached past its prime, but the results were nevertheless very valuable as the system eradicated hard growth like barnacles on the hull, even in the absence of any antifouling. Not having to scrape off hard growth significantly extended the life of the antifouling and made hull cleaning much easier.

I since altered the firmware of the ultrasonic driver module to output a higher average power level and also reach higher frequencies in an attempt to disrupt algae growth, while I had virtually no antifouling paint left over large parts of the hull.

An Approximate Baseline

I had dived on 7 April and peeled off a thick carpet of weed and sponge-like growth which had accumulated since the New Year in Oamaru Harbour in the South Island. The harbour basin is small and the water had reached 25°C at times during the summer. Following this, I returned to the Auckland region via Cook Strait and the West Coast of the North Island before rounding the top and coming back down on the East side, covering about 1100NM. I stopped along the way for about a week in Nelson and then again in Whangaroa Harbour. By 26 April, I was sailing off the Bay of Islands and the boat performance had noticeably reduced already. Following this, I covered the short remaining distance more leisurely and arrived near Auckland on 6 May. By early June, sailing was hardly a proposition due to the state of the hull and I was forced to dive again on 29 June.

Date Description Days since last cleaned
7 April Dived and cleaned the hull  
9 April Sailed off for 1100NM passage 2
26 April Noticeable reduction in performance 19
05 June Sailing is slow and difficult 59
29 June Heavy algae fouling found when diving again 83


In short, the table above shows that it took less than 3 weeks to experience a slow-down due to fouling in spite of covering close to 1000NM in this period. After 2 months, sailing had become problematic and I was forced to dive again less than 3 months after last cleaning.

Firmware Alterations

There is no published scientific research that I am aware of related to the effect of ultrasonic energy in marine antifouling applications. Some research was conducted on the effect of ultrasonic energy on microscopic algae suspended in water [1, 2 and 3] and it showed that:

  • Exposure to ultrasonic energy is capable of causing cellular damage to algae;
  • The effect appears related to cumulative power exposure: it takes longer to achieve the same results at lower power levels;
  • Higher frequencies are more effective.

My system had so far operated like the Silicon Chip design which had inspired the project. The Silicon Chip firmware used frequencies between 19kHz and 41kHz and emitted tone bursts of 1000 cycles. 1000 cycles at 20kHz results in a tone duration of 50ms, which reduces down to 25ms at 40kHz. As output power increases with frequency, this strategy curtailed the amount of power delivered in the higher frequency bands. This hardly seems satisfactory in the light of the above. As a result, I rewrote the code to produce frequencies ranging between 19kHz and 65kHz this time and discovered additional resonance peaks above the nominal 40kHz frequency of the ultrasonic transducers. I also implemented a fixed tone duration scheme, so higher frequencies would not no longer be penalised. This clearly increased the average output power of the system and I had to improve the power management features so the consumption of the system could adapt to the energy available.

The Experiment

The hull badly needed to be repainted, but I wanted to take advantage of these unfavourable circumstances to try and improve performance with regard to algae growth. I was annoyed by the rate at which soft growth covered the hull again after cleaning and I wanted to attempt some alterations to the firmware of the ultrasonic system. By June, I had the new ultrasonic driver in operation, but it was still functionally the same as the prototype unit I had operated since 2014. I dived and cleaned the hull on 29 June and the experiment started about two weeks later, after I commissioned the new firmware on 12 July. As a result, it had to contend with about two weeks of “baseline” fouling. By then, the rudder was covered with sufficient light weed to mask its colour and long strands had attached to the trailing edge.


In the space of a few days only, the blue of the old antifouling seemed to become increasingly apparent over the rudder blade, which was promising, because the rudder is separate from the hull where the transducers are installed and the vibration must travel though the rudder bushes. The rudder gradually shed most of its algae growth over the areas which had antifouling left. The upper part, near the waterline, had no paint left and remained dark. The long strands of weed gradually vanished.

The last paint job was nearing 2 years old. The keel and rudder were in better shape as, three-and-a-half years earlier, a friend had brush-painted them a couple more times while I was rolling antifouling over the hull. Some of this thicker coating still remained and any amount of antifouling, even that old, is still preferable to none.

In September, I started making arrangements to haul out. These fell into place very suddenly and I departed for Tauranga, 120NM away, on the 14th. I had been moored for 6 weeks and intended to dive and clean before leaving, but a good 15-knot following wind sprung up and I couldn’t resist trying to use it. While I had thick long weed around the waterline, it didn’t appear to extend underneath the hull. The rudder looked relatively clean, I surmised that the keel would be the same and so I decided to try sailing and stop in a bay along the way to remediate if necessary. Not only this might save me diving, but it would also allow taking photos of the dirty underwater hull once out.

Tacking out of the anchorage under main alone was somewhat tedious, but I then reached the wind and picked up speed immediately to reach 5-6 knots to my surprise. I sailed 50NM on that day, and 70NM the following one in much lighter conditions with an asymmetric spinnaker. I even exceeded 8 knots at some point. The boat was clearly slow by a good 2-3 knots, but it would have been near-unmovable before.

Date Description Days since last cleaned
29 June Dived and cleaned the hull  
12 July New firmware commissioned 13
14 September Sailed off for 120NM passage 77
20 September Hauled out 83


Coincidentally, I ended up hauling out after the exact same 83-day interval that had separated the previous two dive-and-clean jobs. The 29th of June being very close to mid-winter, the same conditions of light and water temperature prevailed both times. Yet, 77 days after diving and cleaning, this time I had still been able to sail quite reasonably. As the growth appeared to have receded following the alterations made to the ultrasonic driver, I am inclined to think that it was being controlled to an approximately constant level by the system in the prevailing conditions of water temperature and sunlight.


Needless to say, I was extremely curious to discover what the underside of the hull looked like and I had a camera ready when the boat was lifted out. As anticipated, the long thin stringy weed at the waterline didn’t extend further underneath the hull. While the bottom was mostly filthy, the growth was surprisingly thin and short, which explains why I was able to sail. When I had last dived and cleaned after the same period of time, I had peeled off a layer of underwater carpet containing sponge-like growth. There were none this time.

The difficulty in protecting the waterline from slime and algae is a known limitation of ultrasonic antifouling systems. It may be that the ultrasonic energy cannot be transmitted effectively into the algae close to the free-surface. It is interesting to note that barnacles still can’t develop in that area however.

Bottom growth after 83 days with modified antifouling system.

The bottom is only covered by thin short weed interspersed with isolated leaves and tufts. The waterline is showing long stringy algae all around. No hard growth and no sponge-like growth are present.

The keel and rudder had benefitted from a thicker antifouling coating in the past and fared better than the underside of the hull for this reason. The keel bulb in particular had been heavily coated three-and-a-half years earlier. This light-blue paint is an aluminium-compatible ablative antifouling. This suggests that coating the hull more heavily should allow skipping a haul-out by periodically diving and cleaning underwater. The combined feedback I recently received both from shipchandlers and boat owners converged to suggest that this product is also one of the poorest aluminium-compatible formulations on the market at the moment and, this time, I changed to another product.

Thin algae fouling over keel bulb and foil.

The keel bulb was quite heavily antifouled three-and-a-half years earlier and some of this old paint is still present. Note the absence of any shells and the difference with the leading edge of the keel and some areas of the foil, which are down to the dark blue 12-year old base coat keyed in the high-build epoxy paint.

The rudder also showed significant residual antifouling covered by sparse weed. Once out of the water, the weed clings to the surface and the blade appeared more fouled than I was expecting, but the growth is thin and sparse. A better paint or a light wipe of the surface in the water would easily improve on this result. Of interest is the fact that there is no slime at all: the surface either has weed attached, or is clean. If slime is, as often claimed, a necessary precursor for heavier fouling to take hold, then starting with a clean hull could have made a considerable difference, but this remains to be verified.

Algae growth on rudder blade with modified antifouling system.

The rudder looks much worse out of the water than it did while still immersed. Yet the weed is staying very thin and sparse, when it had fully covered the blade only 19 days after underwater cleaning with the original ultrasonic firmware. The light blue paint is three-and-a-half year old antifouling. The dark blue paint is 12-year old antifouling keyed in high-build epoxy at construction. The white areas near the trailing edge are bare high-build epoxy paint. Note the complete absence of slime.

Waterblasting was easier and quicker than ever, because the algae growth was so thin this time. As usual, it produced a smooth and clean surface that could be painted without any further preparation. This has been the case since the ultrasonic system was installed.

Water-blasting yields a clean and readily-paintable surface.

Waterblasting the hull yields a clean surface that can be painted immediately as there is no hard growth at all. The dark blue paint is a semi-hard aluminium-compatible antifouling that was keyed into the high-build epoxy coating during the construction of the hull 12 years earlier.


The experiment didn’t start with a clean hull and, as a result, the modified ultrasonic system wasn’t able to oppose fouling from its inception. The antifouling paint was also past the end of its useful life and large areas had none left. In this regard, the system was put to the test in challenging conditions. On the other hand, the aluminium hull material is probably optimal for such an experiment and the test was conducted in winter when the water is colder and sunlight hours limited. It is very possible that heavier fouling would have resulted in summer.

The alterations to the firmware of the ultrasonic driver produced very impressive results in terms of reducing the thickness of the algae growth. Considerably less algae variety was also present underneath the hull this time. It is interesting to note that, in spite of the presence of the invasive fanworm algae in northern New Zealand waters, I never found any on my hull, even when other vessels moored nearby were being severely affected. If fanworm happened to be sensitive to ultrasonic energy, it would be a valuable piece of knowledge. At the moment (2018), the “fanworm-scare” is being exploited by lobbyists in the local marine industry to have frequent, indiscriminate and costly haul-outs mandated for boat owners in some areas.

The code changes to the ultrasonic driver had three effects:

  1. Increasing the maximum frequency used by the system from 41kHz to 65kHz;
  2. Increasing the amount of energy allocated to the higher frequencies;
  3. Increasing the overall average output power.

In order to differentiate the effect of frequency from the increase in the average delivered power, the test would have had to be staged and a lot more time would have been required. Unfortunately, this didn’t sit very well with my desire to haul out and eliminate the need to dive and clean repeatedly due to the lack of antifouling paint left before the summer. The research mentioned earlier also indicated that the best results should be sought by increasing both the frequency and the energy exposure.

The performance of a properly built and installed ultrasonic system should be a function of:

  1. Its peak power output, as peak power determines the “reach” of the system over the hull surface, as well as the maximum amount of damage it can potentially inflict to marine growth on the hull surface;
  2. The average power delivered, as it seems that damage from low-power ultrasonic energy results from cumulative exposure over time;
  3. The frequencies used, possibly because higher frequencies tend to be intrinsically more energetic, but it may also be that shorter wavelengths are able to excite damaging resonance effects in algal cells.

The table below summarises the consumption of the system with the original firmware as well as the new one, in its two modes of operation. In this test, the new firmware only ever operated in its low-power mode, due to the limited solar energy available at the time. In summer, it will switch to the higher output at least for a part of most days.

  Average Power
Energy Consumption
Original Firmware 5.9 10.7
New Firmware (Low Power) 11.0 19.8
New Firmware (High Power) 14.7 25.6


The firmware changes nearly doubled the average output power of the system, and its consumption. The figures were measured at the battery voltage available at the time, around 13.3V, except for the high power readings which were taken at 13.75V. It is interesting to observe that the figures obtained with the new firmware now significantly exceed the output of many high-priced commercial offerings. If we add to this that many of these systems are installed on hulls that are unlikely to transmit ultrasonic energy as well as an aluminium one and some of the transducers are poorly constructed and installed, it may just be enough to explain why so many ultrasonic systems have been found to be largely ineffective by boat owners. For the sake of guidance, the figures reported above relate to a total underwater area just exceeding 37m2.


The original ultrasonic system allowed me to reach a record two years between haul-outs this time and this limit was largely the result of the hull running out of paint. This time, I applied 50% more of what should hopefully also prove to be a better antifouling paint, with ample drying time between coats as allowing the solvents to fully evaporate is a key critical factor in achieving longevity. I am hoping to reach three years before having to haul out again, provided the new coating doesn’t ablate away too quickly, and four years would represent an exceptional result.

While still providing a degree of foul-release, the silicone coating on the propeller was in rough shape after two years. This can’t be renewed easily without hauling out properly due to the long drying time. The propeller shaft anode could be replaced underwater or by drying up in a suitable location.

The alterations made to the ultrasonic driver firmware appear to have resulted in significant performance improvements with regard to algae growth. Operating the system starting from a newly painted hull should provide a more meaningful answer as to its long-term ability to keep a hull clean, if this is at all possible. Further increasing the output power of the system as well as the operating frequencies would be possible with alterations to the hardware and it would likely further increase effectiveness; however, the need for such developments should first be demonstrated and the power consumption would also quickly become prohibitive for most vessels without access to shore power.

Hard growth was readily eliminated by operation in the lower frequencies at extremely modest power levels, as an average of 0.16W/m2 of wetted surface was already enough to be fully effective. This begs the question of knowing whether keeping these lower frequencies is necessary at all. If operation at 40kHz and above disrupted the development of both hard and soft growth, then a reduction in power consumption could conceivably be achieved by optimising the frequency spectrum to only allocate energy where it is also most effective. While I have sketched schematics for a new and different ultrasonic driver, it may now be some time before I investigate the matter further, at least using my own yacht.


[1] “Effect of ultrasonic frequency and power on the disruption of algal cells”, K. Yamamoto, P. M. King, X. Wub, T. J. Mason and E. M. Joyce, (2015), Ultrasonics Sonochemistry 24, 165–171
[2] “Effect of ultrasonic frequency and power on algae suspensions”, E. M. Joyce , X. Wu and T. J. Mason (2010), Journal of Environmental Science and Health, Part A, 45:7, 863-866
[3] “Ultrasonic Irradiation for Blue-Green Algae Bloom Control”, T. J. Lee , K. Nakano and M. Matsumara (2001), Environmental Technology, 22:4, 383-390

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