Ultrasonic Antifouling ~ Everything You Need to Know

Introduction

The sea is a soup of water and micro-organisms which stick to boats and boat parts and then attract larger organisms. As the layers build they cause damage and create a mass which interfers with the free flowing and function of water across the hull of all vessels of all sizes and types, into water coolers, and sea chests on commercial vessels. Note, the beginning of this problem is tiny and easily removed with a brush, but it quickly gets worse within weeks and months.
We all know antifouling paints don’t work very well, but many boat owners do the same dance every year: lift it – scrape it – sand it – paint it – sand it – paint it – sand it – antifoul it – drop it back into water – pay for it – try to forget about it. Commercial vessels have a more difficult time since they often need a dry dock and the size of the boat means the job is much bigger. A ‘ticket’ to this ‘annual show’ is many $£€100’s – 10,000’s each time. Indeed, the word ‘antifouling’ has become short-hand for ‘antifouling paint’ because some think that paint is the one and only option. Most paints contain or cause ecological harm.

There are now alternatives to painting boats with biocides. Ultrasonic antifouling is a effective and cost effective, sustainable (non-polluting) solution for antifouling for all sizes and types of vessel.

biofouling splash

Biofouling

The menace of biofouling starts with the tiniest of creatures which are ubiquitus in the sea. The sea is a living soup of organisms which are attracted or settle on any underwater surface. Biofouling starts with the accumulation of aquatic microorganisms, also referred to as bio-film. As the aquatic microorganisms accumulate and grow, they create a healthy habitat and food source for larger marine organisms, plants, animals, and others attracting them to the surface as well. This is a food chain with the biofilm and algae at the start.

Current Antifouling Methods

To stop this biofilm forming, antifoul paint is often applied to boat hulls. The creatures die upon contact, find it difficult to attach, or otherwise drop off, as long as there is still active antifoul paint to leach into the water around the boat. To be very effective, antifoul paint generally needs to be very toxic. In fact, it’s nasty stuff generally; the application to a boat hull normally requires protective clothing and respirators. Not just a one-off task, this needs to be done every 10 to 18 months for most boats in most seas.

Effective antifouling is like most problems, deal with it early before it becomes a bigger problem. The most effective way to deter biofouling is to start by eliminating the first stage of the marine food chain, the bio-film.

An ultrasonic antifouling system aims to make a boat’s hull an uninhabitable environment for the bio-film. Eliminating the environment where microorganisms live and thrive as early as the initial attachment stage prevents dispersion and subsequent colonisation.

Simply, stop the biofilm, the algae, and this cuts off the entire food chain of events that worsen to the point of hardened barnacle growth. Stop this early to prevent later issues. It’s more a feather duster, frequent, and often, rather than an intense jet spray.

Antifouling is the positive outcome that all boat owners desire i.e. no fouling, just a smooth clean hull, keel, rudder, propellor, and any other underwater surfaces and intakes. Boat owners want a clean hull for vanity reasons but also to ensure their boats travel with minimum drag through the water. Biofouling is a massive costly problem for the marine leisure and transport industries causing higher fuel consumption, greater maintenance costs, blocked cooling water intacts, and downtime to clean and fix these. The ‘foul’ being masses of marine life firmly attached to a boat’s hull, sea chests and box coolers on ships, unsightly and is costly to remove and keep removed. Biological invasion is pervasive all over the world’s oceans. As a marine vessel moves through the water, small marine animals, plants, and other aquatic pest species cling to the submerged surface and move with it. The more aquatic species attach to the surface below the waterline like the ship’s hull, propeller, and pier pylons, the more they accumulate, grow and multiply. Marine life rapidly degrades a boat hull’s condition and ability to pass through the water efficiently. The problem affects all types of vessel from pleasure craft to commercial ships which are left in the water longer than a day. Kayakers and small launched craft will also be aware if they leave their boats in the water for more than a day or two, things start to attach to the hull. Larger boat owners are very aware of the issue, but most choose to turn a blind eye to what is happening beneath the water surface.

Biofouling has been such a problem for so long that it’s almost a tradition for many boat owners to lift their boats at the end of each season and arrange for the remedial attention to the hull. Eventually, almost all boat owners need to do this or face even bigger maintenance costs or damage to their boats. Some will simply put it off as long as they can despite the ticking time-bomb. What is underwater is out of sight, yet eating away at the hull and the meals that hang from it.

Biofouling and existing antifouling paint is a double kick to the ecology. Biofouling causes greater fossil fuel use on boats (with CO2 emissions), plus often dumps long lasting biocides into our environment.

In boat, yacht, and sailing magazines you will often find articles on ‘…the best antifouling paints…’ Never is the review impressive or conclusive because for antifouling paint to be very effective it needs to be very toxic, and that just is not a great thing to mix with our marine environment. Before long, the toxic paints will be outlawed by world governments, but that hasn’t happened because there has been no widespread econmomical proven alternative adopted. Yet, the biocides are long lasting, stopping their use today will not clean up the biocides already in the marine climate. No matter how the recipe is amended, the purpose of antifouling coatings is to kill and stop marine life attaching to the hull.

You can find a handful of solutions on the market that can help you effectively combat this problem. Most often, boat owners use coatings and antifouling biocide paints to deter marine growth. Coating the bottom of your boat with a combination of ultra-slick or self-polishing surfaces and toxic chemicals can help prevent aquatic organisms from attaching themselves to the submerged surfaces. Eventually, these paints and coatings wear off and the boat needs lifting and the whole hulls needs attention once more. Often this is a yearly exercise for many boat owners and in that year the biocides are leeching into the water.

The idea of marine biocides actually can be attributed to the British invention in the 18th century. A 250-year-old solution is long overdue for innovation especially now the harm that biocides do to the environment has been recognised.

Biocides are a ‘sledgehammer to crack a nut’, a ‘nuclear weapon’ rather than a ‘pillow fight’ that it should be. Biocides are indiscriminate, they kill the plants, the animals that eat the plants, and the animals that eat the animals. Let’s be very clear, the root cause is not a difficult issue. A (leisure) boat owner, or their diver, could simply remove the algae, from an otherwise clean hull, with a soft brush each month, but many boat owners won’t bother to do that. But again, many boat owners won’t do that either because of the cost. Algae is the start of the food chain. Algae is the achilles’ heel of boat owners and operators.

There are now alternatives to biocides, like ultrasonic antifouling which can completely inhibit marine fouling.

biocides are a sledgehammer to crack a nut a nuclear weapon

"A Copper Bottomed Guarantee"

In 1761 the British Royal Navy addressed the issue of worms eating the wooden hulls, by affixing copper plates to their vessels [Reference: wikipedia.org/…Copper_sheathing]. The copper produced a poisonous film, composed mainly of copper oxychloride which inhibited the worm’s growth, and thereby spared the ships. That invention lead to the phrase ‘a copper-bottomed guarantee‘ meaning something that worked well and was trustworthy [Reference: Phrases.org https://www.phrases.org.uk/…]. It was Sir Humphrey Davy who finally identified that the reason for the antifouling from copper sheets was due to the dissolution of copper into seawater. That same concept of using copper as a biocide, all be it in liquid form, is the major means of hull preservation today. In a strange twist of English language, copper, therefore, has a good name, yet the copper plates of 1761 are much different to the copper-based (or other) biocides of 2021.

When iron ships came along in the 18th century, biofouling once again returned since iron does not have the same toxic effect on bio-organisms, and so once more the search was on for new antifouling coatings.

Sound & Ultra-sound (Ultrasonics)

Sound, is generally invisible to the naked eye, but can be observed with microscopic and slow-motion photography; it can even be felt when surfaces creating sound are touched or body parts reverberate at certain frequencies. Our ears feel sound that is at an audible frequency. Sound, can be considered as the energy that physical objects have when they are vibrating i.e. colliding into and stretching away from each other. Though we talk about sound waves, these are not like the curving waves of the ocean. Sound waves are more like pulsing objects bouncing against one another causing random movements in those objects as well as directional movements, in a series of high, low, positive and negative pressures. In other words, sound is a pressure wave, the force of one object pressing against or pulling away from another that moves that other object in a series of collisions and repulsions. The vibrating or pulsing causes positive, and negative pressure on the objects affected. Squeezing and pulling. Sound is the travel of energy through a medium such as air, water, steel, biological cells, muscle, wood, anything (but not a vacuum). Sound only travels through mass or mediums. At audible frequencies these movements affect our ears and are then converted to electrical signals which are recognised by the brain.

Sound Frequency

TThe speed of vibrations is known as the frequency; how many peaks of pressure occur in a period of time. The degree of movement of the vibration is dependant on the intensity of the pressure, known as the loudness in audible terms.

he sound frequency spectrum is divided into three major ranges: infrasound (lower frequency sound), sound (human audible), and ultrasound (in-audible to humans). Ultrasound (or ultrasonic) is simply sound with a high frequency, higher than audible to humans. Ultrasound is not physically different from audible sound, except in that humans cannot hear it. Human hearing functions from 20 Hz (20 pressure waves or vibrations per second – a deep low rumble) as long as of adequte energy (e.g. normal breathing is around 10 decibels to around 16kHz to 20kHz (20,000 pressure waves per second – a high pitch whistle). Just think for a moment how quiet human breathing is, definitely audible yet definitely on the low end). Normally younger people can hear higher frequency sound than older people.

Ultrasound devices operate with frequencies start from 20 kHz up to several gigahertz, and is inaudible to humans.

Infra-sound

  • (to be updated)

Sound

  • Bumble bee @ 10Hz
  • Middle C @ 256Hz
  • Mosquito @ 1.5kHz
  • Grasshopper @ 7kHz

Ultra-sound

  • Bats @ 70kHz

The diagram below shows the frequencies of these on a logarithmic scale

Sound Intensity

Sound has kinetic energy and potential energy: objects are moving or have the potential to move. Sound intensity is defined as the sound power per unit area, measured in watts/ m2 measured in decibels.

Decibels (dB) are named after Alexander Graham Bell, the inventor of the telephone and audiometer.

A deci-bel is 1/10th of a bel. A decibel is not a unit of loudness, it is a ratio comparing one number to another, and the scale is not linear, it is a logarithmic scale where

+3dB is 2 times the power,

+10dB is 10 times the power,

+20dB is 10,000 times the power, and

+60dB is 1,000,000 times the power.

The total energy in a specific sound can be measured but normally sound is reported as an intensity compared to a reference. The energy of sound is defined as power (energy per unit time) per unit area. The units of sound intensity are Watts/ m2 (watts per square meter) or per cm2 Sound meters measure “Sound intensity level” expressed in decibels and compared to a defined standard of one picowatt/ m2. (In standard units, total energy is measured in joules, power in watts, intensity in watts per square meter or decibels.)

Here are some familar noises in the audible frequency range:

Sources of soundIntensity (W m-2)Intensity level (dB)
Nearby jet airplane 103150
Fast train, Threshold of pain!101130
Siren100120
Lawn mover, Jack hammer10-2100
Vacuum cleaner, Listening to radio or watching TV10-570
Mosquito buzzing10-840
Whisper10-930
Rusting of leaves, Normal breathing10-1110
Faintest audible sound10-120

Just as audible sound is pleasant at some volume, so it can be unpleasant and dangerous at other levels. Sound can harm, as we all may know from spending time in noisy night clubs. Both the intensity of the sound (the loudness), and the length of time affect the damage done. Those persons who use noisy equipment will also need ear defenders to protect their hearing.

The degree of damage, impact or affect of sound on another object depends upon both the:

1. Source of sound:

  • intensity (energy, loudness, volume, amplitude, sound pressure
    levels) of the sound source when it reaches an object. (Objects further
    away receive lower intensity sound.)

and

2. The object in the path of the sound waves:

    • the material & density e.g. air, water, steel, biological cells
    • the object’s structure (which can lead to resonance, reflection, absorbtion)

and

3. Time

  • the duration of the sound (time)

For human hearing, intense sound will cause damage at the level and time stated below, notice this is regardless of frequency, this intensity at any frequency will cause harm. It is the intensity not the frequency that causes harm.

    • 85 dBA of sound (e.g. loud music) if you are exposed to it for at least 8 hours
    • 100 dBA (e.g. an alarm or very loud music) and you are exposed to it for at least 14 minutes. Check the chart above, imagine 14 minutes of constant ‘fast train’ noise in your ear.
    • 110 dBA (e.g. a siren) and you are exposed to it for at least 2 minutes
References:

Resonance: Watch the amazing “Gallopin’ Gertie” November 7, 1940 film clip which shows this fabulous bridge destroy itself during a windstorm. Like a piano wire being plucked the bridge builds up an oscillation that has so much energy the materials break. The sound is very low frequency yet has enough energy to damage and destroy the bridge.

This device is very familiar to most people. Objects collide with one another and spring back, this is just how sound travels through a medium. Sound affects physical objects in a similar way. Imagine one ball was the hull of a boat, and another ball were a water molecule. Watch the different patterns of resonance e.g. from time 2 minutes 35 seconds onwards. Imagine these as cells in a marine organism and the frequency of movements is ten-thousand times greater.

Resonance: Speaks for itself. The boy resonates the glass, and continues to do so probably at the exact resonant frequencies of the glass which causes so much vibration that it fractures. It’s not ultrasound, but demonstrates how sound waves can cause damage.

Demonstration of how gentle ultrasound can be even to blow out a candle.

Though the frequency of this plate is not ultrasonic it nicely demonstrates how particles will be physically moved to parts of the plate. Try to imagine the ‘hills’ between these streams of sand. The hills are the higher pressure areas that exclude the sand from all sides. The shapes are just due to the shape of the plate and frequencies. Imagine if this were on a microscoptic level and the (empty) spaces were just where there were cells of an organism, it would experience this pressure in that place. Ouch! Depending on the intensity, and duration, this could harm the organism.

Ultrasonic Applications

There are many applications that ultrasound technology has, for instance:

  • welding (e.g. metal and plastic) [see video below]
  • medical imaging (i.e. sonography, viewing a fetus in the womb) [see video below]
  • cleaning ( e.g. jewellery in a sonic bath) [see video below]
  • rodent repellants (e.g. garden cat repellant),
  • echocardiography (viewing the heart),
  • avoiding objects through echo location (i.e. bats and submarines)
  • sonar (i.e. use of reflected sound waves to measure depth)

Some animals make ultrasonic noise whilst others listen to it e.g. insects may listen to the ultrasonics of bats to avoid being eaten. Baleen whale sounds range from 10 Hz to 31 kHz. [Reference: https://en.wikipedia.org/wiki/Whale_vocalization]

Below are some interesting videos that demonstrate types of ultrasound.

Ultrasonic welding causes so much vibration and then heat that materials fuse together into a united material.

Sonography, using the reflections of high-frequency sound waves to construct an image of a body organ (a sonogram); commonly used to observe fetal growth or study bodily organs.

Ultrasonic cleaning baths can be used to clean fine jewellery such as diamond rings. Here the ‘dirt’ is rapidly physically detached through the vibrations and microscopic pressures in the liquid.

Treating abcesses with ultrasound in a non-invasive manner.

Cleaning ponds with ultrasound

Killing bacteria with sound (sadly no details to support the title)

Measuring Flow with an ultrasonic flow meter.

Water and sound: super strange effects

Music & Water: super strange effects 2

Ultrasonic Antifouling

Ultrasonic antifouling technology produces high-frequency soundwaves that are transmitted through rigid surfaces. These soundwaves are delivered to a marine vessel via a series of transducers. These transducers are connected to a fully automatic control unit and are attached to the inner part of the bilge. Since the transducers are simply attached to the bilge, there is no need to drill through the hull. A control unit is used to generate signals within the frequency bands. It is intelligently designed to provide optimum power and to allow maximum cleaning efficiency. Effectively, the transducer is tapping & vibrating the boat’s hull like a drummer beats a drum.

Often, the frequencies used are varied across a spectrum, since the organisms also vary in size and type across a broad spectrum. In order to have the most effect, these frequencies are played across the surface from deeper to higher (ultrasonic) notes in order to deter, upset, & kill as many of the initial organisms as possible.

The rhythmical oscillation of the transducers simultaneously generates alternating positive and negative pressure patterns in the water and the organisms which are also water-based. Depending on the intensity the situation may be as in ultrasonic welding where such high pressure and heat is caused at a microscopic level that the water becomes unable to support life, or at lower intensity the resonance disrupts the functions of organisms such they can not live. (Imagine a loud deep sound incident on your heart, you can believe that though the sound itself is doing no damage, the upset it causes to your heart’s function would cause damage). As the rhythmical oscillation circumambulates the perimeter of a boat’s submerged surfaces, it builds a microscopic shield or barrier that protects the hull from the bio-film. The frequencies and high-pressure space along the hull are too strong for the microorganisms to survive and pass through. This makes ultrasonic antifouling technology effective in eliminating existing biofilm, preventing further growth, and in blocking microorganisms in the water from passing through and attaching themselves to a surface.

The secret to successful application of ultrasonic technology is in selecting the appropriate frequency and intensity as per the examples above.

12 month marine test from one manufacturer of an ultrasonic hull cleaner.

Nice introduction on the technlogy which was first noticed by the US Navy.

Frequencies Used in Ultrasonic Antifouling

Ultrasonic antifouling technology produces soundwaves in different frequencies. Each range affects biofouling in different ways. The range of ultrasound frequencies for antifouling is as follows:

    • 20 kHz to 40 kHz
      This frequency range is designed for hard fouling. The signal pulses produced in this range can help deter the adhesion of barnacles, tube worms, molluscs, and other similar organisms into a boat’s hull surface. Hard fouling includes making the hull surface virtually slick, thus preventing organism attachment.
    • 40 kHz to 60 kHz
      This frequency range is used for soft fouling. This range produces signal pulses that are enough to alter the cellular structures of the biofilm, hydroids, seaweed, and algae. Soft fouling is also used to disrupt the reproductive and dispersal cycles of these aquatic organisms.

References:

An ultrasonic transducer installed on the inside of a hull.

Photo courtesy of Ultrasonic Marine ultrasonicmarine.co.uk

An ultrasonic transducer. When attached to a surface it vibrates that surface at a high frequency.

Photo courtesy of Ultrasonic Marine ultrasonicmarine.co.uk

An ultrasonic control box. It sends the signals to a transducer of a defined frequency.

Photo courtesy of Ultrasonic Marine ultrasonicmarine.co.uk

Instead of ‘speaker’ imagine it’s the boat hull being vibrated ultrasonically.  Instead of paint, imagine it’s biofilm or microorganism or algae or just the thinest water film next to the boat hull.  Notice how the different colours move in different ways, just as different organisms might.  The paints here mix very rapidly, just as all the organisms near ultrasound might be thrown around.  Also notice the intensity is limited, the paint reaches only a certain height from its resting position – it does not get throw into the clouds, and the sound is not unpleasant to the guys nearby.

Note: a speaker cone is designed to vibrate in this way, it is not damaged by the vibrations, the material lasts decades.

Visually this is probably the best analogy to what happens in the case of ultrasonic antifouling.

Conclusion

We see that ultrasound is already an established part of the animal world for insects to mamals, as well as having been adopted in industries also for everyday cleaning to medical and industrial welding. As with any technology ultrasonic antifouling has matured greatly and now there exists a whealth of scientific & engineering knowledge and pratical experience about the tuning of this technology to achieve the desired affects.

With the twilight of biocide paint coming soon, great attention has been placed on eco alternatives to this centuries old concept. Modern ultrasonic antifouling is a proven option and already adopted especially by those, commercial, vessels that feel the biggest effects of drag, delays, increased fuel consumption, and down-time due to maintenance.

You can take a look at hull cleaning in our shop. Over the coming weeks, we are promiting all kinds of alternatives to biocides from the high tech effective ultrasonic to the low tech boat-brush on a long stick. In another article we will also highlight other techologies which are being researched though not yet come to market. For a dozen options of antifouling, read our article

If you have experience, views, and suggestions on antifouling, let us know in the comments on this page, or contact us on Instagram, email, or WhatsApp. We trully would love to hear from you.

Written by John Dickerson MBA BSc(Hons) MInstP, Dream Sails.