Benefits of a Clean Hull – In Detail

Grandezza 26

Grandezza 26 after 1st season with SonicShield II Installed


There is very little that thrills us more than blasting around in a well-sorted powerboat, leaving nothing but spray in the air and a smile on our face. At slower speeds, the same balanced, lightweight hull can provide a suitably efficient cruise, however getting up on the plane requires a great deal of power and therefore fuel. And as we all know, fuel is expensive, so you need to do everything you can to reduce those costs. But what is the best way?

Keep your hull clean

The basic physics

When a planing hull is at rest or very slow speeds, it displaces water to keep itself afloat, and this is known as buoyancy, or the ‘displacement mode’ of the craft. So in order to move, the boat has to move this same amount of water out of the way to progress by a single boat length. But as the speed increases, the hull’s shape, along with vertical transom and hard chine lines create increasingly powerful lift forces, that reduce the effective displacement of the boat and lifting the hull further out of the water.

Eventually, the hull will create enough lift that the boats’ weight is nearly supported by lift alone, reducing the displacement to near zero, allowing the hull to skim along on a relatively small planing surface, next to the transom. Removing the hull from the water reduces drag massively, meaning you can go faster. By trimming the boat, you alter the angle of attack of this planing surface, much like the wing on a plane, and there is a fine balance of trim needed to maintain a planing craft’s speed.

Too high a trim (up at the bow) often results in porpoising, which in extreme cases can be very dangerous, but too little trim, and a larger than normal area will be in contact with the water, increasing drag and slowing you down, reducing lift and slowing you further. Here it is plain to see that too much drag costs you speed, fuel and fun.


The impact of growth.

Growth of marine flora and fauna on your hull adds both weight and drag to your boat, both things that you want to avoid at all costs. It all starts when micro-organisms such as algae attach themselves loosely to the surface, but the bond quickly becomes much stronger. They multiply to form a green slime, which attracts other, larger organisms such as bacteria and larvae to the surface, which create a wonderful source of food for much larger things such as barnacles and tubeworms.

All of these layers increase drag through increasing surface area and hull roughness, as well as causing transition from laminar to turbulent flow earlier than necessary. In fact, research carried out by the IMO (International Maritime Organisation) has shown that cleaning a commercial ship’s hull can reduce fuel consumption by up to 12%, but the effect on planing craft can be much greater due to the higher speed and relative amount of fouling on a much smaller hull.

The effects of fouling accumulate over time, so you may lose 2-3 knots of top speed over the course of the season, with some cruisers reporting a reduction in efficiency (as measured by either fuel flow or top speed achieved) of between 15 and 50%.

This is a serious dent to both your bank account and your fun, so what can be done?


The solutions

Obviously, the best way of stopping fouling is to remove your boat from the water when not in use, however it may be impractical to launch and recover every time, or impossible due to the size/weight of the boat and available facilities. If the boat must remain in the water, there are several things that can be done.

The traditional method of applying antifouling paint of any kind still works, albeit less effectively now the IMO have banned the ‘toxic warfare’ chemicals in the paints of old.

Soft paints that self-polish to reveal new, fresh layers tend to work if the boat moves reasonably often, and the harder paints with copper can work in low to moderate fouling areas; however fouling will always happen to a certain extent, especially on impossible to protect surfaces such as propellers and metalwork, as most of you will know.

New types of paint and coating that adopt strategies of making the hull too slippery or soft for life to stick to show promise, however they can be easily damaged and must be professionally applied, at which point the cost can easily run into the thousands.

Becoming increasingly practical and cost effective is a technology called ultrasonic antifouling. Actually discovered by the US navy during sonar trials in the 1950’s, it is becoming increasingly practical and cost effective for boat owners/operators – thanks to digital technology and low-power circuit boards. The various systems available work by pulsing ultrasonic sound waves through a boat’s hull, at a range of frequencies much higher than the human hearing range that resonate and burst the cell structure of algae, making sure the first bond never becomes permanent, and removing the first link in the food chain of fouling. This is prevention, not cure as with the traditional paints.

The better systems cost about the same as a season’s lift and anti-foul with traditional paint, but can pay for themselves very quickly and many times over with the fuel savings, the vastly reduced lift and paint costs and the fact that it will keep your hull clean for years at a time. This is why we think that these systems can represent a clean, modern solution for today’s boat owners and operators.


Other considerations

If you haven’t bought your boat yet, it could pay to research the most efficient hull form and propulsion systems available in your budget or desired craft – lower weights and cleaner lines will help save those litres of fuel, and they all add up. Stepped and ventilated planing hulls, hydrofoils and trim tabs will help too, but you should also consider how you will drive it.

Make sure you adjust the trim settings to get as level a boat as possible, accelerate gently and avoid hard turns at high speed. Loose extra weight if you’re carrying it, and make sure your propellers are in good condition. Any damage to the surface, chips or imbalance can make for efficiency losses as well as an uncomfortable ride. If in doubt, have an engineer check it, or send it to be repaired. You’re burning fuel to spin it, so why have it less than perfect?



For those with small boats, there is no excuse. If you can’t take your boat out and clean it by hand, there are hundreds of products around to clean your hull, from simple chemical cleaners to performance waxes that can help keep your hull clean, your fuel bill down and the smile on your face!

For those with larger boats, the task is a little tougher, but you will notice the dirty hull where it matters most, your wallet. It’s probably worth the effort, even if you have to go for a swim and brush off that slime every now and again. Remember, no-one will want a ride in a slow boat with a green beard!

For more information on the SonicShield Ultrasonic Antifouling system click here

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