Propellers are very sensitive components. Even minor damage can significantly reduce their efficiency. This leads to poorer handling characteristics and increased fuel consumption.
In the event of an imbalance - for example due to damaged propeller blades or mussel growth - shafts, bearings and gearboxes can suffer serious damage. Heavy contamination also has similar effects. Although they do not necessarily cause damage, they increase the water resistance and thus reduce the efficiency of the propeller, which is not optimal anyway. To prevent fouling on the propeller and the associated problems, there are special antifouling coatings for propellers.
- Trilux Prop-O-Drev >> available here
- SDV Antifouling
- Hempel Ecopower >> available here
- Silic One Propeller Kit >> available here
- Propspeed >> available here
The best propeller fouling protection is smoothness
To put this into perspective: the tip of a 14-inch propeller working behind a gearbox with a 2:1 ratio, for example, moves through the water at 67 km/h at 2000 engine revolutions per minute. Depending on the gear ratio, this speed is linear to the engine speed. In this example, doubling the engine revolutions means a good 130 km/h through the water.
This is less relevant for rather slow-turning displacement diesels, but is certainly relevant for high-speed outboards or sterndrives. Because: "The smoother the surface of the propeller, the higher its efficiency," says Jens Dohrand, Sales Manager at propeller manufacturer SPW. And this is where the problem begins: it is not easy to get a propeller fouling protection to hold permanently on such a fast-moving object with a smooth surface in an abrasive medium such as seawater mixed with sand. At the same time, this rapid movement is precisely a property that some products utilise, but more on that later.
More about antifouling:
Different principles of action in Propeller fouling protection
As with the products for the hull, there are also different active principles for propeller fouling protection: Biocide-containing as hard or soft antifouling, as well as biocide-free. These coatings usually have very smooth surfaces from which the fouling simply slides off during fast movement. Alternatively, the manufacturer Hempel states that its Silic One This creates a surface that aquatic organisms and vegetation regard as water. Logically, however, they do not settle on it.
In the case of products containing biocides, it is important to check whether these are authorised in the respective home territory. In the Netherlands, for example, there are very strict authorisation regulations for propeller fouling protection. These include those for propellers. Products without an approval number may not be used or processed, and there are no special biocide-containing propeller antifoulings with an NL approval number, meaning that biocide-free products must always be used. Alternatively, you can also try an approved antifouling for the hull.
Whether a biocide-containing product is necessary at all is a question of the area. The fouling pressure in freshwater areas, for example, is usually significantly lower than in salt water. In turn, there is less growth in flowing areas than in stagnant water. "However, the fouling situation is very complex. It can change from year to year, it's just nature. Sometimes the pox bloom and everything is overgrown, sometimes the water is too cold and the vegetation is much weaker. The tubeworm is also on the increase," reports Dohrand from conversations with many of his customers. It is therefore important to develop a feeling for how the fouling behaves in the home waters. As a rule of thumb, however, what works as a system on the hull will also be effective on the propeller. Of course, it always helps to ask local water sports enthusiasts about their experiences when moving to a new area.
As with the paints for the hull, there is no permanent solution for the drive. Some products, especially those based on silicone, promise to be effective for several years, others can be applied in several layers and thus last longer. However, if the boat is regularly taken ashore for winter storage, durability is of course less important.
The propeller material is also important
Finally, the material of the propeller and the drive also plays a role in the selection of suitable propeller fouling protection. As with the hull, not every paint can be used on an aluminium propeller. Products containing copper can lead to electrolytic corrosion and should not be used. Some manufacturers state that this is still possible if a primer is used. This is true as long as all surfaces are completely covered with the primer. If this is no longer the case, for example due to damage caused by flotsam or ground contact with the drive, damage due to electrolysis can result.
The application of propeller antifouling is at least similar for all systems. In order to achieve good adhesion to the smooth, fast-moving surface, the surface must first be very well prepared. A bright metallic finish for bronze propellers or a good adhesive coating on the drive or on aluminium propellers are the prerequisites. Then degrease first. For some products, this must not be done with acetone, but with white spirit. The substrate should then be sanded. The grain size varies depending on the product. Sometimes preparation with a Scotchbrite fluid is also sufficient.
Then clean and degrease again. The Propspeed kit even includes special cloths for this purpose to provide an optimum surface for the first layer. This primer ensures good adhesion to the propeller and creates a surface to which the actual antifouling adheres well. This is no different from the layer structure on the rest of the hull. With Hempels Silic One, a third layer called tie coat is required between the primer and the top coat. The primers are often two-component and based on epoxy resin. They therefore have specific temperature and processing windows that must be strictly adhered to. The recoating intervals also vary depending on the product. Hempels Ecopower Spray, for example, must be applied while the primer is still tacky.
The manufacturer's data sheets, which explain the application process in detail, also provide good advice. These should be read carefully before application and a plan made as to how the layer structure can be optimised. The weather also plays a role here: the humidity must not be too high and the temperatures must not be too low. The recoating intervals vary depending on the temperature, so if you want to avoid having to apply the next coat in the middle of the night, you need a good plan. Unlike coating the entire fuselage, there is one advantage with the propeller: it can be dismantled and taken back to the workshop at home. There, the coating can be applied under controlled environmental conditions and with short distances to work.
In inland areas it also works without antifouling
So which product should you choose if you have problems with fouling? First of all: If there are no serious fouling problems: None at all. According to the expert, even the coating itself reduces efficiency, even if only to a small extent. In areas with low fouling pressure, there is no need to shoot sparrows with cannons. An annual cleaning and polish, possibly Teflon-based, can be enough to get the prop through the season fouling-free. So-called propeller grease has also proved to be a good propeller fouling protection.
However, if you are located in an area where smallpox or tubeworms quickly attach themselves to exposed areas, you must take action. If the drive and propeller are made of aluminium, products containing copper are out of the question. The risk of electrolytic corrosion in the event of damage to the primer is too great. In this case, only biocide-free paints or silicone-based products are available. If the drive consists of a shaft and screw made of bronze, paints containing biocides can also be used. In this case, pay attention to the approval in the home area. A self-polishing antifouling on the fast-moving propeller only makes sense if very few engine hours are to be expected, as the wear is then roughly identical to that on the hull. If a lot of sailing is expected, the paint will soon wear off and the effect will be lost. Then only hard antifouling remains, which in turn only works thanks to its biocide content.
Silicone-based antifoulings
Non-stick coatings, usually silicone-based, can be applied to all drive types and areas. They are biocide-free, so there are no approval or corrosion problems. The idea of these paints seems particularly suitable on propellers: the surface offers fouling hardly any opportunity to anchor to it because it is very smooth. Add to this the fast rotation and it is easy to imagine how the surface virtually frees itself of fouling as soon as the propeller turns.
A broad-based study in the Netherlands in 2020 showed that Hempel's Silic One also achieved quite good results in current-running saltwater areas. Unfortunately, it had little effect on stagnant salt water. However, tests were carried out on plates suspended in the water, which is a completely different application compared to the moving propeller surfaces. Nevertheless, if there is movement along the surface, the product was able to prove its effectiveness.
Disadvantage of silicone products: If, for whatever reason, they are not effective on your own boat, it is difficult to switch to other products, as silicone causes problems when recoating even after removal.
However, the study in the Netherlands showed very clearly that milking grease on the test plates was sufficient to prevent fouling, particularly in freshwater areas. In other words, there are areas with very low fouling pressure that should not be unnecessarily burdened with biocides.
However, the manufacturers' detailed instructions clearly state one thing: if you decide in favour of an anti-fouling coating for the propeller, you need to take great care when applying it. When applied correctly, however, the protective coating should ensure good efficiency under engine power while also saving fuel and reducing wear.
