Chris Johnson explains how to choose the right bearings for the job. Electric boats are gaining ground – even on water. Future boat companies are committing to some form of electrification in boats, with one being named as the ‘Tesla of the sea’ along with its founder, Konrad Bergström, hailed as a nautical Elon Musk.
Given the extreme conditions, it is easy to assume that choosing a bearing made out of a harder material, such as chrome steel, would ensure the best bearing performance. However, because of sea water’s extreme corrosive power, a compromise between load capacity and corrosion resistance is required to maximise the lifespan of bearings.
Advantages and disadvantages of stainless steel bearings
Bearings used in many marine applications, including electric boat motors, are often made from less corrosion-resistant materials, such as 440 grade stainless steel, due to its low noise and low vibration properties. However, these bearings need to have added protection from seawater contact in order to extend their useful life.
The standard approach to minimise corrosion risk in these bearings is to use housings or end caps, and a packing of grease to shield steel bearings from the elements. Without these, any moisture that gets into the bearing will cause rapid corrosion and deterioration. Although this is a logical route to take, users must be aware that this approach can greatly increase bearing torque, hindering performance.
When assessing the suitability and quality of bearings, it is also important to look beyond the external aesthetic. Users who have 440 grade stainless steel bearings in their electric boats may start to see signs of corrosion on the outside of their bearings. However, while the instant reaction may be to assume that this will cause the bearings to fail, external corrosion may have no effect on the bearing’s internal rollability and general performance.
Ceramic bearings may be optimum choice
Inside, however, marine bearings must be protected from ingress of contaminates. Wherever there is potential contact with salt, water and the elements, bearing material choice is extremely important. Although full ceramic bearings are more expensive due to the greater difficulty in machining to precision tolerances and the cost of the raw material, they are an excellent choice for electric boats due to their impressive corrosion resistance properties, even when fully submerged.
What role does bearing lubricant play?
Bearing lubricants work by simply reducing friction and dissipating heat to stop elements from overheating — the two most basic types being oil or grease. Regardless of which you choose, extreme temperatures can dramatically impact the effectiveness of a lubricant. Operating in reduced temperatures, without the correct lubricant, will result in a higher viscosity lubricant; which can increase frictional torque significantly.
Where marine bearings are immersed in water however, there may be very little heat build-up, so lubrication is not always necessary - particularly with full ceramic bearings. Fortunately, if lubrication is required, there are water- and saltwater-resistant lubricants available; even for submerged bearings.
Finding the balance
Many of the dilemmas faced when choosing bearings for electric boats apply to all marine applications - should load rating or corrosion resistance be the priority? It’s a fine balance that must be achieved by considering the long-term demands of the bearings, as well as the harsh environment they will be used in.
Either way, electric boats are on the rise, and with pioneers and major companies committing to reducing emissions, the future is electric.
The author Chris Johnson is managing director of marine supplier SMB Bearings
