“A silent revolution beneath”
Combustion engine outboards are noisy, require expensive maintenance, cost a small fortune to run, and emit more CO2 per kilometer than most road vehicles.
The revolutionary Candela C-POD, an electric pod drive developed by Stockholm-based tech firm Candela, Europe’s leading electric boat manufacturer, will change all this.
– What would be the best boat propulsion one could imagine? Here are some ideas: electric, completely silent, without oil changes, and with an almost unlimited lifetime. Add to that almost no losses – converting most of the energy to thrust, says Gustav Hasselskog, Candela’s Founder and CEO
In traditional boat motors, whether electric or fossil fuel, the motor typically sits in a box above the waterline, transferring thrust to the propeller through a complex system of shafts, bearings, and gears. The Candela C-POD eliminates gears entirely. Instead, it mounts two ultra-compact yet powerful electric motors underwater in a torpedo-like socket, directly driving the propellers. Each motor directly connects to a propeller, minimizing friction losses, while contra-rotating propellers further enhance propeller efficiency.
– The engineering challenge was to make the electric motors compact enough. Being submerged, they have to have a very small diameter in order to cause minimal drag, explains Hasselskog.
To accomplish this, Candela’s engineers had to rethink electric motor design bottom-up. Electric motor volume is not driven by power. Instead, it is largely proportional to torque. And power is torque times rpm. For the Candela C-POD motors, Candela opted to increase the rpm and lower the torque to boost the Candela C-POD’s power density. The best way was to split the thrust needed on two propellers. The reason is that the maximum rpm is limited by the speed of the propeller tip moving through water. At around 45 m/s propeller speed, the pressure in the flow around the propeller reaches vacuum and starts to boil. That creates noise, damage, and inefficiencies. The speed of the tip of the propeller is a product of rpm and propeller diameter. Therefore, splitting load on two propellers allows for propellers with a smaller diameter and thereby higher rpm – and in turn smaller motors.
By placing the motors under the water, Candela also elegantly solved the problem all-electric motors face: heat. The Candela C-POD engineers designed the slim pod drive to be directly and efficiently cooled by the flow of seawater, thereby enabling higher operating temperatures and extracting more power from the motors.
– The first obstacle towards a very small high-power motor is heat. You can take any motor and give it three times more electricity than it is rated for. It will work. But only for a few seconds. Then it melts. With Candela C-POD, we have almost unlimited cooling power – we just need to get heat from the coils to the surrounding water flow, says Hasselskog.