Candela electric boat motor: big efficiency with 2 motors, 2 props

When the team at Candela was tasked with designing a new electric boat motor, they used the same principal they did when creating their revolutionary hydrofoiling electric speedboat: start with a completely blank piece of paper and deliver the best solution to the problem.

The result is the Candela C-pod, an underwater, torpeedo-shaped double electric motor with contra-rotating propellers that delivers 50 kiloWatts of power in a 50 kilogram package. That’s a lot to unpack, so let’s start at the beginning.

Candela hydrofoiling boat – like nothing ever before

Candela was founded in 2014 by Gustav Hasselskog, a mechanical engineer and boater who wanted to do something good for our planet by speeding up the (then) slow-moving transition to fossil fuel-free lakes and oceans.

He felt that the best way it could be done was by building a boat with performance not just equal to, but better, than fossil fuel competitors.

Candela C-7 hydrofoiling electric boatA quick analysis of the physics of a fast, heavy boat moving through water made him realize that just throwing an electric motor on a boat made for a fossil fuel engine wouldn’t work. The goal wasn’t to build a different boat for electric propulsion, but to build the best boat for electric propulsion. Since the hull of every boat has enormous water resistance, even when planing, he decided to lift the hull out of the water with hydrofoiling.

So he gathered a task force of leading experts on subjects starting with hydrodynamics, structural engineering and electrical machines and went on to include dynamic modelling, image & signal processing, hardware design, control theory and machine learning.

The result was the Candela 7 recreational speedboat with a software and mechanics system that monitors the boat’s movements and adjusts the foils 100 times per second. The system has now also been modified for use in the company’s proposed water taxis and public transit vessels – the Candela  P12 and P30.

The Candela 7 is now the best selling electric boat in Europe, but Hasselskog has known for some time that the next piece of the puzzle was to develop an electric boat motor specifically for his hydrofoiling vessels – so he set the talented Candela crew to work on it in 2018.

“What would be the best boat motor you could imagine?”

Once again, they looked at the problem not with the idea of adapting something else, but by asking themselves “What would be the best boat propulsion one could imagine?”

Electric to begin with, of course, because quiet is a good thing on the water and quiet is electric’s first advantage over a fossil fuel motor. The noise of a big gas-burning outboard can hit 100 decibels – not just difficult to shout over, but loud enough to damage the human ear.

Candela electric boat motor exploded view of motor inline with shaftThen there’s efficiency. The cylinders and pistons and fuel mix systems and explosions and exhaust and timing mechanisms of a combustion engine make for an extremely complicated way to spin a shaft. A good combustion engine converts only 30% of the energy in fuel to mechanical energy that can be put to work while a modern electrical drivetrain converts 90-95% of its electrical energy to mechanical.

Gears also reduce the efficiency of energy and power transfer.. Some electric outboards use the same configuration as ICE outboards, an ‘L’ shape with the powerhead at the top of a vertical shaft and gearing in the lower unit to transfer the movement to a horizontal propeller shaft. So the first design proposal for the Candela electric boat motor was putting it underwater and inline with the propeller shaft.

Being underwater makes it even quieter than a combustion engine (not that you could put a gas burner underwater even if you wanted to), and eliminating the energy loss of the gears and bearings also eliminates the sound of them whirring against each other – and saves the owner the cost and trouble of lubricating those moving parts.

Reducing size and maintaining power output

This kind of electric motor under the water is called a pod motor, but most of the existing pods are designed as auxiliary power for sailboats (check the Plugboats Guide to Pods and Saildrives). The power demands to get a 7 metre boat up into the air and then continue hydrofoiling are quite different. You end up with a football-sized thing under water creating a lot of drag – which counters the whole idea of hydrofoiling in the first place.

Some basic physics: When it comes to amount of work a motor can do over a set period of time (the definition of power), the term ‘torque’ is used for rotational work, like in a motor. And the equation for power is torque times RPM (revolutions per minute). An electric motor of X diameter spinning at Y rpms generates a power of XY kiloWatts.

So when you solve the drag problem by building a motor half the diameter –  .5X  –  you have to make it spin twice as fast (2Y) to get the same power. That causes other problems, one of which is heat.

As Hasselskog says “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 burns. Water provides a constant opportunity for cooling, so we rearranged a design to get all heat generation as close to the shell of the motor as possible.”

Getting maximum propulsion

Even when you do have a low torque / high RPM motor, spinning a shaft quickly in water won’t provide much propulsion on its own. For that you need a propeller blade to produce thrust – which is essentially a measurement of how hard the water moved by the propeller is pushing against the water behind it. That’s what moves the boat.

The speed of the propeller tip moving in the water is another limiting factor in the speed of a motor for a boat. The bigger the diameter of the propeller blade the faster the tip is moving through the water per rotation and at a certain RPM the water flow around the propeller creates a vacuum and the water starts to boil – cavitation – which causes inefficiencies at the least but also noise and possibly physical prop damage.

A smaller propeller gives slower speed at the propeller tip, but also less thrust. However, if you have two propellers, they can combine to produce the required thrust. So to create their ideal boat propulsion the Candela team decided to run each of those propellers with the smaller diameter motors placed end to end. Hasselskog says “We now had some four times the power density compared to the best available electric boat motors on the market.”

Double propellers further increase efficiency

When you look at the photo of the Candela electric boat motor you wouldn’t be the first to think its long thin tube resembles the torpedos launched from submarines in warfare. There is another similarity. To give the maximum possible speed within a limited diameter and also to counteract the torque of a propeller that would make the armament veer towards one direction, torpedoes have commonly used something called contra-rotating propellers – two propellers spinning in opposite directions.

contra-rotating propellers diagram of flowThere is another benefit to contra-rotating propellers – more efficiency, added to the efficiency advantages of the electric motor and underwater inline drive.  A conventional single prop setup has losses of more than 30% because when the propeller spins it is moving the water not just behind it, but also beside it, which provides no thrust.

By putting one propeller behind another and having them run in opposite rotational directions, the flow after the second propeller can be made to go straight, like in a tunnel. Then you end up with roughly 30% losses in the leading propeller and only 10% in the trailing one – moving the overall efficiency from 70% to 80%.

For the boat owner that translates into a 14% increase in range.

Candela electric boat motor – another big step

As with electric vehicles, the range is the thing that needs to be addressed for widespread adoption of e-boats. The Candela 7 made a huge leap in increasing range by lowering the amount of energy required to move a boat across the water.

And while electric motors for boats have some tremendous advantages over fossil fuel, there is still a challenge for boat owners in that a kilogram or litre of fossil fuel can hold a lot more energy than the best and most advanced batteries, even though the combustion engine wastes 30% of that energy.

Now the new Candela electric boat motor is providing another big step in closing the range gap by further improving the overall propulsion efficiency. If you look at the chart below, you can see that the double propellers and double motors of the C-Pod deliver 72% of the energy in a battery to actually moving the boat, while a fossil fuel motor delivers a total efficiency of only 19%.

Stay tuned!! We’re going places!!

Candela electric boat motor compared to fossil fuel motor: chart


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