The appropriately named Dutch company Rim Drive Technology is the only company that offers a rim drive electric outboard motor, and is developing a 15 kiloWatt (≈25 HP) model to extend its current line-up of 4 outboards. It also offers a similar range of electric rim drives in pod motor and thruster configuration.
What is a rim drive electric motor? It is sort of a ‘motor with a hole’, the hole being the place where you would expect to see a propeller shaft and blades. With a rim drive, there is no central hub or propeller shaft, instead the prop blades are attached to a circular outer rim – hence the name.
Propeller blades directly attached to motor
The ever trusty Wikipedia tells us that “The concept was proposed by Ludwig Kort around 1940, but only became commercially practical in the early 21st century due to advances in DC motor controller technology.”
For a bit more of a short history lesson, Wikipedia also tells that “Kort was a German fluid dynamicist known for developing the ducted propeller, or ‘Kort nozzle‘.” Herr Kort was looking at ways to reduce erosion in canals from the water turbulence of boat traffic. When he tried enclosing a motor’s propeller inside a short tube or tunnel to lessen the turbulence he discovered that it also increased the thrust.
In a rim drive electric motor, this construction is in effect the motor itself. The tube/tunnel/cylinder is the stator (non-moving part) of the electric motor and the propeller blades are attached to the rotor – the spinning part. It’s a bit redundant to say the ‘electric’ motor because there is no way a fossil fuel motor could work in this way.
Having said that, there are ICE motors that look like rim motors – they are called thrusters – but when you look more closely you see that it is actually just a standard propeller shaft and blades surrounded by that tunnel – the Kort nozzle.
The fossil fuel thrusters have been used for decades on large boats and huge ships, mounted at the bow or stern and switched on for manoeuvring and docking because they are much more responsive than the main engines. The electric rim provides some additional advanages.
But enough history – now the present and the future!
From defense use to recreation and commercial
Since the introduction of rim motors in the early 2000s, one of their main uses has been in military/defense applications, partly because the reduced vibrations and noise make it more difficult for monitoring technology to detect and track the vessels.
There are some ship units as big as 1.5 megawatts (≈ 2,000 HP), but on the smaller end of the scale they are popular for submersibles and underwater drones. A benefit for underwater use is that they run on batteries – no exhaust – and are also easily adapted for remote steering or autonomous operation.
That’s where Nick Ruinard saw the motors in action, realized their benefits for the consumer market, and decided to start Rim Drive Technologies (RMT).
Unlike many founders of electric boat and boating startups, Nick has no sailing or marine background. He studied business administration and was working for a company that supplied naval equipment, including rim motors. He saw an opportunity for them in non-military boating and started to do some marketing research about where the industry was headed, especially in light of what is happening in electric cars.
He saw a market not just in recreational boating, but also workboats and the growing number of pilotless autonomous vessels used both above and below the water.
Rim drive electric outboard safe and snag-free
Along with the quiet, exhaust free operation and low maintenance that is an advantage of all electric motors, the rim motor has some additional benefits. There is no propeller hub for weeds or other water debris to wrap around. There is also the safety aspect of the propeller blades being enclosed by the motor itself.
Those are the advantages for electric boat users and owners. As a manufacturer of electric boat motors, Nick liked the possibilities for scalability in manufacturing. The same basic rim drive motor can be easily adapted for a variety of applications simply by adjusting the way it is connected to a boat.
RimDrive Technology has 7 basic motor sizes with power outputs ranging from .5kw to 30 kw (≈1HP to 52HP). Each of them come in 6 configurations, including: a stationary pod style motor that can fit underneath the hull of a boat, a steerable azimuth pod that can rotate 200º or a standard bow thruster with extended Kort nozzle tunnel. There are other companies that do these, but not a rim drive electric outboard.
Until now, the company has only made the outboards available in four sizes – from .5kW to 11kW. The reason is simply because once you get into the higher power models the size of the rim becomes awkward for an outboard. Nick and his team, though, have modified the 11kW model to provide 15kW of power without increasing overall motor size and weight.
‘One of our key characteristics is to have all knowledge and experience in-house.” says Nick. Whichever the model, it comes complete with motor controllers, (standard or lightweight), throttle controls and cables. Rim Drive Technology assembles its own 12V, 24V, 48V and 96V LiFePO4 (lithium iron phosphate) batteries with energy storage of .7kWh to 9.6kWh and supplies all of the necessary monitors and chargers.
With the wide range of models and configurations, Rim Drive Technology is opening eyes about the capabilities of electric rim drives for all kinds of boats and applications.
Two companies that turned to RMT for electric propulsion solutions are Amperaboats for their 610 sloop and Danadynamics for their remotely operated ASV-E drone that collects water samples for environmental monitoring. Rim Drive is working with a French company to deliver customized hybrid systems and have just received Netherlands Enterprise Agency grant to develop a standardized hybrid drivetrain.
For those looking for the benefits of electric rim drive motor as an outboard using, check out the Rim Drive Technology site for specifications on the 4 models available now and updates on when the 15kW will be available.