A retrofit system that can turn even an outboard powered boat into an electric hybrid one promises a quick-fix solution for owners and builders alike. Hugo Andreae investigates...
Most boat builders recognise that an all-electric future isn’t yet viable for planing boats over a certain size – the weight and cost of the batteries required to give a sensible cruising range simply don’t add up.
However, hybrid drivetrains with a limited electric-only range for low-speed cruising in sensitive areas such as rivers, harbours and marine parks, and petrol or diesel engines for faster, longer range work make perfect sense.
The trouble is they are notoriously tricky to install unless the boat was designed with a hybrid drivetrain in mind – trying to integrate an electric motor, battery pack and control unit into the space of an existing engine room is no easy task on a shaft-driven boat. As for trying to adapt a sterndrive or outboard powered craft, it was a non-starter… until now.
Spin doctors
French company BlueNav has bypassed the difficulty of trying to integrate an electric drive with existing outboard engines by creating a completely separate package that works independently of the current drivetrain.
BlueSpin Outboard, as it is called, simply bolts onto the transom either side of the outboard engine(s) with its own electric motors, power supply and throttles. The only shared helm control is the steering wheel, but as this simply requires an electronic sensor to read how far and fast it is being turned, rather than an extra cable or ram, that is easily done.
All the skipper has to do is select whether they are using petrol or electric power, engage the relevant throttles, disengage the redundant ones and carry on as usual. Drive comes from a pair of 15kW (25hp) hubless rim-drive electric motors with propeller blades extending inwards from the circular housing.
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These spin round on a rotating ring driven by electro magnets for minimum resistance and drag. Electric drivetrain manufacturer RAD Propulsion tried a similar set-up for its own electric outboard and found it very efficient at lower speeds but not so good at faster speeds.
Because the BlueSpin Outboard system is only designed for low-speed cruising between 6 and 10 knots this makes perfect sense, maximising its electric-only range.
Retractable drives
Ultimately, the range will depend on the size and number of batteries fitted but BlueNav recommends 6 LFP batteries of 7.2kWh each for a total of 43kWh. These can be installed anywhere inside the hull and should give 2-3 hours running time at 7-8 knots – around 20nm.
Like an outboard motor, steering is achieved by vectoring the thrust from side to side but, unlike an outboard, these drives can rotate through 360 degrees, enabling them to operate in astern by spinning 180 degrees or act as unusually powerful stern thrusters by pushing sideways in tandem at 90 degrees.
Bespoke throttles and software give independent control of the electric drives to minimise interference with the main engines
In fact, with the help of a suitably powerful bow thruster, it should be possible to move the whole boat sideways onto or off a pontoon without any assistance from the main engines.
The usual disadvantage of having an extra set of drives hanging off the transom, namely the drag created by the propellers when they aren’t being used, has also been neatly side-stepped by making the unit retractable.
When not in use the rim-drives simply lift up into the housing on the transom, keeping them out of the way of the waterflow passing under the hull. The only thing this might affect is the positioning of the trim tabs, which are usually mounted in roughly the same place on the transom.
This aside, BlueSpin Outboard looks like a very convenient solution for boat builders or owners wishing to retrofit a hybrid drive solution to a new or used boat without having to change the existing engine set up.
According to BlueNav, in most cases “installation requires no alteration to the existing propulsion system or the boat’s structure, providing hassle-free transition to electric power.”
Alternative options
At the moment the BlueSpin Outboard model is limited to 15kW per side and needs to be installed in pairs due to the offset mounting positions, but a 30kW (40hp) version is under development as well as a smaller 8kW (11hp) drive.
BlueNav also offers a retractable Inhull design and a Stationary fixed version. The Inhull one is designed to be fitted further forward on the centreline of a boat and retracts to leave a flush-fitting plate in its place.
A typical installation on a 30-45ft twin outboard day boat
This looks best suited to flat-bottomed craft like sailing yachts or perhaps a round-bilged motor boat. The Stationary version is a fixed solution that is designed to sit under the hull of slow moving displacement boats where drag is not as much of an issue.
BlueNav is being a little coy about revealing retail prices to the press, saying only that prices are available on request. We’ve not yet had a chance to see or experience it in action but as a quick fix solution to creating a hybrid drive option on an existing boat, it certainly seems like a very neat idea.
First published in the October 2023 issue of MBY.
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