Going 100% electric: the Motor

A few people have been asking for product information about our electric plans. Bear in mind that this isn’t yet fully implemented and certainly not proven by us. Also that our choice is to be fossil fuel free and live with the impacts. If it means we can’t go as far or as fast, or if we have to do a lot of active management in order to be fossil fuel free then that is ok with us.

Note that we are not qualified to offer any advice, this is our own journey, learning as we go.

Finally, this isn’t a rush job. we are planning to get this sorted over a number of years. I have a minimum of 3 more years in my current appointment with extensions possible before we retire to live-aboard. Therefore we have time to get everything sorted, until then our sailing will be odd days and holidays.


We bought our motor and it’s controller in the UK from Falcon Electric. They work with electric car projects but we chose a motor that is sold in the US for yachts by Thunderstruck Motors. We couldn’t find a EU marine dealer for these motors.

What we bought is a package of motor, controller, wiring loom and meter. So the motor is a HPEVS AC-34 and the Controller to go with it is a Curtis 1236SE-5621 (48v, 600a, 40hp).

We looked at many other motors. Either they were out of our price range or they were not brushless. We also like this being air-cooled as we want as few holes and as little complexity as possible. We know this might mean needing forced air ventilation for the motor compartment in hot weather, however for us that is a lot cheaper and simpler than water cooling.

We have bought the Curtis throttle from Kit Elec Shop we are going to make our own handle for it. This was tricky to find, with expertise we could probably have bought a non-Curtis item.

We are not yet sure to what extent the controller will need programming for best performance, the devices to do this are quite expensive so not rushing to buy one. We also expect to need to add a larger cooling plate to the controller.


We are reusing the existing 3 bladed propeller (design max speed 1600 rpm). Our hope is that, rather than this being a big drag slowing us down while sailing, it will prove powerful driving the motor in regen mode. Upgrading to a folding prop that would have less drag while sailing a slow speeds while still being able to unfold for regen is a long term possibility (although expensive).

We are replacing the cutlass bearing due to normal wear and tear. The length of the propeller shaft and the position of the skeg require the propeller shaft to be taken out inwards. Having to remove the engine for this work was a motivation in switching from diesel now.

The original stuffing box had been leaking and needed a lot of work. We decided that as we want to minimise salt water near the electric motor we would replace it with a PSS Pro Dripless seal we chose this a) because it does not require a pressurised water feed (with an air cooled motor we don’t have one) b) it has a wide variety of sizes. We are re-using the flange that screws onto the stern tube that the stuffing box used to be attached to (have ground and sanded it smooth). The PSS Pro is therefore for a shaft of 1-1/4″ and a flange of 2-3/4″. We will be adding their Hy-Vent to provide water to the seal.

The propeller shaft will be connected to an Aquadrive system CVB10.10 (all the drivetrain components are being supplied by T.Norris Marine where Jonathan has been really helpful).

From the Aquadrive (via a coupling from T. Norris) we have a shaft that goes through our motor frame to an 80 tooth pulley for a 30mm wide timing belt (two bearings are fitted, one at each end of the frame). The motor frame is now at the forward end of the motor compartment so we are probably going to add an additional bearing for the shaft near to the Aquadrive (with the coupling etc there is quite a lot of weight on that end).

The motor sits above the shaft in a custom frame we have built and we have a 56 tooth timing belt on it. This reduction gear (56T to 80T) should allow the motor to run at peak torque with the propeller at design maximum speed. Given the motor is, at least in straight numbers, more powerful than the 29HP diesel with more torque available at low speeds we expect to not need to push the motor very hard. The motor can be moved up and down in the frame to tension the belt.

We bought the pulleys and timing belt from Bearing Boys, the stainless steel bearings from Simply Bearings, the Stainless steel for the frame from Metals4U and all the bolts from Accu. See various posts about the motor frame.

We have 4 engine mounts coming from T.Norris.

This means we should have few alignment issues for the motor and as the thrust is taken by the Aquadrive the motor should be free to float on it’s mounts which will reduce vibration and noise.

Motor Battery Bank

We are running the motor with a 48 volt battery bank (we decided we did not want to go for a higher voltage as then there are safety issues and also you need so many batteries in the bank).

We are building this bank from four KS Energy KS-LT300B 12V 300Ah LiFePo4 batteries. So 4 x 300Ah is 1200Ah. We chose them as being cheap, open about the technology and high density. It should be a lot simpler to wire just 4 batteries together rather than a larger number of smaller batteries (and was cheaper).

This battery bank will fit above the shaft just aft of the motor frame, forward of the aquadrive. It will be 2 layers of 2 batteries. as a 2×2 block. This keeps weight low and central with short wiring runs.

The batteries will be encased in an epoxy coated plywood box for protection.

We don’t have space to add additional batteries to this bank. However, in an (slow) emergency our house bank could be rewired as a replacement motor battery bank (by changing from parallel to series connection).

Our very rough estimate is about 1 hour at close to hull speed and about a day at slow speed (2 or 3 knots). Hopefully up to about 20 miles.

I’ll cover cabling in a later post. We are making our own cables and have the proper crimp tool. For efficiency and reliability we are using thicker cabling than is required. There will be a shunt (for measuring battery bank). There will be a shut off switch and a fuse. We will be fitting 3 Victron battery balancers (as they recommend for a bank of 4).

That is all I can think if for the moment. Have I missed out anything useful?

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