DIY Electric Outboard: Sparky Jr is Born

It’s been over a year since I started dreaming about an electric outboard. Life gets busy and as you might expect, and the outboard project has gone on and off the back burner. I have made some progress over the past year and figured it’s time to share.

Donor Outboard Leg

Thanks to the great folks at Seapower, I was given a 1967 Johnson 5hp Sea Horse. The powerhead had seized a long time ago, but the lower end gearbox is in perfect working order. Sparky Jr is finally coming to life.

It turns out that the 1967 Sea Horse has some KISS design details that are quite beneficial for this conversion compared to a modern outboard. On this outboard, there is no reverse! If you wanted to go backward, you simply turned the engine around, and backwards you went. What this translates to is a simple direct drive to the propeller. The propeller itself is also a more efficient, simpler design.

KISS Design Benefits

Simpler Gearbox Configuration

Modern 5 HP outboards have gear shifters with a cog system in the gearbox. That cog system is similar to that of a 10 speed bicycle hub. If you pedal the bicycle backwards, the cogs disengage, and the pedals can turn freely in the backwards direction. When this happens, no power is transferred to the wheel, the bicycle just coasts. For the outboard motor to transfer power to the propeller in the forward to and reverse direction, the gearbox must be shifted so the cogs can engage and drive the propeller. In practice shifting from forward to reverse often requires the operator to turn around and look away from where they are heading to reach back and operate the shifter. This is often done just as you are approaching a dock or another vessel, creating an opportunity for an impact that can cause damage.

Since the direction the new electric motor spins is controller driven, a simple flick of a switch and the motor spins in the opposite direction. No extra mechanical shifters are required, and the operator does not have to take their eyes away from where they are heading.

Propeller Design

Another perk to the old Sea Horse is the propeller design. Modern 5 HP outboards funnel the engine exhaust down the leg and out vents in the center of the propeller. Because of this, the center of the propeller is larger in diameter.  This creates more drag and reduces efficiency. The old Sea Horse does not have vents in the propeller, the center hub is smaller and thus reduces drag and theoretically improves efficiency.

The simple propeller and direct drive design of the old Sea Horse adapts very well to a modern electric powerhead.


Battery Pack

The battery pack build is still in process. I was able to score some secondhand LiFePO4 cells from Cameron at TransMarine. The secondhand cells originally boasted 73 Ah of capacity, and after testing I discovered they still have 67-70 Ah remaining. I am very excited about this because getting new lithium batteries shipped to NZ is prohibitively expensive. It also falls perfectly in line with our sustainability principles, Renew Reuse Recycle!

Battery Chemistry

I chose the LiFePO4 chemistry because it doesn’t have the fire risk of other lithium chemistries. What they sacrifice in energy density is made up for in cell stability and that is very important in the marine environment. The cells I will be using were part of a battery pack that failed due to improper use. The cool thing is, even though the pack failed, there were still enough good cells in the pack to have a second life as Sparky Jr’s power supply!

Size and capacity

The new 15 cell battery pack will take up as much space as a typical 20L petrol tank and weigh somewhere between 40 and 50 lbs. In theory this battery pack should let us run approximately 50 minutes at wide open throttle. That should be plenty big enough for our application.

The Build

After removing the old powerhead from the Sea Horse, I started by making a composite adapter plate that converts the motor’s bolt pattern to that of the Sea Horse leg. 

A big thanks to Ted Cary on this one! Ted helped us by not only giving us shop space to work, but also providing endless guidance in all things boat building, thanks Ted!

After the epoxy cured on the adapter plate, I took the outboard leg, new electric motor, and old motor drive shaft to Opua Marine Engineering. At OME, Mark and Carl cut the original drive shaft and welded on an adapter. The motor output shaft now connects to the drive shaft, Sparky Jr is born.

Once the physical motor linkage was sorted, I set about to wire up the motor controller and throttle. The RoboteQ motor controller I chose for this project is highly configurable and can drive many different types of motors. They have free software available that allows you to see/program a whole list of parameters. The software even charts them live so that you can see the input/outputs of the controller in real time.

It’s not the most intuitive software, but it doesn’t take too long to click around and understand how to use it.

Another benefit to RoboteQ controller is that it can operate from 12-60V. This means it can take a wide variety of batteries. I was able to do the preliminary configuration with a standard 12V battery at Ted’s Shed.

Sparky Jr v1.0

In the new shop space, and thanks to Terry B Unlimited, we now have two 48V power supplies to continue testing/development. One power supply is can spin up the motor to 25% of its output. If we wire the second power supply in parallel, we can simulate to 60% of Sparky Jr’s max output. 

Another big shout out to one of our new flat mates, Ben. Ben helped me come up with a good solid mount solution for the controller inside the outboard cowling. 

As I was testing the motor the other day, I noticed it was not spinning as fast as I expected it to. After a day of reading and troubleshooting I discovered that I had one of the motor parameters configured wrong in the software. Once set properly, the noise from the motor and excessive current draw has gone away and Sparky Jr v1.0 is alive and functioning properly!

The motor itself is very quiet and the controller only produces a slight hum. The drive shaft adapter seems to be the loudest component of the outboard. Since noise is loss, I imagine the next iteration of Sparky Jr will involve a more precise adapter plate. Some sort of rubber adapter to manage the slight misalignment of the drive shaft to the motor would also be helpful.

Stay Tuned for the battery build and sea trials!