DIY electric outboard
Olly Epsom built his own 1kW electric outboard motor for under £600 How (and why) to build your own ElEctric OUtBOarD ENGiNE
A reader shows how he makes his own batterypowered outboard motor
For most people, building your own electric outboard motor is unnecessary, time consuming, and may well result in a product that is both inferior to, and more expensive than a new commercial electric outboard. It will almost always be possible to buy a petrol outboard for less. So why did I do it? Well, my motivations were as follows:
1 To keep myself occupied at home, alone, over the long nights of a Scottish winter (over previous winters I’ve constructed a navigation system, an autopilot and solar water heaters).
2 Because I wanted a system that was more powerful than the cheap trolling motors, but much cheaper than equivalent commercial units (an equivalent ePropulsion Spirit currently retails at around £1,500)
3 Because I have longer-term aims of making the system fully remote controlled (essentially a drone, perhaps for wildlife photography), and for that I needed complete mastery of the control system, which is achievable if I build it myself.
There may be other reasons to embark on such a project, and if so, fill your boots.
Preparation and design
To embark on a project like this requires a basic knowledge of electronics and mechanics, but should be within reach of most practical people with a willingness to learn. The voltages are low enough that they should not present an electrocution risk but take all appropriate precautions when working with electricity and moving mechanical parts.
My project utilised computer control and a lot of new-to-me equipment such as touch screens, Bluetooth, Arduinos and oscilloscopes. I also had to learn how to code the software. With the learning and setbacks the project took me about six weeks of evenings. You may wish to structure your project differently – a significant simplification could be made by discarding computer control.
I had the rough design in my head after an evening of research online. I decided to use the bottom end of a conventional, second-hand outboard motor as the basis of the machine. Above this, in place of the engine would be an electric motor, directly coupled to the driveshaft. Electronic control systems would be mounted alongside the motor. The battery would be separately mounted and connected via flexible electric cable.
I settled on a power of about 1kW because it is significantly more than two rowers could achieve and would result in either a fast dinghy, or a dinghy with significant reserve power.
Batteries cannot be fully discharged without damage, so to power this for a minimum of 40 minutes at full power would require roughly 1kWh of batteries.
The motor, control systems and batteries are the main components, but figure on another £100-£200 to cover enclosures, transition pieces, wire, switches, connectors, and, of course, things going up in smoke.
This will bring the total budget to around £500-£600. Plus weeks of your time. Suddenly rowing may seem less of a hardship! But I am single (hard to believe I know!) and live alone and have nothing better to do with my time and money so onwards I went.
I located a 1974 Johnson 2hp outboard on ebay for £24. It cost me more in fuel to collect it. I removed everything north of the driveshaft housing and re-sold those components for £5.
These old engines had very high specification mechanical components and the gearbox in particular was in excellent condition. That said, the motor was forward-speed only, with no clutch. It is possible to make an electric motor go backwards as well as forwards but be cautious doing this on an engine which originally had no reverse because the gears won’t be designed to operate backwards and there won’t be a thrust bearing to keep cogs in check.
I decided on a brushless DC motor because they are generally reliable and don’t require maintenance of brushes. Usually they are slightly more efficient than permanent magnet DC motors as well, although in practice this depends entirely on the specific motors being compared.
Most motors in this power range run at 48V or above. While it’s possible to get 12V motors in this power range, they would draw over 90A at full power, requiring unacceptably heavy wires. In addition there was a far wider range of motors available at 48V.
Likewise battery packs at 48V were more numerous. I settled on lithium-ion batteries due to their high capacity, low weight and high cycling ability. A 1kWh lithium battery is roughly 20Ah at 48V – about the same amount of energy held in an 80Ah car battery at a quarter of the weight.
The downside to lithium-ion is the cost. There are various battery packs on
The finished electric outboard on the bench at home
The old outboard motor stem
Scruffy, but well-built 1974 2hp Johnson
ABOVE Unused petrol engine partsLEFT Propeller and drive are to be retained BELOW Original cooling impeller can be discarded