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

Practical Boat Owner - - Contents -

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

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