Electric boat conversion - drive motor

(Sorry - terrible picture - I must have shaken!)

Talking with people about my ideas of converting to electric drive, it was very curious to find that almost anybody to do with narrowboats assumed an electric motor couldn't "really" be as powerful as a direct-drive diesel. I didn't have this misapprehension for being fully aware that the biggest and fastest ships are... driven by electric motors!!

No, no, all that matters is that the motor has the same potential power as trad diesel - of itself an electric motor is simply as (potentially) powerful as you choose to fit!

Question, though, what power rating would suit our boat, to be at least as powerful as the (then) existing 1.8l BMC.

It was quite hard to know how to find a suitable figure - all the conversions I'd heard of had, by report, not really been as powerful as conventional drive - although it was hard to be sure if people meant actual immediate power was lacking or that they ran out of power for the batteries losing charge and/or that a weaker motor was fitted in order to conserve battery charge....

However, I had by now been on a trip on a neighbour's smaller, lighter, overnight re-charging fibreglass day boat - fitted with a 4kW motor, top speed more than adequate, but above all, the "punch" of acceleration quite startling.

I did some purely theoretical sums about accelerating our 10 ton (or tonne - they're much the same!) boat, allowing only 50% efficiency... and decided 5kW should be enough. So, from the range of powers I'd come across, 8kW should be definitely OK. Therefore choose 12kW to be absolutely sure!

Please be clear, there is no disadvantage in having a potentially high power motor - it will only use more power if you supply it! And the extra cost for higher power is not really very significant.

I should perhaps mention here that a great many people are fooled by "nominal" power ratings for diesel engines - and this can be why some think electric drive can't be as powerful! Our 1.8l BMC was nominally rated as 42 bhp. And numerically 12kW is only 16 bhp (or so.) But this is actual power as opposed to a rating dreamt up ... I'm not quite sure how! To jump ahead, let me be clear that 12kW electric is categorically as powerful as 42 bhp nominal - and acceleration is at least ten times as great! In saying that... our 12kW motor actually draws 14kW "flat out"... but that's still only 19 bhp.

I investigated Lynch motors specifically, first, because that is the type fitted to our neighbour's boat - and very neat and small it is. They are not desperately expensive, are wonderfully simple using extremely powerful permanent magnets, and are amazingly light for their power ratings.

But, I searched around on the Web and also discovered "SEM" (Seperately Excited (field) Motors) motors - and somebody who'd been using them for conversions of wherries on the Norfolk broads. They cost about the same for the same power, although they weigh nearly twice as much! In fact they are high power servo motors where speed and direction is all (highly complex!) electronic - so there is no multi-split commutator (liable to cause more brush wear and failure for shorting in the commutator breaks) and reverse is entirely electronic, whereas the Lynch-type motor needs high current contactors for reverse - and anything electro-mechanical is more likely to fail than pure electronic.

However, in choosing between the two types, what really decided me was that

(1) the supplier I'd found, Rupert Latham of Stelco Yachttechnik, had fitted both Lynch and SEM motors, but in his experience the Lynch motors had a pronounced whine... NOT something I wanted! and

(2) Various other reports of conversions I'd heard of all seemed to recommend a Lynch motor had a reduction pulley (or gear) for best performance.... whether correctly, who knows! I came across a lot of disinformation in my researches!... whereas the SEM type could drive the propellor directly.

(3) The SEM motors work at nominal 72V (volts) whereas the Lynch devices mostly work at 48V. The currents the SEM motors take are therby lower for the same power - and we are talking whopping currents! 200A in our case - would be 300A at 48V! - so short lengths of 400A cable are OK. But for 300A.... cable losses would easily escalate (and they might get hot!)... one would quite possibly have to be starting to consider busbars

To avoid the cost and potential failure of any sort of pulley or gearing mechanism struck me as well worth the extra weight of an SEM motor.

Rupert, incidentally, said according to his data, 8kW should be more than adequate for our boat. On what basis, I'm not sure, since nothing I'd been able to track down seemed to have convincing arguments people actually knew what power a 38' narrowboat might need!

Incidentally, another reason for choosing an SEM motor is that they are the type used for fork lift trucks, crane motors, etc. - been used for yonks as sturdy high power drive units whereas Lynch motors, yes, have proved to be extremely good, but are promoted not least for their light weight. Assuming my ballasting calculations worked out, in a narrowboat a bit of weight wouldn't matter - especially if it merely replaced previous ballasting blocks!

Technical spec of this motor is:

10kW SEM motor upgraded to 12kW for the fan fitted (visible at the left-hand end.) To be honest, I'd wanted a genuine 12kW rated unit and Rupert rather rushed me into accepting this particular version - not that it doesn't work very well!

Nominal working voltage - 72V (so max current approx 200A)

Size: (length, diameter), 539, 304 mm or 21.5", 12"

Weight: 102kg (225lb.)

Approx cost (pre Vat) £1,800 + £100 for the fan.

Supplied by Rupert as above, email rupertlatham@tiscali.co.uk

I'll deal with the drive-shaft details and control unit and overall boat performance in other postings, but, be clear, this tiny motor performs quite incredibly well. It is basically totally silent - although at certain low speeds between it and the control unit there is a slight audible slight tone...

For mounting, Rupert provided "laser cut" mountings - why it should be so grand they are laser cut I don't know!.... perhaps because that way they should have no hidden metallic faults. Alan bolted these mountings to his adapted frames using standard rubber engine mounts - the motor doesn't really vibrate at all, though.

The motor is coupled to the propellor via two flexible bearings (ball races) and a shaft, where the stern-most transmits all the linear thrust from the propellor directly to the hull - so the motor is only providing torque.