When the current lockdown is over I WAS going to pick up the carcass of a big big outboard I got for next to nothing, pull it apart and see if it could accept a little 10kw electric motor, without the need for a sophisticated engineering shop. I was thinking its pretty robust gearbox might be suitable for the forces of a larger diameter slower propeller. Then there's the question of finding a way to graft this larger propeller onto the output shaft of the outboard gearbox.

I am satisfied that a Yamaha 9.9 Hi Thrust 4-stroke petrol motor (even with its little 9.25" x 7" propeller) would do a sufficient job for my needs (a 4-5 ton 26' scow sailboat) so I am theorising that a 10kw home-made electric outboard with a bigger diameter propeller and suitable reduction, via pully-drive and its gearbox, would be at least as effective, if not better. It is even conceivable that its own original 14" dia high pitch propeller (for high revs and high speed) might be not too far away in pitch from matching my low revs low speed requirement, but maybe that's daydreaming.

This rush of blood to the head came about when a GoldenMotor 10kw brushless DC motor with Kelly controller, appeared on trademe, with (at the time) no bidders, at NZ$1000.

It is 72V, but the vendor assures me that is way more efficient for the purpose than 48V (less amps, less heat generated) and required smaller diameter wiring. The thought of a home-made electric outboard motor for not much more than $1,000 appealed (and still does). I have noticed that all of the marine electric propulsions systems you can find on google seem to be 48V or less, which concerned me enough to ask some of the more electrically savvy JRA members their opinions, and I’m glad I did. (See “Electric Auxiiary to Antarctica” thread.) I am told that this motor could still be used at 48V. It is rated at 160 Amps continuous, which I suppose means it could still function nicely enough at 48V as a 7.5kw motor, and that would still be enough. However, there is another bidder now, and also I am not knowledgeable enough to know what other electrical parts would be needed to complete the system, or even if they would be available off the shelf somewhere. I wish I had more knowledge.

I still think a home-made 10kw electric outboard motor may be possible, and ThunderStruck (an American company – same brand as Kurt has on Mehitibel) have a marine conversion kit that is made for 48V and revs at 2,400, including controller, mounting beds and belt drive reduction. It is is meant as an inboard motor but would serve just as well if it could be belt-driven onto an L Drive such as a suitable outboard motor leg. ThunderStruck seem to be very responsive and helpful. The cost of these electrical components imported into New Zealand is around the order of $5,000 or a little more. I thought this might be a better proposition than simply purchasing a 10kw Torqueedo outboard, which at more than $20,000 in New Zealand is totally outside what I could afford anyway.

I am prepared to leave batteries and solar panels out of the equation for the present, because at my rate of building it will be at least a year or two (or three) before I will need them, and then (if I don’t beat the project to the finish line) they might be cheaper – and then, if the live-aboard project is successful enough to warrant it, I could perhaps justify putting the last of my savings into a decent battery bank and offsetting some of the cost against the domestic requirements of the vessel, for off-the-grid iving. What is needed at this point in time, really, is just a decision whether to provide for an electric motor, or my original plan for a Yamaha 9.9 petrol motor of which a good one is available right now for about that same figure of $5,000.

So, the next thought exercise is to use the Yamaha as a benchmark for performance, and see what else might be available (apart from a home-built outboard) for about the same price. Kurt has suggested a pair of electric trolling motors. I am not sure if two small propellers equals one medium propeller, in terms of effective thrust. Its a good thought, but  I’ve only provided one outboard well, and I’m past the age where I can hang over the back of a transom fooling around with outboard motors.

Yesterday I took a second look at an electric outboard I had previously dismissed on account of its flashy advertising: the delightfully slim and compact Navy Evo 6.0. Its got a 6kw motor and runs on 48V.

Advertised in New Zealand at about $5,200 and touted as a replacement for a 9.9hp petrol motor. Well, that’s less than 10kw, but ought to match a Yamaha 9.9 for price and performance (except for range, of course.) I don’t believe the price – there appear to be none in the country right now and the next lot to come in are sure to do so with an increase in price, who knows. And by the way it is being marketed I suspect there are some required accoutrements which are not included in the package. We’ll see.

Edit 23/9/2021 got a quote from Beacon Marine in NZ :wait time 2 weeks long shaft Navy 6.0 plus side mount control NZ$5232 - plus GST = $5,886

But as this motor, performance-wise, fits so nicely into the small coastal cruising junk rigged boat category (as indeed does the Yamaha 9.9) I thought I would do a comparison, and hopefully attract some further comments and advice from other JRA members out there who are always generous with their knowledge.

The price for each of these motors is roughly equal – I am looking at present at a Yamaha 9.9 which is a couple of years old but still in the box, never been used. Lets say a bit over NZ$5,000 for each motor (and remember, the cost of components for a home-built 10kw electric outboard is also likely to be of the same order.)

The Yamaha 9.9hp long shaft “Hi Thrust” model is Yamaha’s answer to the need for pushing a displacement boat at hull speed. I have used one in the past on a 6m launch, and it was the sweetest motor I ever had. Two of our members have used one of these successfully as auxiliaries on their well-known junk rigged keelboats Footprints and Shoestring.

The propeller on a Yamaha HT is 9.25” and the pitch is about 7”.

It is too small in diameter to be efficient – but in practice, it has proved to be effective enough for a sailboat auxiliary. The motor runs at 5,000 rpm max, but at (say) 3,500 rpm let us say it would give a hull speed of 5 knots in flat conditions. The reduction gear is about 2:1 so putting that through some calculations we can see that it is not a very efficient arrangement – the propeller slip is about 56%. Still, that sort of figure is not uncommon in dispacement boats, and “the proof of the pudding…” as they say.

The Evo 9.9 comes with a choice of two propellers, and the lower pitched one is 13.5” diameter by 8.5” pitch. A much better diameter – but perhaps a bit over-pitched for our requirements. This motor revs at 1,550 max. and being in a submerged pod, is directly coupled to the propeller (no reduction.) Let us say that at 1,200 rpm it is capable of pushing the same boat at the same 5 knots in calm conditions. In that case the propeller slip works out at 45%. A few more revs might be needed to reach 5 knots - maybe 50% slip is closer to the mark.

I am not sure to what extent propeller slip can be regarded as a measure of inefficiency (but always open to being educated) - anyway,  I think what this tells us is that the two motors are going to be very much in the same ball park regarding performance – with the potential edge perhaps going to the Evo, since its propeller is considerably larger and its revolutions are lower. This, of course, for only as long as the batteries have power, hence the use of the word "potential".

The question of range, and the downside of having to rely on battery power, is a whole ‘nuther subject. Suffice it to say, from my point of view, I think I could live with the limitations of a decent size bank of batteries and solar panel array, taking a leaf out of Kurt’s book and using the motor only very sparingly and almost never trying to motor at more than 3 knots.

The possibility of regen has been discussed on another thread. I don't think it would amount to much, but it may be a ittle bit of "icing on the cake".

One last point – this 6kw Evo (9.9 hp), if its price is real (I have yet to hear back from the distributor) – kind of kills the idea of trying to make a 10kw outboard from a big old outboard leg.