Resizing a prop for a small electric outboard.

Hello,

I am up to version 3 of my electric Franken-outboard-motor and it’s going well.

I started with the leg of a blown 2.5HP Mercury outboard, discarded at my marina. Version one was to attach a 12 volt motor from a discarded macerator pump (yeah, a bit gross) and grab the car battery. Motor was about 100 watts. It ran for about a minute before overheating and cutting out, at which point I realised I had forgotten to bring the oars. A bit embarrassing as I drifted back to the nearest marina finger while my neighbours enjoyed the show from their cockpits.

But it was enough to make me think it was worth spending some actual money rather than ratting around the marina skip, so I bought a good quality 350 Watt 36 volt motor and a speed controller. I chose the wattage and voltage because that’s the most current I dare draw from my 36 volt electric bike batteries, and I don’t want to buy or store more batteries on my boat.

So version 2 was to try the 36 v motor and controller which ran magnificently for about 30 seconds before my poorly designed drive shaft attachment exploded in a shower of aluminium filings.

This time I had remembered to pack the oars.

Version 3 was basically version 2 but I asked a few brain cells to take part in the bit where I attached the drive shaft to the motor. Eventually they came to the party resulting in a text-book trial run around the marina, slowly increasing the power over time to the point where I dared to use full throttle. And, to my great and happy surprise, and the mild disappointment of my neighbours, it all worked.

Now I’m thinking about the prop. It must be way oversized. The Mercury engine was designed to run at around 3600 rpm at a guess and my electric motor runs at 2700 rpm max. Gut feel is, despite the lower rpm of the electric motor, that I’m badly over-propped. No worries, it’s a plastic prop and I own a lot of cutting, scraping and fairing tools, so a smaller prop should be no problem.

But how to do it sensibly? Do I make the blade chord shorter or the prop diameter smaller?

Should I leave it alone? I mean, it works now, nothing blew up, but my instincts tell me I’m probably missing out on a bit of power by limiting the electric motor rpm. Or is that IC engine thinking?

Over to the sort of people who remember to bring the oars on the first attemp.

Matt

 

Without any essential details on motor torque characteristics, prop pitch and diameter, boat hull shape/ dimensions, Target top boat speed / range Etc- everything will be a guess on our end.

That outboard leg may be pretty lossy, because of appendage drag and especially if there's thick gear case oil lubrication which creates viscosity frictional losses. An Asian long tail type design is generally more efficient.

This video gives you an idea about how a long tail motor works, if you haven't seen it already,

https://m.youtube.com/shorts/vv76m78Z3W0

If the electric motor is getting very hot quickly from a cold start at Full Throttle, that might indicate that it is over propped. The solution is to engage The throttle control which can increase the efficiency tremendously and run the motor at slightly warm to the touch, assuming the resulting speed decrease is acceptable for your use. If you want to experiment with propeller diameters and pitches, probably the easiest and cheapest way- is to use APC model airplane props, some of which are priced well under $10.

 

Well, I did actually state it was a 350 Watt motor with peak revs of 2700 rpm. And DC electric motor curves are basically flat.

As for the boat and hull form? At 350 watts you could probably figure out we are not talking a trawler. Maybe a dinghy? Perhaps?

I mean, seriously.

And frankly, what does it matter how an outboard works, clearly Mercury knew what they were doing when they fed 2.5 HP in the top at around 3500 rpm. They will have figured out the losses already. Now I’m feeding 350 Watts in the top at 2700 rpm, there’s the relevant data.

But it’s ok, you proved how much you know,

 

Matthew,

You could decrease the cord on the blades by removing material from the trailing edges and see if it runs better.
Don't remove too much as it's really hard to put back on!

Good luck with your project.

BB

 

I also felt chord was the better one to reduce. I could probably rig up some kind of bollard pull scale…

 

That’s your response to someone who took the time to answer your post asking for input?

 

No, that’s my response to someone so anxious to prove what they know that they didn’t actually read what I’d written.

 

If we skip the rather small influence of forward speed, the diameter required can roughly be written as: D=const*Power^0,2/(rotational speed)^0,6. If your numbers are correct (which I doubt....), then the prop dia should be reduced as follows: Dnew=Dold*(350/1840)^0,2/(2700/3500)^0,6.

All provided your electromotor will produce 350 W at an output shaft speed of 2700 rpm, and the original ob actually did turn out 2,5 hp (~1840 W) at 3500 rpm.

 

Well, that’s very useful.

But why do you doubt the numbers?

 

Because some are obviously wrong, and some need clarification in spite of your attitude......

 

Ok, so which are wrong and which need clarification?

 

Start by showing us exactly which 2,5 hp engine that produces its rated power at 3500 rpm.

 

Well, frankly, I can’t. That was a number given to me by a small engine mechanic who lives at the marina. Unfortunately whoever threw out the outboard neglected to include the owner’s manual.

What do you think it might be turning over at?

Any other numbers worry you?

 

Matt those little two stroke outboards only achieve useful torque by having the ring reamed out of them throttle wise and are designed to run at up to about 6000 rpm at the crankshaft and HP rated at ~5500RPM. Your electric will try to keep up with the torque requirements while it can obtain enough current while the petrol will only have useful torque in a very narrow range. Modern vehicles are able to use fairly tiny engines because constantly variable electronic transmissions can keep them with a specific rev range. We are only just getting under way with electric boat propulsion in a serious manner and Minn Kota style makes the most sense currently, submerging the motor so that there is direct power transmission to the prop and water cooling. On a small setup you will lose significantly through the seals, bearings and bevel gears. If you left the impeller in place that will triple or more.

 

Yes, I was a little surprised by the 3600 rpm, but then this guy was a small engine specialist so I went with his suggestion.

At the end of the day, if he is wrong, it doesn’t change the main issue, except in magnitude I guess. That is, I know it is over propped, but by how much? Maybe much more than I thought.

As for the rest of it… yes, I know about the losses, and of course I removed the impellor. I went over the whole leg to find any sources of drag, including fresh higher viscosity oil, since it looked like it still had the original oil (tar) from the factory.

But a decent new electric outboard starts at around AU$2K, whereas I’m not even AU$200 into this one and it is working very well. I’ve still got the thumping great 2 stroke to get the Highfield 3.1m tender up to around 20 knots when needed, but after years of cruising I learned that 99% of my travel is between my boat and other anchored boats, balancing crockery, glasses, a cooked dish and few bottles of booze on my lap. Then heading back home to my boat five hours later a little… er… less well coordinated. Plus most of the anchorage is usually asleep by then. So this quiet little thing is perfect.

Early tests suggest a good 35 minutes of run time at full power, probably an hour at efficient power, giving me a range of a one to two miles in good conditions. (Forgot to bring the gps so I’m guessing at speed, it felt around 1.5 to 2 knots.) Certainly enough for a night of revolving boat dinners (one course on each boat) and enough to run ashore for the groceries.

Would I like a proper motor like a Torqueedo? Sure. A friend loaned me his and I loved it. But the $3k is better spent cruising. This one will be fine.

But, it would be nice to get the very most out of it, hence my propellor question.

The rest of the issues are academic.