Sizing a propeller to match an electric motor

Given that some found the graphs presented by the OP confusing, I'll display a couple graphs that are typical of 1. an ICE outboard ...



... and 2. a BLDC motor.


Hmmm. Maybe that doesn't show up? Well, here's the link to the MIT article that shows the corresponding curve shape at the end of the article.: D.C. Motor Torque/Speed Curve Tutorial:::Understanding Motor Characteristics http://lancet.mit.edu/motors/motors3.html

What you will see is that the power curves have the appearance of a quadratic curve in both cases (a camel's hump) . If you showed only the part of the curve for the ICE that Gerr shows (chopped off at the power peak) then the general shape of the x-eng curve and Gerr's curve are extremely similar. The BLDC power curve from MIT, treated the same, way is also very similar in shape.

You could superimpose Gerr's "correct" prop curve on either and find, of course, that there is just one point at which the output curve and the prop curves cross... namely at peak hp, in both the ICE and BLDC case. So, there is no difference in picking a prop for either an ICE or a BLDC. Unless the prop (for either BLDC or ICE) is variable pitch, it is "correct" in just one one place on the power curve. But that has been the fact for more than a century. This is why every boater who cares tests his or her prop to get the pitch right. Certainly, if one were unhappy with the natural curve of a BLDC motor, then tweeking the controller parameters can alter the shape of the output curve, something that cannot be realistically done with an ICE. There is no prop curve that "matches" the shape of either the ICE power curve or the BLDC curve. The prop curve is naturally inflected in the wrong direction for that... in other words it is a hollow, whereas the power curve is a hump (quadratic)

Extending this thought a little: Down at the "low end" part of the ICE power curve, the prop is operating at a lower speed, and the engine is producing far less power, so clearly a prop for (say) 100 hp at peak is obviously completely wrong for the 20 hp of "low end" power. So... just be happy that the prop curve and power curve match at peak hp. And as I said before, just call High School Harry and have him match up the prop and the engine. Many boat dealers are out there every day, doing just that, and the process is described in many outboard motor owner's manuals.

Once you have the craft doing 40 knots, and want it to go 41.5 knots, come back and ask Hardiman some more questions. Or examine the prop on a winning small hydroplane.

BTW, you cannot "burn out" a bldc motor by picking the wrong prop, as was claimed above. The controller prevents that, no matter how hard you try. The same is true, in a practical sense for any brushed PM motor too: the controller prevents that... unless you are entirely oblivious to it all and fail to read either the motor manual or controller manual. This is true even for loads that are much less "gentle" than driving a prop, where at low speed the prop simply cannot absorb enough power or require enough torque to heavily load the motor. Put your electric motorcycle against a wall and "floor it" (really "twist it") eventually after you've smoked away half your rear tire, the motor just stops, in response to overheat or simple a timing of too much amperage for too long.

So... just build the boat and prop it just as you would for an ICE of the same HP. (It is, incidentally easy to find a BLDC motor with its power peak at the same rpm as that of a stock comparable outboard motor engine.) Having found that motor, then, as Gonzo correctly said, there is no difference at all. A 6.5 hp electric-motor-powered-foiler will go exactly the same speed as a 6.5 HP ICE boat, given the same displacement and appendage shapes. Big heavy boats with clunky stock lower units and crummy looking aluminum props, both of which that look like they were carved by ax are on lakes every day going 40 and 50 mph.

 

And to further bloviate:
Also, any electric motor producing the target power will also make the boat go the same speed: Brushed PMDC, Synchronous, Non synchronous, PMAC axial flux , radial flux , transvers flux (like a Mercury electric) something you wound in your basement and magnetized with those little dogs whose noses stick together or repel... any of those will work, given the target output power.

 

Thanks for the good info, wing guy!

Thought you might find this link interesting, which speaks to using a different propeller with the same motor, as well as changing other significant considerations and the effects on speed and range, post 252,

Efficient electric boat https://www.boatdesign.net/threads/efficient-electric-boat.27996/page-17

 

I agree that the controller should prevent the electric motor from failing. However, for people mix and matching components, failure are very possible unless they have the technical background. I believe that buying a motor and controller from the same manufacturer is a safer approach. That is, from a reputable manufacturer.