Sizing a propeller to match an electric motor

Propping a boat for top speed is not that hard. Any high school harry hot rodder can do it. The first thing needed is the boat. The second thing is the motor or engine.

I am struck by the confusion in this thread regarding how output curves are read. Some frequently sane person thought that the eletric motor curves showed zero torque at zero revs. That is not at all what they show.

Probably Torqeedo and EPropulsion have done almost as much to muddy the waters regarding the performance of electric motors as the makers of shop vacs, where it is common to claim 5 hp output from a motor that cannot consume more than 120 volts at 15 amps without blowing a fuse. 120 v x 15 amps is 1800 watts, which is 2.4 hp. In the US, when electric motors are spec'd with an HP rating, that rating is output (shaft), not input, so given the dismal efficiency of vacuum cleaner motors (65%?) you could expect a reasonable output rating to be 1.56 hp, which we can round to 1 1/2 hp. A far cry from 5hp.

Fortunately Mercury and Tohatsu are in the electric outboard motor biz now, so you will see electric hp spec'd accurately, measured at the shaft, just as marine engines are measured. It is worth taking a look at Aiqidi motors, from China. Their 7 HP rated motor has an input rating of 5.1kw (about 6.8 hp.) To their credit they are not using the "1 or 1.1 kw being 'equivalent' to 3hp" rule of fraud that both EPropulsion and Torqeedo have done. The 7 hp is a reasonable rating, given that the motor can be easily overdriven (beyond it's continuous 5.1kw rating.) Actual continuous shaft hp is about 6.2 hp, after slight controller and gearing losses... and even that is in the right ball park, a lot more reasonable than the 15hp that the fraudsters would claim for that input power.

Why do I mention this? Because the prop on my Aiqidi (mine is actually Servovision, from Thailand -- but it's the same motor) is the standard prop for a 6hp Yamaha outboard, from Yamaha, from which the entire motor chassis comes. Its tiny: 8 x 7.5, I believe. A prop from a 6 hp gas engine works just fine on a 6.2 hp electric motor. If, in the future, I decided to optimize efficiency at a particular speed in my catamaran or decided to motorsail at fairly high speeds (unloading the prop) am might change the prop pitch, and that process would be just like repropping any outboard motor for a boat of a particular weight and hull characteristics. Just call high school Harry. Done.

Controllable pitch marine props are expensive, but can be useful for owners who want a particular set of performance characteristics. Every boat should have one, just as every high performance general aviation airplane has one (or two).
In the typical 18 foot outboard powered boat, the prop for the best hole shots is not the prop for best top speed... so the "best" prop needs to be adjustable pitch. But isn't that getting needlessly esoteric?? This discussion has been needlessly esoteric, because tested resistance curves for this vessel have not been produced.

For any discussion beyond "Well, lets see, a twenty hp small hydroplane that does 40 knots has a 9.25 x 14 prop -- try that," we need real resistance curves, motor output curves for the selected motor, and the performance criteria of interest: maximum speed? maximum range? Earlier or later foiling point?

So OP: what is the competition about? What is going to be measured? What is the budget? Are the competitors all mechanical engineering students? How is the submersion of the propeller adjusted for foiling? Is full autonomy to be demonstrated over a particular course? Is this all just a thought experiment?

 

Whoops. I thought I remembered a mass of 220 kg. It's really 100 kg. Mea culpa.

 

There is a common misconception, fueled by sales hype, that all electric motors are the same. On top of that, that the HP (of kW) output of an electric motor is equivalent to a higher one in an ICE. As far as AC motors, the proper power measurement is Volt-Amperes.

 

In my world Volt-Amperes is used to measure the apparent power which includes the wattless power (reactive power) and it determines the dimensions motor cables.
But the electric energy which can be converted in mechanical energy is measured in Watt to distinguish it from apparent power.

 

That is correct. There is also starting current, which is higher. Sales brochures mix them all together and use incorrect units to make things look better.

 

Contrary to your incorrect claim that VA is the "proper" measure for an AC motor, Heimfried correctly states that watts in vs watts out (or hp out) is the standard for measuring electric motor efficiency. This applies to all electric motors of all types. Apparent power is of concern to power distribution companies, but not to the EE who is specifying (or selling) a motor for a particular application like an outboard motor. Many buyers are interested in efficiency, and it is always shown as watts out over watts in.

Also, starting current current is not higher (than what??) for electric outboard motors. Coincidentally, I recently measured starting current on my electric outboard, and it was 5 amps for a small fraction of a second and then settled to 1 amp, as the motor idled at 500 rpm under the load of moving water in a test tank. The outboard will operate at high speeds at 100 amps. 100 amps is more than 5 amps.

Although sales brochures are pretty sketchy for Torqeedo and EPropulsion, most real electric motor vendors do not "mix them all together" (mix all what together?) and "use incorrect units to make things look better." Reliance, GE, Westinghouse, and even the small vendors of motors used in the sailing market (such as Motenergy) all use the correct units to enable engineers to make selections, and all can produce dyno curves to show actual performance. Even the Asian brands with unfamiliar names use the correct terms on their graphs.

Motenergy describes their motors clearly, and provide dyno curves for them. When you buy one, the dyno curves are those produced for your particular serial number rather than smoothed and generalized curves. It takes a few seconds to run a dyno curve on a motor and spit out a print.

Here is a typical curve for a motor in the size range that the OP might want to use for this mystery competition.
https://sep.turbifycdn.com/ty/cdn/yhst-129399866319704/ME1716ME1717Plots.pdf?t=1709531008&

 

What is brake horsepower? | BHP vs HP https://mechstuff.com/what-is-brake-horsepower-bhp-vs-hp/

 

VA is how much stored capacity is needed to run an electric motor for the required range. I see most posts are using the output for calculations. Further, I correctly specified what the VA is needed for. Specifically, to compare an electric motor to an outboard which is rated at the output shaft.

This agrees with my statement. If the starting current is 5A, then the wiring and battery power have to match the requirement.

 

That's the worst article I have read in a long time. It is full-on bloviation, going on and on with unsupportable (and often wildly wrong) statements, when a simple definition from Webster's would provide far more useful information. And who, in this thread, asked for the difference between bhp and the various other forms of quoting hp?

Webster's definition is very close to the definition you would find in the Bosch Automotive handbook that every automotive or marine engineer has:
>> : the power of an engine or other motor as calculated from the force exerted on a friction brake or absorption dynamometer applied to the flywheel or the shaft.

That's exactly right.

Brake HP has numerous flavors (such as SAE Net, SAE Gross, DIN, etc) but they are all measured by a "brake" with a reaction arm (If the reaction arm is one foot long and the scale at the end reads 100#, then the engine is producing 100 lb ft of torque at that speed and throttle setting (generally wide open during the test).
Electric motors are measured with brakes, too, as you can see from the plots from Motenergy that I referenced earlier. (The "brakes' used can be eddy current reactors, generators, water drums, hydraulic motors, etc. etc: anything that can be easily regulated to load the engine or motor variably in torque.) You can be a human dynamometer with a tiny, 1.5 volt hobby motor: just grab the shaft with your thumb and forefinger, and compare how hard you have to squeeze (to slow it down) with the motor you are comparing it to.

HP is, for the most part, exactly the same thing. Every electric motor of consequence has been measured on a dyno, but many manufacturers do not use the B to show that. Ditto for car manufacturers and marine engine manufacturers: many just use HP with the B understood.

(If you tend to obsess about such things, then, the DOE in the US produces a guide to measuring electric motor efficiency, and they are fairly serious about it, because losses via inefficient motors costs billions of dollars and wastes resources. )

The term BHP came about to make a clear distinction with "indicated HP" which is calculated from mean effective pressures:
Webster's definition of indicated HP is also good:
>> : the power developed in the cylinders of an engine as calculated from the average pressure of the working fluid, the piston area, the stroke, and the number of working strokes per minute.

Because Indicated HP does not account for friction and the affects of various geometries, it is not a figure used outside of combustion labs or classrooms where the issues that contribute to the difference between indicated and BHP (actual output) can be enlightening.

But the difference between HP and BHP is not germane this thread. It's just more noise.

In outboard motors (since 1981) the quoted HP is shaft HP which is also BHP. In electric motors, shaft HP is measured by a dyno, and is therefore BHP. (In very small motors, the output that shows up in a computer simulation can be used, because nobody puts fractional HP motors on a dyno. )

The "difference" in horsepower that is germane to this thread is EHP vs BHP. EHP, as has been explained above, is the HP that actually has to be delivered to the water to move the boat at the desired speed. For the desired EHP to be delivered (say 20 EHP), the engine must produce more (say 30 BHP) because props vary from moderately inefficient to fairly dramatically inefficient. If the goal is to operate the system at best efficiency, then one pitch and area will work best given the constraints of space. If the goal is to operate the system at maximum speed, then a different prop will be required. (Then, in addition, if operating difficulties such as cavitation are projected to occur, then more tweeking is required.

But none of that has anything to do with HP vs BHP. Nor does the article linked, in any real sense.

 

Yes, that article was not meant to be taken seriously or analyzed at all. It was an example illustrating sales hype (bloviation) that can also occur with IC. Hype is not just Limited to electric propulsion as referenced in post #33 and some previous posts on different electric propulsion aspects. Apologies, I should have included this note with the posting.

 

Look through this link:
Something strange here about Motenergy... https://www.diyelectriccar.com/threads/something-strange-here-about-motenergy.178849/
I have spoken with John Fiorenza many times (although not in the last decade or so). He has designed a slew of really excellent motors, and sells them at extremely good prices. The B&S Etek motors were loved by thousands of people making electric micro cars, motorcycles, go karts, and boats. If I recall correctly, John designed the Etek. The Motenergy motors have both good documentation and completely reliable ratings. In fact, every motor I have received from them comes with a dyno printout for that specific serial number. That printout tells you precisely what you can expect from that motor. (The images of dyno curves provided for each motor model on the Motenergy website are, in my experience, extremely close to the actual chart for each motor received: most are within one percent of the published image I would guess, without actually trying to find one of the paper printouts). The Motenergy motors are made in China, but they are certainly not "no-name" (nor are they like so many things ordered through Amazon: a crap shoot... much of it functionally OK, but with awful documentation.)

 

Gonzo: Good news! I found one sentence that you wrote that is technically accurate! I knew there must be one!

You then launch into a fabulous misinterpretation of the graph, which is very clearly labeled and very, very, simple. How on earth could you conclude that the graph shows zero torque at zero rpm, when the graph is so clearly labeled? The general form of the graph - torque on the x axis - is extremely common.

 

Sarcasm will get you nowhere, try buying me a beer.

 

Having built or specified various battery and motor systems of less than 30 Kw output, I have put together spreadsheets to compare batteries, motors, etc.
I stumbled across some entries from about 8 years ago... or maybe less, like five. The figures are certainly out of date, but might be instructive for sourcing batteries/cells for this experiment.

I'd looked into packs and cells from the Leaf, Chevy Volt and Tesla, on the used market. I also checked prices on vendors that I had used or followed, for new (generally LiFePo4) cells or batteries. The Chevy Volt and Tesla modules are 48 volts (and then series connected, when in those cars, to get hundreds of volts). That 48 v voltage level makes them viable for experimenting. (You can still blow your hand off, or burn down your house by dropping a wrench across output terminals, but you are not likely to be electrocuted by 48 v... its right at the lower edge of what OSHA considers lethal. ) The Tesla (and maybe the Volt) are probably most dangerous, in terms of thermal runaway. The Leaf packs I don't know -- you can look them up -- I was not too seriously considering them for use at the time. So here are the numbers, USD, cost per KWh: Tesla: 211, Leaf: 130, Volt: 217, GBS: 572, Battleborn: 791 Fortune: 416, LG Chem: 212
At the time, I selected GBS, because I had experience with them and was adding to a pack that already contained those cells. But then, had I been building an autonomous little vehicle, (and not putting them in a boat that I would occupy) I probably would have gone with the Tesla cells (in module form ... you'd need considerable patience to make your own Tesla pack out of cells -- there are just too many of them. I think that, then, the Tesla cells could be discharged at a higher rate than some others. Now... you'd want to do some homework. If a person just wanted to throw together a pack of 4) 100 Ah 12.8 batteries in series(new, now) , I'd recommend Yeagulch. They seem fine, and are really cheap, and come very well packed and with adequate documentation. (The LiFePo4 cells from some Teslas are probably too new to find on the used market... but maybe not. They are less likely to burst into flames than the pre-2023 cells. )