Hi Myles,
I'm flattered, especially coming from you with all your electric powered experience. Thank you.
Of course you are absolutely correct too concerning copper losses. In my initial sheet I did take into account resistance, but for simplicity in the final calculation I didn't for fear of confusing folk, but it is good that you have stated:
ShaftRPM = (Vm – Im * Rm) * NoLoadMotorRPM/volt (Kv) See link below for more on motor on Kv and Kt:
https://en.wikipedia.org/wiki/Motor_constants
Here are my reasons and explanations of why I displayed the data the way I did.
1) MMIs - My little joke, as whenever I was at sea as an ROV pilot if we had trouble with the ROV and had to stop our task, the skipper would ask what the hold up was. We'd tell him it was the MMIs and he'd say, OK just tell me when they are fixed. Sometime later he asked what MMI stood for. With a grin we told him the Man to Machine Interface ;)
2) Based on 1, I used these tools in the attached images and note there will be small errors in the tools and the human and timing of measurement. One tool not shown in the images in this post is a Clamp meter to measure current right near the motor terminals.
Sealey TM101 9V Function Professional NCVD Digital Multimeter.
Price: £31.47 UK pounds.
A digital tacho and an infrared thermometer with laser pointer. Inexpensive at £19 UK pounds for both.
Note I did try to wire up my KTY temperature sensor which is in the motor core, but I was unable to get it to work, hence the infrared thermometer to check various points on the motor, which is less than ideal but a good indication along with my controller temperature which rarely ever exceeds 50C. Note my motor has never been too hot to touch.
3) The no load speed of my motor or motor voltage constant is quoted by the manufacturer and that can vary a little too, hence I wanted to compare my shaft RPM with the tacho (which is quite a difficult location for me to measure and the the numbers vary some so I took many readings and averages). My point being is there are small percentage errors in all the calcs so they can compound, and it was better for me to take lots of shaft RPM readings and lots of motor voltage readings (voltage right at the motor terminals Myles) and current readings on the cables with the clamp meter as close as possible to the motor terminals. Having done all that I came up with averages and adjusted the sheet accordingly to demonstrate what is happening in the serial efficiency from battery to prop, be that bollard pull, open flat water and electric to diesel comparison. I wasn't so concerned at readings at very low speeds or max speeds, more the speed at which I normally go at.
4) Having said all that, the best way for me to illustrate Myles point is by images and graphs comparing motor efficiency vs Voltage vs Current and efficiencies at particular boat speeds. Note the actual voltage is at the motor terminals (DC motor) and not the nominal or open circuit voltage at the battery or elsewhere. The shaded blue area I have added shows the range at that motor running voltage that the amperage must be in to get the best efficiency (minimal losses). Of course the motor current depends on the load and the shaft RPM slows at higher loads (amps). The load being created by the prop and sailing conditions.
Hopefully though people will see immediately from this compound image/graph I made just how voltage vs RPM vs amps affects efficiency. In terms of motors and controllers etc. Think of efficiency this way. If I take measurements and see that the motor is producing V x I of say 1,000 Watts and I note the voltage and current at any given point of the motor I can then clearly see how and where I must choose my voltage/amps combination for a measured boat speed in knots - and that is done by choosing an appropriate prop which affects all this, as of course does gearing in a geared system. Easier playing with gearing note than new props. If motor was at 1,000 Watts and 90% efficient then of course you'd be losing 100 Watts of propulsive power to the shaft as heat. Anyhow I hope this helps folk see visually what Myles refers to.
5) So what does all this mean? To me it is about choosing a prop with gearing or not and the balance of a good motor voltage vs motor amperage at you normal cruise speed in calm waters to obtain the best overall efficiency at that speed. The place where you spend the most time at. My speed range where I spend most of the time is 3.5 to 4.25 knots, so I'm optimising for that as best I can without resorting to more prop fiddling or gearing, if I had it. The bottom line is I'm happy with what I have as a decent compromise using as few batteries as possible (cost) but sufficient for my needs of 15 to 20 miles range, with a small back up genny for serial running if needed. Sailing yacht after all!
The sheet I made with measurements which needs to be understood in terms of the points above, is here:
http://john.rushworth.com/Elektra_Performance_June_2016.xlsx
A final note, If folk want to get a good overview and other insights I suggest James Lambden's PDF ebook which he's created from hard won practical experience over about 15 r so years. Minimal cost.
http://electroprop.com/the-electric-boat-book/
John R
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