Re: Thanks Coots, getting started on "stuff"
It would be an interesting experiment to compare running one vs two motors driving the boat at a given speed. It's likely a wash in terms of loss in the controller and given near identical prop characteristics between the two, you're talking about doubling the prop loading in the 1-motor scenario compared to 2. At low loads (and speeds), that might not amount to much of an efficiency reduction to drive with just 1 prop, and might even result in the prop operating closer to its higher efficiency zone (Gerr's prop manual might inform here). But as the boat drag goes up with the square of the speed, at the higher speeds, having the dual props and dual motors sharing the load likely wins out big time assuming we're talking about speeds where the loading nears the high end of the motor's rating. And motor torque via motor current delivers that force.toggle quoted messageShow quoted text
Motor heat loss (2mtrs) ~ 2 * k2 * Rmtr * (Fdrag/2)^2
= k2 * (Rmtr * Fdrag ^ 2) / 2
Motor heat loss (1mtr) ~ k1* Rmtr * Fdrag ^ 2
Assuming k1 ~ k2 (conversion losses in controller, prop loading, etc. are near same), we should expect the single-motor configuration to have about 2-times the heat loss as for the 2-motor case.
So even if prop efficiency for the 1- or 2-motor scenario were identical (unlikely) at the high speeds, we should expect twice the heat loss.
Now, how much of the total power to the controller(s) is lost to motor heat due to motor current? It's a lot, but the efficiency of the propeller, combined with the drivetrain is the shortest stick---this could be as high as 70% at speed or as low as 50-55%. This, compared to a difference between 90% vs 80% efficiency for a lightly, vs highly loaded motor. Controller efficiency is likely in the 95% zone. Cable and battery losses also factor in.
Add to this, dragging an undriven prop thru the water---best to drive that prop with just the right amount of power that its current draw just starts to increase. At that point, it is presenting minimal drag.
Bottom line: Prop efficiency is paramount. Loading the prop to the point that its operating point moves out of the higher efficiency zones quickly defeats other gains you might attain elsewhere (e.g. by driving with just 1 motor). Add to this, I^2R losses in the motor and battery/cabling quickly conspire against you. But it all depends on the loading. At low speeds and loads, the single prop drive likely wins out. As speed and load increases, single prop loses.
Still, let the data speak :-) .
From: email@example.com [mailto:firstname.lastname@example.org] On Behalf Of John Kohnen
Sent: Wednesday, September 2, 2020 10:55 PM
Subject: Re: [oregoncoots] Thanks Coots, getting started on "stuff"
I visited Cal today. His ammeter was hooked up somehow so it measured the current going to the motors, at least, but he was using a 200 amp shunt for a 100 amp gauge. which I now know isn't kosher. We moved the shunt to the common ground and replaced the shunt with a 100 amp one Cal had lying around. Maybe even the one that originally came with the gauge. <g> He should be getting more accurate and complete current readings now -- if the shunt's mV rating matches the gauge -- but we can't tell until we get the boat in the water, and Cal bunged up his knee so that'll be later, after he gets it rested up.
The dial ammeter Cal's been using isn't very precise. It has a range up to 100 amps, and the swing of the needle is only about 90 degrees. We really should keep trying to talk him into getting a digital gauge so he can keep better track of the current draw under different conditions.
Like trying different propellers.
Cal's idea about switching off one motor to save juice is an interesting one to ponder while sitting in our armchairs. Since he started dreaming up Surprise I thought Cal only needed one of the 80 lb. motors for her.
But now he's got two bolted to her bottom. Let's say he's cruising along with both motors at 3 mph. the switches one off and "throttles" up the other until he's going 3 mph again. Will he use less juice? The motors have a fairly efficient pulse width modulated speed control, so not much juice is wasted when running both motors at a reduced speed. If he switches off one motor it's still there under the boat with its propeller free wheeling causing drag. Two motors running slow, or one motor running harder and dragging a dead motor. Which will give longer range? If we can rig up a more precise ammeter in Surprise we can have fun doing some experimenting. :o)
I looked at the documentation that came with Cal's motors. Their instructions for installing them are probably OK, but I was looking for specifications. In vain. <sigh> But I did find something interesting.
They said that the rule of thumb for current draw for their 12 volt motors is 1 amp per pound of thrust, but the rule of thumb for 24 volt motors is 3/4 amp per pound of thrust! I wonder whty it isn't just half the draw of the 12 volt jobs, as you'd expect... So Tuffy;s 50ish amp draw at full chat isn't above her rating. Cal's motors could draw 120 amps at full power,
On 9/2/2020 7:56 AM, Electri-Cal wrote:
A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It is a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. (Jimmy
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