Re: [Electric Boats] Re: Kipawa prop thoughts.

Have you tried measuring the voltage to the motor from the kelley controller when it runs at or close to 40A?
I'm not sure how a regular digital multimeter would handle the PWM chopped waveform. Would it read max voltage experience (ie: 24V) or would it average out? Perhaps an analog gauge would work a little better in this case.
Anyway, the voltage from the kelley controller to the motor will tell you what duty-cycle the PWM is working at and therefore the effective voltage being sent to the motor at max amp-draw.
If you have the rpm/volt constant for your motor, you can then obtain RPM and coupled with boat speed in knots you can determine the effective pitch of the propeller (assuming a perfect prop with zero loss). AT that point, it's anybody's guess as to how efficient the Kipawa props are. Personally, I never assume more than 50% efficiency. So double that number for the resulting pitch.

To recap:
1) get rpm/volt constant of your motor.
2) Determine effective volts from controller to motor when operating at 40A
3) get speed through water reading in kts, or use GPS speed averaged over two marekd-distance runs in opposite directions.
4) calculate!
a) (speed in kts) * 72913 / 60 <== This converts speed from kts to inches per hour, divide by 60 to convert to inches per minute.
b) (motor constant rpm/v) * (voltage from controller to motor) This gives motor RPM value
c) divide a) by b) to get real inches advanced per revolution.
d) divide by 50% efficiency (divide by 0.5 or just multiply by 2) to get a good estimate of prop pitch.

This is an exercise that can be done when you know the real pitch but want to know the efficiency at various speeds. A good thing to know for your boat, in my opinion. It will allow to objectively quantify improvements to your drive system over time.

Cheers,

/Jason

On Jun 18, 2015, at 5:09 PM, gwest@bmolaw.com [electricboats] wrote:

>
> I have a Minn Kota Endura 40 turning a 314 Kipawa prop on a small 16' launch. I have had that configuration for probably five years now. The motor is connected to a 24V Kelly controller (and two AGM batteries in series) and the controller is programed to limit 40A to the motor so the motor wouldn't burn out. The prop spins faster and moves the boat significantly faster than the normal Minn Kota at 12V, and the controller lets the motor on my boat run at 18 or 19V usually before hitting 40A. On a lighter boat, it would probably let is spin faster. (As I understand the Minn Kota names, the 40 is usually designed as pounds of thrust, but it also seems to represent the maximum amps to the motor before it burns up.)
>
>
>
> When I described this on this listserv a few years ago, I remember that a few people seemed to think it was a really bad idea and that the motor wasn't desi gned for higher volts and higher speeds. But it seems to run faster fine as long as I limit the amps. And if I have to replace a cheap $130 motor every few years, it's not the end of the world. (As an aside, I have the Kipawa on the boat under protest, since I really hate that the company will not disclose any information at all about the prop specs except that it "increases speed." I certainly asked, even nicely at first, for information about the prop, but to no avail.)
>
>
>



------------------------------------

------------------------------------


------------------------------------

Yahoo Groups Links

<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/electricboats/

<*> Your email settings:
Individual Email | Traditional

<*> To change settings online go to:
http://groups.yahoo.com/group/electricboats/join
(Yahoo! ID required)

<*> To change settings via email:
electricboats-digest@yahoogroups.com
electricboats-fullfeatured@yahoogroups.com

<*> To unsubscribe from this group, send an email to:
electricboats-unsubscribe@yahoogroups.com

<*> Your use of Yahoo Groups is subject to:
https://info.yahoo.com/legal/us/yahoo/utos/terms/