Need help understanding amps/thrust for electric motors

[The Greenhorn]

My wife and I recently purchased a used 17' bass boat. It came with a 115hp Yamaha motor. Unfortunately, in my area the majority of the lakes are electric motor use only. In fact, on my favorite resorvoir, a boater cannot even have a gas tank or lines in/on the boat. So we took off the motor and the tank/lines. I have a Minn Kota 55lb. thrust on the back and a Minn Kota 70lb. thrust on the bow. Both are new motors.
The problem that I am having is understanding the batteries. The 70 thrust is a 24 volt system so I have set the batteries up in the proper setup. The 55 lb thrust is a 12 volt system. I am trying to figure out how to read a battery so that I know how much time I am going to have on the water running at the 5 forward speeds. All of the information that I have been able to find deals with marine batteries where there is also a gas engine involved. This is not helping me.
Can someone explain to me how I can figure out the time on the water that I will have running at the various speeds?
For instance, if I have a battery with a reserve capacity of 205, does that mean that I can run my 55lb. thrust at half-speed for approx 205 minutes(with a 20 amp. draw)? What about the 24-volt, 2 battery setup for the 70 lb. thrust? If I have two batteries each with a reserve of 205, does that double and give me 410 minutes on a 20 amp draw? I am very confused and would really appreciate some help. Thanks for taking the time to read this.

 

[Texasmark]

Re: Need help understanding amps/thrust for electric motors

Surely battery OEM sites would explain this.

Consumption rate is paramount in figuring time to run. Part of that is battery power lost in heat from the current generated. The higher the current the hotter the system and the higher the resistive losses. Current x resistance is power dissipated (that was stored in your battery as energy) that is not usable for the job at hand. It's not linear, like half the current will get you 4x the running time and vice versa.

Part of the problem in trying to answer your question is defining the system losses which the battery mfgr doesn't know in your situation......like is your TM on the end of 20' of #10 wire, or 10' of #4 type thing which can make a tremendous difference in run time.

Other things come into play, size of charge in battery and ambient temperature.

I think I had a Minn-Kota back in '89 that had a consumption graph on top of the troller. Answered your question but they had to estimate the variables I mentioned in order to make the chart.

24v systems usually give you better service as the 24v, for the same thrust would only use half the current and do the same job considerably longer.

HTH

Mark

 

[Zackman]

Re: Need help understanding amps/thrust for electric motors

I agree 100% with what Mark wrote that the discharge rate will not be linear, but I found this question very interesting...

To give you an idea of the calcuation processes... an estimate calculation I think (emphasis on think) that these trolling motor are rated in force (e.g. pounds of thrust). From this you need to calculate the torque, so you would need to know the size of the impeller and the amount of slip. Next you would need to know the speed of the prop so that you could calcuate the horsepower. Once you knew the horsepower you can convert that to Watts then divide that by the volts to get the amp draw. But you still wouldn't know since the battery is more than likely going to draw down much faster than its rating, as Mark has stated.

 

[jtexas]

Re: Need help understanding amps/thrust for electric motors

on the 24V system, 2 batteries in series, you get 24 volts and the amp-hours of the lesser one.

You can double your run time by making 2 parallel banks of 24 volts using 4 batteries - more than double in fact, since a slower drain rate means more power.

Actually, you could run both motors off 4 batteries in the configuration I described.

I guess weight isn't an issue on your boat........comments about hole shot come to mind, but I'm not going there............

 

[Texasmark]

Re: Need help understanding amps/thrust for electric motors

Well Zack, I totally agree with you but I don't have those figures on the motor to make the calculation and I'd bet you don't either. I did think about the calculation you mentioned but abandoned it for the reasons given.

Additionally, the initial thrust (current draw) required to get the boat moving would be high (lot of water resistance to overcome) while once the boat was moving right along, it wouldn't take much thrust to maintain speed, comparatively speaking. So there is yet another variable and couple with that the fact that the wires ran cooler at the lower current so there was less wiring resistance, hence more power available to operate the motor which amounted to reduced battery power consumption for a given amount of thrust....................... on and on it goes.

Mark

 

[Zackman]

Re: Need help understanding amps/thrust for electric motors

Mark, I didn't mean to imply that you or Greenhorn could / should do the calculations, just some thoughts on what might be required to do the calculatons he was considering.

 

[Texasmark]

Re: Need help understanding amps/thrust for electric motors

Actually I wanted to do the calculation as I like to do that sort of thing, but I don't have the numbers to plug into an equation, nor the equation, in an attempt to get an answer. If I had some battery/motor spec sheets then I could make some assumptions about some of the variables and come up with something for a ROM guess, but I don't so I abandoned the idea.

Thanks for the consideration Zack.

Mark