12v trolling motor used as a 24v set up??
If you chain batteries such that pos meets pos meets the device, you are in parallel and the voltage remains essentially the same, just more juice. (This does not make it 24 volt, just a "bigger battery").
If you chain the batteries such that pos meets neg (on the next battery) and then the remaining open leads (pos and neg) are fed to the device, you have a series, like a flashlight, which gives a higher voltage (12, 24, 36)depending upon how many batteries are in the series.
You can do one, but not the other.
I did this with a Minn Kota 55PD, worked great.