Re: 12 volt vs 24 volt
Fact is, two batteries in series for a 24 volt system has whatever reserve capacity or AHr rating as one battery. Two batteries in parallel still provides 12 volts but reserve capacity or AHr rating doubles. So what the OP didn't tell us is how big the 24 volt motor is and what it's maximum current draw is. I can safely assume the 12 volt motor is 55# since that's the largest available for 12 volt operation. For a MinnKota 55 PD-V2 max current draw is 46 amps. To make a fair comparison a 24 volt 65# MinnKota draws 40 amps max and a 70# draws 42. So, you can draw your own conclusions. Battery capacity and current draw are the key numbers -- not so much whether the motor is 12 or 24 volts. To put this in perspective, lets run some numbers. If both motors ran the same size batteries (let's use a group 27 deep cycle with 200 minutes RC @ 23 amps which is a standard spec).
At 46 amps (12 volt motor) that is twice the current draw as the spec listed above so that cuts the reserve capacity in half resulting in a 100 minute run time -- AT MAXIMUM CURRENT DRAW. But since we have two batteries that puts us back at 200 minutes.
At 40 amps (24 volts) the current draw is 1.74 times the above spec vs 2.0 for the 12 volt motor. So 200/1.74 = 114 minutes. Keep in mind here that the two batteries in series DOES NOT double the capacity -- it doubles the voltage. Therefore run time AT MAXIMUM CURRENT DRAW remains at 114 minutes.
So the net result is that the 24 volt motor will theoretically run far less time than the 12 using two batteries AT MAXIMUM CURRENT DRAW. Since there is no published information on current draw at slower speeds because of all the variables, it cannot be determined with any accuracy what slow speed comparisons are but current draw is fairly predictable. It must be remembered that this comparison is like comparing two cars with similar HP but one has two fuel tanks so run time favors that two-battery configuration. On the other hand, the big motor may be able to be operated at slightly lower power settings than the smaller motor, so some run time savings would be achieved but it would not likely make up the difference.
Put another way, if the 12 volt motor was operated on one battery then the 24 volt motor has a 14 minute run time advantage.
