Gambler, Mark has had a look and the problem is 24 volt motors are running on only 12 volts. They will not have the torque that the motors should provide, less than half, but double the current consumption.
Marks solution is IF the speed controllers are 24 volt rated then you need a 24 volt alternator and your current batteries wired as series. Batteries may be low on capacity but the voltage will allow the motors to run at the correct efficiency.
If the speed controllers are 12 volt you will need 12 volt motors
On a side note the motors will generally consume 20/25 amps at full load which means the existing batteries will have a short full load run time.
Mark is happy to help with any further info should you require it and he feels the 6hp motor is more that adequate
Hope this helps
