The wires should be big enough. The wires are sized to the amount of current they will be carrying and the 24 volt trolling motor should pull no more current than the 12 volt motor. You can check the current draw on both motors to be sure. Make sure that the wires you connect to the 24 volt system is only going to the trolling motor and not to any other devices like depth finder, lights, etc.. I believe that you will get through the fishing day with plenty of battery left because the 24 volt motor will not have to work as hard as the 12 volt motor to do the same job.