36 volts on Aluminum hull

[Bill Reynolds]

My friend has a 17 ft aluminum boat with a 36 volt Ulterra. With limited space in the bilge he has to use the starting battery as one of the three to get 36 volts. I know the motor creates a ground path through the transom bolts and have always assumed it to be a high resistance ground so never thought too much of it on 12 volts.
However, this same scenario at 36 volts makes me nervous.
The starting battery is bat1 in the wiring order, would changing it to bat3 make a difference?
Is there a way to isolate ground and still maintain the 36 volt series?

[Woody b]

I don't think bat1 or bat3 would make any difference. Electricity doesn't "know". What group 12 volts batteries does he have? My 50 amp hour Ionic (12 volt) lithiums are tiny. I'm sure 3 would be smaller than 2 group 27's.

[CatFan]

The starting battery needs to be the low one in the string. Any other position will cause current to flow between metal things that touch the water and cause rapid corrosion or possible current to flow through the hull. You don’t want any of those things.

[Bill Reynolds]

I don't think bat1 or bat3 would make any difference. Electricity doesn't "know". What group 12 volts batteries does he have? My 50 amp hour Ionic (12 volt) lithiums are tiny. I'm sure 3 would be smaller than 2 group 27's.

I am not sure if they are 24’s or 27’s but we can measure the space. I have 50 AH Lithium’s in my boat so we can go by those dimensions. Sure hope they fit.

[Bill Reynolds]

The starting battery needs to be the low one in the string. Any other position will cause current to flow between metal things that touch the water and cause rapid corrosion or possible current to flow through the hull. You don’t want any of those things.

Thanks, that it where it is wired, Bat1 which is the motor battery.