Earthing an electric outboard motor.

[Alan S]

I am installing a 48 Volt outboard motor in my recently bought grp motor cruiser previously fitted with a petrol outboard. I have connected the 48V neg to the 12V neg but there is no connection from the 12V neg to the shore power earth. The ali and stainless body of the motor is isolated from the 48V supply. There are anodes on the motor but no anodes or any underwater metal on the boat.
Does the team think this arrangement is satisfactory as regards safety and corrosion? The manual for the motor does not mention earthing.

 

[William_H]

I think your concern should be with the earthing of the mains supply, presumably for battery charging. Ideally a charger should be "double insulated" ie no connection mains wiring including earth wire to the output wires to battery. If then the charger is the only device on the mains then the mains earth does not need to be connected to the ships negative earth. If however it is, then a galvanic isolator is needed to stop tiny leakage currents which cause or are caused by electrolysis corrosion. Current can come from the marina earth to the sea via your boat metal work so is a risk.
If on the other hand your charge is not left connected long term to batteries in the boat in the water and batteries ( Both) are isolated then should be no concern.
A complex subject so i hope we can get better opinions than mine. ol'will

 

[jamie N]

I'd disconnect the motor from the 12VDC battery entirely, as it's nothing to do with your new outboard. Did you feel the need to connect your petrol outboard in this way?
The anode on the new outboard was put there by the manufacturer, and is the one fitted to combat corrosion by the people who make it, with all of their engineering knowledge about the motor.
They know better.

 

[Alan S]

I think your concern should be with the earthing of the mains supply, presumably for battery charging. Ideally a charger should be "double insulated" ie no connection mains wiring including earth wire to the output wires to battery. If then the charger is the only device on the mains then the mains earth does not need to be connected to the ships negative earth. If however it is, then a galvanic isolator is needed to stop tiny leakage currents which cause or are caused by electrolysis corrosion. Current can come from the marina earth to the sea via your boat metal work so is a risk.
If on the other hand your charge is not left connected long term to batteries in the boat in the water and batteries ( Both) are isolated then should be no concern.
A complex subject so i hope we can get better opinions than mine. ol'will

Thanks Will, Charger is is fully isolated and never left on when I am not aboard. Shore power is used only for the charger and very occasionally for double insulated power tools so I think I'll leave out any connection between mains earth and DC neg. Seems less likely to cause corrosion problems, and any possible electric shock risk should be eliminated by the RCD.

 

[Alan S]

I'd disconnect the motor from the 12VDC battery entirely, as it's nothing to do with your new outboard. Did you feel the need to connect your petrol outboard in this way?
The anode on the new outboard was put there by the manufacturer, and is the one fitted to combat corrosion by the people who make it, with all of their engineering knowledge about the motor.
They know better.

The petrol outboard was connected to the 12V system. In fact it was the main consumer and provider of energy. Not sure to be honest though why I connected the 12 and 48 Volt negs. Just seemed logical. After all in a conventional set up with separate start and domestic batteries the negs are connected. Just doesn't feel right having a powerful 48 Volt system floating with no reference to earth.