Using engine as a negative terminal

[elias]

Hi there !
I m planning to use the engine as a negative pole for 2 wc motors at 25amps so I don’t use too much cable run direct from the batteries . I think it should be ok, any other experience ?

[SYJetzt]

I'm sure, your toilets would work fine but....

Usually all circuits should be wired with seperate + and - cabling and have the proper size of fuse for the current flowing in the lines.
The engine should have only one ground point (usually on the starter motor).
Usually the saildrive should be electically isolated from the engine, but if there is any unintended connection between both, you'll get currents between any earthed stuff on shore and your saildrive, which will cause electrolysis on the saildrive and kill it in short time.
No professional should do this. 

[JEN-et-ROSS]

I don't know the answer..But.
 Motor vehicles use the chassis/bodyshell as a Negative terminal for almost all the onboard services. Saves weight, copper and lowers final cost of the vehicle.
So, I don't see why you can't do something similar on a boat..........

[IslandAlchemy]

What engine?  A lot of VP engines are isolated earth.

Make sure you connect it to the return strap, not the engine.

[Salty]

If you are plumbing and wiring in two electric toilets, then it’s probably 99% sure you are going to place them within a toilet compartment. Also, in those same toilet compartments you will most likely have a shower and an electric pump used to empty the shower drain. That pump will have both positive and negative cabling already in place, and it is quite possible that the shower drain pump may have a similar demand for current as your Electric toilet, so if the cabling is of the right size, then why not share the negative line that is already almost to the same point as your new toilet.
As for connecting the negative line to your engine, I wouldn’t do that, and I wouldn’t recommend doing so to anyone, and particularly not when it’s most likely you have a ready to use negative line already in place.

[elias]

The motors are 25amps ... running from the batteries is around 7meters , so I installed 6mm2 cross section cable. I can’t add to an existing cable more load. For the moment I wired on the engine block where the alternator negative is mounted

[Salty]

If you are plumbing and wiring in two electric toilets, then it’s probably 99% sure you are going to place them within a toilet compartment. . . . .

. . . and for the 1% chance it might be somewhere else, herewith a photo from a posting seen on one of the social media sites !!!

[elias]

Haha ! It sounded strange the 99% ... now I understand !