Looking at adding Fireboy-Xintex liquid level monitor for my freshwater and waste water tanks.  No problem with application for poly freshwater tank, however I have an aluminum wastewater tank mounted between pontoons on my 35' Crest.  Mfgr says the tank needs to be grounded to the neg battery.  Currently the 12VDC is all isolated from the AL frame & pontoons.  What are the problems and consequences of grounding the (-) side of both battery banks to the pontoons???

Yes the tank needs to be grounded to the negative of your DC system but, the hull and structure of your boat should not.  If you do this you will get stray currents and bad galvanic corrosion. Parts of your boat will start disappearing.

 

To do this right it sounds like the tank needs to be mounted such that it is isolated from the hull and structure.  Then you can run a ground from the tank to the DC negative. If you don’t do it that way you’re going to have big trouble down the road.

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Ike
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thanks Ike - It is impossible to isolate the holding tank as it is welded along the top edge to the aluminum cross members and to both pontoons. Can't ground the tank without grounding the hull.  Need to look for a different level sensor for my aluminum tank.

Are you sure that the pontoons are isolated from the 12 volt negative?


Such isolation would be a fairly difficult thing to achieve when the boat was built and something that would be way to easy to change as time went on and things were added to the boat.

The pontoons weld to the overall frame of the boat.  Eventually this frame gets to the motor mounts which connect the block of the motor to the frame.  

If you have outboards bolted to the back of the pontoon then the pontoons would be grounded via the outboards.  

At some point you may have mounted a small courtesy light by drilling a hole into the frame and fastening the light down.  This could ground the frame of the boat through the light's ground.

What about your gas tanks?  Are they metal tanks?  If they are metal I would expect them to be grounded, through the sending unit if nothing else.

Do you have a propane system? Are the propane lines rubber hose or copper?  Copper lines would be connected to the battery ground at the water heater and refrigerator.  If the copper lines come in contact with the frame at any point then you create a ground connection there.

Anchor windlass?  I would certainly expect that to connect to the frame and be connected to battery ground.

If you have a test light it would be a very quick test to see if the frame is grounded or not.  Simply connect the ground of the test light to the frame then touch the probe to the battery plus.  If the test light lights up then you already have the connection.

Rod

Rod- Thanks for in-depth analysis.  I'll take a 12V test lamp (or my digital VOM) and see if hull is grounded.  Copper lines do run from LP tank to stove.  None of the - 12 volt cables I've seen appear to be connected to the hull at any place? I can get to all the power lines. I'll check it out later this week.  Been on vacation and didn't get on the forum until tonight.  Thanks again.

I checked for any voltage from - 12V terminal to the pontoons, and found a minimal .002 VDC.  As you and others mentioned, the pontoons are at  -12 VDC ground potential through the outboard ground, it's mounting bracket and water.

Thanks all!!