Topic: Earth Connection to Aluminium Hull

[mako101]

Hi all, i have a question someone here may be able to answer, should the negative terminal of the battery be earthed to the hull on an aluminium boat?

[ITGEEK]

Absolutely NOT.
This could cause your hull to corrode.

Attach all negative wires directly to the negative
posts on your battery(s), or get fuse blocks where
the negative cable of the battery hooks to,
then attach the negative wires to the block.

[mako101]

Thanks for your answer ITGEEK, the posts below appeared recently on the internet so i thought i would ask the question to clarify the issue. Any further comments would be greatly appreciated.

Copied from the internet:

"I'll shed some light on the whole earthing the hull questions.
It's not actually "earthing" a hull, it's refered to as "Bonding" in the marine industry. As 'xxx' has eluded to all electrical items must run a positive and negative wire to each individual component, with the neg. running to a common Bus Bar. The negative wire going from the battery to the hull is there to allow stray currents in the hull a path of return to the battery. Otherwise these minor stray currents will isolate through spots in the hull and escape through to the ocean (which as you will recall from basic science salt water will produce a small amount of current). These spots will create corrosion real quick. In fact if you have a SS filler this bonding is a must to stop static electricity during filling.
However if you boat realy dosn't see a whole lot of water time and you don't have a complex electrical system then you will get away without bonding the hull. But there is a method to the madness, they are not incorrectly wired by having a "Bond Strap" to your hull."

"What I meant was...every aluminum boat Ive had has been earthed via the outboard motor. If you remove the negative cable from where it has been bolted to the hull in your renegade, leaving the outboard motor connected to the battery, then do a continuity test with a multimeter you will find the hull is still earthed. The outboard is connected to the battery then when it's bolted to the hull, the hull also becomes earthed (albeit poorly). This earth to me is not very "clean" due to it being connected to a nice new painted outboard and then bolted and siliconed to the hull with stainless steel bolts. Quintrex (and probably other manufacturers) then put a nice clean earth to the hull (to fix this) via a large tinned copper cable which then makes the hull earthed cleanly same as the engine. They then run 2 core cabling to every electronic device, lights, sounders etc, so the return is via a cable and nothing is using the hull as an earth return (like in a car). I think by removing the earth cable to your hull you would be doing be more harm than good and encouraging electrolysis."

[ITGEEK]

mako:
When I first got my boat, I thought to ground everything to the hull, like a car, but
people told me NOT to do it.
By grounding the hull sitting in water (especially salt water),
the electricity flow will cause the hull to corrode.

The negative wire going from the battery to the hull is there to allow stray currents in the hull a path of return to the battery.

What stray currents?
We are talking about simple wiring on pleasure craft, not a combat battleship.

The outboard is connected to the battery then when it's bolted to the hull, the hull also becomes earthed (albeit poorly).

The battery should be sitting in a battery box, and the
battery not grounded to the hull.

I hope someone else will chime in here.

[Humminbird_Greg]

I thought that the zinc bits we have on our boats and motors were there to keep an electrolysis (galvanic) reaction from happening.

[ITGEEK]

Greg,
You're right.

Those bits are softer than boat hulls and will degrade faster
than the boat hull material.

I think introducing current flow through the hull will
wear them out much quicker, and could lead to hull
damage if they are not replaced soon enough.