North Australian FISH FINDER TM forums

Hi all, had an auto leccy mate give me a hand with some wiring, Iv got a new console with all new wiring throughout the boat, and realised there's bits of electrolysis starting already. (Only 6months old). Out of curiosity we checked the hull for earth and it had a real strong earth. Now none of the negative wireing is earthed out to hull and all goes back to a buzz bar and then goes back to the battery. We started unhooking things one by one and found that my sonic hub and my Yamaha 150 was causing the earth. Is it normal for outboards to do this? Being an aluminium hull this is a huge no no. Has anyone checked there's?

Barnzy, my Yamaha is earthed to the boat. To my knowledge all Quintrex with Yamaha are earthed. Not sure about other makes. Im not happy about it but it's 10 years old and still floats


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Iv got a bar crusher. there's no earth directly to the boat but it must come through the engine mounts. I'm thinking of sleving my engine bolts.

I think you would have to sleeve the whole transom mount, not just the bolts I would imagine. I'm not sure how you would isolate the motor from the boat


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Most mounts already have bread board plastic I between the motor and transom

Every outboard earths to the hull, only way to stop it is to insulate the bolts and use a backing board. Quintrex use a earth point on the hull because of that reason

cheers guys, I'm going to look into it and see if I can get some bushes for my transom bolts. Il put a post up when I get it all sorted

Scottie5.3 wrote:Every outboard earths to the hull, only way to stop it is to insulate the bolts and use a backing board. Quintrex use a earth point on the hull because of that reason

%100 agree with Scottie my Quintrex has an earth point connected to the outboard and I run all my other earth extras back to that point. I do have a main battery switch that I turn off when boat is not in use. This kills all power to the electric's.

Cheers grumpy

Here we go again, it has been a couple of years since this topic was lasted debated I suppose...

The negative of the battery will be connected to the body of the outboard which in turn is in contact with the hull.

So expect to see a "good earth" via the motor.

The corrosion problem is called electrolysis which is galvanic action with the water. Galvanic action is used for things like electroplating etc.

Any part of your boat in contact with seawater will give a galvanic joint and corrosion ( electrolysis) will occur.

What is needed is an alternate galvanic joint so that it will corrode and not your boat/engine parts.

That "alternate galvanic joint" is called a sacrificial anode and its chemical composition is such that IT corrodes and not your boat parts. Its made of material that is considered more active than aluminium on the galvanic chemical table so it'll corrode before the aluminium hull or motor. That's why its called sacrificial.

If you look above your prop you'll see a little round plate with what looks like a fin hanging down. That's the sacrificial anode.

That sacrificial anode's fin also has the other purpose of being the propeller trim tab which combats the tendency of the boat's stern to turn sideways under the influence of the left or right hand propeller thrust. Some motors have anodes of differing shapes, positions and dual uses.

Look up the internet about sacrificial anodes on boats. Very helpful.

The yamaha's also have anodes in them. Those need to be changed and checked during servicing. A lot of back yardies and other companies other then In and Outboard don't change them during servicing. Some of the older motors came out with anodes that were too hard and not corroding as they should ( attracting the electrolysis). This has caused some motors to corrode internally letting water into the cylinders/ oil. Make sure the anodes are decaying as they should or it may be a costly repair. The older 100''s came out with the hard anodes mainly but insure that they are working if you are servicing them yourself or at least get them checked by a dealer.

Spot on, Cuddles.

A perfect example of a sacrificial anode is a hot-dipped galvanised boat trailer.

Note the use of the word galvanised (that chemical table I mentioned).

The coating is zinc which corrodes before the steel of the trailer.

Be careful using stainless steel screws/bolts or other fittings in harsh environments on aluminium hulls. eg bowrails to hulls, transducers direct to hulls, bung fittings onto transoms.

If you screw stainless screws into an aluminium hull in a saltwater environment you have the perfect galvanic joint. The aluminium will corrode as you've got 2 dissimilar metals in contact with each other and both in contact with saltwater (electrolyte).

The process of galvanic action (electrolysis) is similar to a battery but at much lower voltage differences. On a battery the process is reversed by charging.

On a boat its all downhill from the minute that the hull hits the water. So make sure you get it right.

The motor manufacturer does his bit with anodes on the motor but its your responsibility about other dissimilar metal-metal contact on the hull itself.

I'd be pretty wary insulating the motor from the hull. No connection means no anodes protecting the hull. You'd want to makes sure your boat wiring is 100% reliable for the long term.

Barnzy

Listen to Maxpowers advice. Don't go sleeving the o/b motor mounting brackets.

Heres why

I used the term electrolysis before because that's what everybody thinks that the problem is called.

Its not.

With electrolysis you need a voltage source (like a battery) with an electrolyte (like seawater or acid) and the current flows through the electrolyte eating away the battery components (plates).

The problem is actually called galvanic corrosion and there is no battery source involved for it to occur.

The 2 dissimilar metals and the seawater form their own galvanic battery of very small voltages.

The further that the 2 metals are apart on the Galvanic Table then the more voltage involved although very small. Minute currents flow between the metals and one gets eaten (corroded) away. The more "active" metal is eaten away.

Not understanding the actual process involved has caused many a boatie (and auto electrician) have learned some very expensive lessons in trying to apply a electrolysis remedy to a galvanic corrosion problem.

You can take your battery out of the boat completely and still have galvanic corrosion at the hull and fittings.

So complicated earthing and bonding mechanisms and double pole isolating switches are not the solution that the boating industry has come up with.

Hence the sacrificial anode protect system.

Galvanic corrosion also occurs with motor vehicles and they tend to use what they call electronic rust protection attempts to negate corrosion.

Aluminium screws into an aluminium hull are the way to go. However, nearly everybody goes for stainless screws because they won't rust. Sure, that's right but you are simply creating another problem of galvanic corrosion in doing so.

Talk to your auto elecie mate. Show him these posts.

PM me if you wish.

Well said ronje!