earthing aluminium hulls

[gucci84]

hi all
just want to know opinions of running the earth lead from your battery to an aluminium hull, the way i understand it is thats going to speed up corrosion buy inducing a current into the dissimilar metals???
any help would be greatly appreciated

casey

[Bros]

The hull is earthed at the motor so I don't know what is to be gained.

[boatie_72]

All i can say is dont do it. Unless you dont want your boat anymore and like to have white powder and pin holes every where.

[finga]

All i can say is dont do it. Unless you dont want your boat anymore and like to have white powder and pin holes every where.

I disagree with that a wee bit.
First of all are you wanting to run a negative polarity wire from your battery to the hull so you can simply connect the positive wire to appliances and accessories and use the hull as the negative return path for the current of the appliance/accessory??
If so then you'll end up with more strife then Speed Gordon could handle.
Simply put....DON'T.

Now if you run a positive and a negative wire to all appliances/accessories and a decent sized wire from your outboard (which is directly connected to the negative terminal of the battery) to the hull and every metal (or alloy) surface in the hull then that may reduce the amount of corrosion occurring.
The key point on that idea is the equipotential bonding of all metal (or alloy) in the boat.
That is that all metal (or alloy) surfaces in the boat will be at the same potential (or voltage).
The reason corrosion occurs in hulls is that some surfaces are not at the same potential (ie potential difference). This is more apparent in older hulls which have the seats etc riveted into place.
A slight potential difference will exist between these riveted surfaces and this is where the corrosion will start because where there's surfaces of potential difference there's a resistance between the surfaces and where there's potential difference (voltage) and resistance there's a current flowing and this current is the corrosion maker.
This is why you rarely see corrosion where welds are but every where where there's two bits of metal (or alloy) bolted, screwed or riveted together).
As a side note this is also why you sometimes get tingles in the shower in older homes (especially if you have cuts on your hands and it's dry dirt around the house).
The waste water (where your feet are normally) via the old sewer pipes are at a different potential to the water pipes of the house and when you turn the taps on (with your hands with the cut) your the equipotential bonding conductor between the old sewer pipes to the water pipes ie you get a tingle in your cut.
I have measured some of these tingles to be well over 100V in some homes.

Outboard manufacturers use this equipotential bonding on their outboard to reduce corrosion. They're the little bits of wire everywhere that look like little traces connecting different parts of the outboard together.

Clear as mud???Sorry

[oldboot]

There was a " vigourously discussed" thread on this matter a while ago.

Finga has coverd the basics of it.

There is a lot of supersticious hokus pokus going arround on this matter.

Have a look for that thread.

As a matter of interest, some of the aluminium boat manufacturers are installing a "bonding point" to tie the negative to the hull.

There is an argument that the physical connection of the outboard body to the transom may not be a good quality electrical connection and that installing a "formal" bonding point can reduce corrosion particulary arround engine mounting hardware.

As I said there are some very strongly held beliefs about this matter that simply do not stand up to basic technical scrutiny.

cheers

[Fatenhappy]

Hey Casie ...

Just to endorse further what finga and oldboot are saying, we use exactly the same sort of technology with relation to aircraft to prevent static build up or as finga puts it potential diference.

There, we tend to use bonding straps and the like between adjoining areas of the airframe and anything else that is metal based. Even so you have to be extremely careful to not using dissimilar metals because of corrosion ...

Cheers

[gucci84]

cheers guys for all your ideas still abit unsure, telwater has earthed the hull and i dont have any corrosion any where yet i have treated all the screws with duralac and plan on bonding some pannels with straps from work if your not shopping green your paying to much
have a good one and thanks again

casey

[Spaniard_King]

I disagree with that a wee bit.
First of all are you wanting to run a negative polarity wire from your battery to the hull so you can simply connect the positive wire to appliances and accessories and use the hull as the negative return path for the current of the appliance/accessory??
If so then you'll end up with more strife then Speed Gordon could handle.
Simply put....DON'T.

Now if you run a positive and a negative wire to all appliances/accessories and a decent sized wire from your outboard (which is directly connected to the negative terminal of the battery) to the hull and every metal (or alloy) surface in the hull then that may reduce the amount of corrosion occurring.
The key point on that idea is the equipotential bonding of all metal (or alloy) in the boat.
That is that all metal (or alloy) surfaces in the boat will be at the same potential (or voltage).
The reason corrosion occurs in hulls is that some surfaces are not at the same potential (ie potential difference). This is more apparent in older hulls which have the seats etc riveted into place.
A slight potential difference will exist between these riveted surfaces and this is where the corrosion will start because where there's surfaces of potential difference there's a resistance between the surfaces and where there's potential difference (voltage) and resistance there's a current flowing and this current is the corrosion maker.
This is why you rarely see corrosion where welds are but every where where there's two bits of metal (or alloy) bolted, screwed or riveted together).
As a side note this is also why you sometimes get tingles in the shower in older homes (especially if you have cuts on your hands and it's dry dirt around the house).
The waste water (where your feet are normally) via the old sewer pipes are at a different potential to the water pipes of the house and when you turn the taps on (with your hands with the cut) your the equipotential bonding conductor between the old sewer pipes to the water pipes ie you get a tingle in your cut.
I have measured some of these tingles to be well over 100V in some homes.

Outboard manufacturers use this equipotential bonding on their outboard to reduce corrosion. They're the little bits of wire everywhere that look like little traces connecting different parts of the outboard together.

Clear as mud???Sorry

Mr Einstein I got an issue with a engine that keeps burning out a bonding strap between the trunk and the steering tube/pivot tube. Its a 75 on a 5m plate tinny(its done 2 hrs) WHen are you available to come sort it out

[finga]

Mr Einstein I got an issue with a engine that keeps burning out a bonding strap between the trunk and the steering tube/pivot tube. Its a 75 on a 5m plate tinny(its done 2 hrs) WHen are you available to come sort it out

Mate, you need my magic little meter that tells where the currents are running.
When you say it burns out a bonding strap does it melt or corrode away?

Easiest way to fix is bung a nice new Honda on and sell me the old motor that's cactus....maybe??

I'm hoping to go past in the very near future on the way to Evans

[Spaniard_King]

Not sure whats going on Scott. Its done all of 2 hrs and the bonding strap is no more.. will email you a pic tomorrow. And it is a new Honda

[black runner]

Ditto Finga/Oldboot - Hull bonding and using the hull as an earth return circuit are two completely different beasts.

There are quite a few threads on this site and the web about ally hulls and earthing. The main thing to ensure is that all -ve cables are physically connected usually at a common point and this can, in many cases be on a plate that is connected to the hull. This doesn't mean that the hull is being used as an earth return circuit. But it does ensure that the outboard (through a bonding strap or starter -ve cable) has the same electrical potential as the hull and that the hull is also connected to the outboard anodes which offer sacrificial protection.

Cheers