bonding the boat ground(240v earth) to the 12v negative terminal

[rogerthebodger]

Am I missing something, all this talk of galvanic corrosion and dissimilar metals, what ever happened to plain old rust?

Yes you are missing something.

Rust is when when Iron in the steel combines with Oxygen to form iron oxide and there are chemically 2 types Fe2O3 and Fe3O4.

The Fe is the iron and the O is of cause Oxygen.

Most metals also Oxidise to same extent by combining the metal and Oxygen but most produce a stable and therefore protective layer on the metal. Aluminium oxide and Chromium oxide of 2 examples.

Rust or Iron oxide is not stable and is porous so flakes off and allows oxygen to again attack the base metal continuously.

Galvanic corrosion is in effect the transfer of metal from a anodic metal to a cathodic metal by and electrolytic process. Any form of metal plating is an example of this. Zinc, Nickel, Chromium and even Gold and Silver can all be electroplated in this way. One job I had was designing a plant to refine gold by using the electroplating process for a local gold refinery

The problem is that on a steel boat with yellow (copper based alloy) metal fittings if there is an electrical contact between the steel and the yellow metal the material process is for the yellow metal the be plated with the iron in the steel thus destroying the structure thus strength of the steel. We therefore use zinc anodes the will electroplate the yellow in preference to the iron.

The general way to protect steel/iron from rusting is by preventing the Oxygen getting to the surface of the steel/iron by painting the steel of even coating it with a more stable metal like zinc. nickel, cadmium or several more stable metals.

Stainless steel is interesting because the protection of the iron in the stainless steel from rusting is with a layer of chromium oxide on the layer that forms in the presence of oxygen. But that is not the whole story and too complicated to discuss here.

The position on the galvanic scale (this determines how electrically reactive metals are to each in simple terms) of stainless steel and mild steel is quite close so the effect is there but small enough to ignore. There are even special welding rods the allow stainless steel to be welded to mild steel reducing this effect at that joint.

 

[rogerthebodger]

that is a serious boat! where have you / do you plan on sailing too?

Depend which way the wind blows but some where nice and warm.

Off to the boat on Tuesday for 5 weeks but not going too far this time as safety inspection is needed soon and need to get ready for that.

 

[yesod]

I was wrong about the VHF requiring an earth - they don't I was mixing it up with SSB transmitters that do need a sea water ground plane.

no they don't.

 

[kashurst]

can you explain ? or give me a link to how you do SSB without a ground plane, thanks

 

[rogerthebodger]

can you explain ? or give me a link to how you do SSB without a ground plane, thanks

Well you could but what you do need is a counter poise. You have use a half wave dipole but that will only work for efficiently for one frequency and for the SSB frequencies would be very long (246 ft at 2Mhz)

You can use a quarter dipole with a ground connection again only one frequency for any efficiency this tine 123 ft at 2 Mhz.

Generally SSB use a random length aerial with a ATU (Aerial tuning unit) to make a quarter dipole but again needing an earth as the counter poise.

As I said before his does not need to be a DC earth but a meta (copper strip below the water line that dives an AC connection to ground.

If there is another way I would also like to know.

He may be talking about this.

http://www.kiss-ssb.com/about.html

But as I said strips of copper below the water does the same with an ATU.

 

[yesod]

can you explain ? or give me a link to how you do SSB without a ground plane, thanks

what rogershaw said - counterpoise or use copper inside the hull to capacitively couple to the sea water. an actual physical connection to the sea water is not required. ask any decent ham