Hi Kam
That s why I need one of the guys with more expert knowledge of why it happens rather then me, but no it doesn't have to be in contact with the water itself.
My very limited understanding (I have seen the effects) of electrolysis is that it isn't actually "rust" in the normal sense of the word, but an electrolytic current (posh word, not sure exactly what it means) that runs through the water and when it makes contact with a metal surface will begin to corrode it. If that metal surface is itself in contact with something else metal then the effect will in time pass on t that and then on to the next bit and so on. Again sorry for the limited understanding, but most boats have wiring througout them. For example an outboard is wired to throttle controls and ignition switches and so on and so the effect can pass through the whole boat in time.
Soft zinc anodes help prevent the effects of electrolysis as the corrosion effects the softest metal surface on the boat first (the zinc anode). The anodes must be connected into the whole boats system for this to work, if they are simply bolted to the boat without being connected in they will corrode slowly (more slowly), but the electrolysis will be effecting the rest of the boat at the same time.
You can slow down the effects by not plugging in to shore power needlessly and by turning off your batteries when you leave the boat, however the effects are speeded up by poor marina wiring, boats on shorepower all around you and metal in the water such as marina piles etc.
Is that any clearer?
Like I said, I am a layman and that is my basic understanding. I am hoping one of the more expert members of the club, may be able to contribute
Tom
Are you saying VHF, fishfinder and plotter will just melt into a pile of rust?