Corrosion and piles

[dralex]

Is it true that if you are moored near a pile in a marina, underwater corrosion is greater. I seem to remember there can be a significant potential in marina water. I was trying to work out why my prop folding teeth corroded so badly over the space of one year, with a fresh shaft anode last year. THe anode had gone- I wonder if it fell off or corroded fast.

The other question if the above is true- is it worth getting another dangly anode to use in the marina, smilar to what aluminium boats have to do, and hang it over the stern when berthed.

 

[boatmike]

Complex subject, but the short answer is that it could certainly have an effect. For an anode to be effective it must be connected directly to, and as near as possible to the item being protected. In your case the prop. The only way a "dangly" anode would help is if the other end were connected directly to the prop. A cable with an alligator clip to the propshaft might help...

 

[tillergirl]

And then there's this argument:

http://www.galvanicisolators.com/final%20flyer/index.htm

 

[Ships_Cat]

This is one of the common marina myths.

There is no way that a pile can cause corrosion on your boat unless you are connected to it electrically by a conductive path other than the sea. The usual only path that can exist is through the earth conductor in your shore power cable, should you be using shore power. That goes to earth at the supply on land and maybe through some conductor to the pile (assuming the pile is metal) to complete an conductive circuit to the pile, the pile then forming the other electrode (usually the cathode), the sea the electrolyte and your anode and other metals on the boat less noble than the pile the other electrodes (anodes).

Another path can exist with a shore power connection in that the ac neutral is grounded on shore, but a circuit to marina ground via the neutral from the boat can only exist if there is an electrical fault on your boat connecting the neutral on the boat to the boat earth while shore power is connected (neutral on the boat must be earthed when no shore power connected, assuming the boat has an invertor or generator, but must be automatically broken when shore power is connected).

If such a path exists then your anodes will suffer first and then the less noble metals on your boat. Also the same thing can happen with no pile but simply through being connected to other boats on shore power through yours and their shore cable earth conductors and the marina earth conductor.

If you have no shore power cable, and there is no other electrical connection between the metals of your boat and the shore, then (and it is hardly likely there is but is possible through other services to the boat, or for an aluminium boat if it is moored with its fender rail lying against the pile so electrically connecting the boat to the pile) so you are just floating near the pile, the pile will have no effect on your boat. For those who think otherwise, and I am sure there will be many, they may wish to describe the physics involved in other than imaginative terms /forums/images/graemlins/smile.gif.

If you have a shore cable, there are two alternatives - one is to not connect it and the other is to fit a galvanic isolator in the earth conductor on the boat - as per Tillergirl - (or fit an isolating transformer which is generally not warranted on smaller boats, especially if not of metal construction).

Also the talk of problems from potentials in marina water is generally a load of hogwash and you will find no commercial or professionally managed aluminium vessels hanging anodes off themselves by wire (check out a few superyachts and you will see what I mean - if there was a problem they have most to lose). I have commercial clients with fleets of aluminium vessels moored in commercial docks with steel piles all around (including the docks they build for themselves) and they do not need to hang anodes in the water off wires and would probably be amazed to find that amateurs do. As Boatmike points out the anode on a wire will still have to be electrically connected to your prop shaft and hung close to the problem metals, even if just trying to make a temporary fix.

Usually, if there is no shore cable involved, corrosion problems on the boat are caused by either inferior materials used for the components involved, insufficient anodic protection for connected incompatible metals (eg the anode fell off) or else an electrical fault on the boat. Note that the electrical fault may be a passive wiring problem, it need not necessarily be livened from or connected to the boat's DC.

John

 

[boatmike]

There you are! Told you it was a complex subject! The Cat is absolutely right of course and the talk of marina water being somehow the cause is daft. I would however only comment that I once had an old ferry moored against a steel jetty
with gert big steel sampson posts welded to it and the anodes were "fizzing" so badly that we took every shore connection off the boat..... No difference.... Then we replaced the soaking wet flax mooring ropes with dry nylon ones..... no more fizz.
As I said... a complex subject!
Whatever the cause additional anodes near the prop can't do any harm but must be connected to it.
/forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[Ships_Cat]

Hey old fella, I am too young to know about flax ropes /forums/images/graemlins/cool.gif

John