Anode

[guydickinson]

Its strange. On the skeg of my recently acquired V d Stadt is an anode. Looks like its been there a while, but not v pitted really. Stranger is that it isn't wired to anything at all! Stranger still, really, is that the prop (an elderly Sabb variable pitch - bronze I guess) is not affected at all it seems. The question is - should I resite it to the hull and wire it up to the engine etc as suggested by the surveyor, or leave exactly as it is. Why do you think it is like this and why no erosion?!!! Is there a danger things could change?

 

[SailingDoc]

Is this a GRP boat? I know that there are diametrically opposed views about anodes in GRP. Essentially, if a metallic part (propshaft/propeller, or through-hull fittings) is not part of a 'circuit' ie is not connected electrically to any other part that's in contact with sea-water, it shouldn't corrode. You must have a complete circuit to allow galvanic current to flow. If the anode is not connected to anything, it won't corrode. If your propshaft is connected only by a non-conducting link to the engine, and the engine has no direct electrical connection to the water, other than the raw water, it should be fine.

So what would the pro-anodists say?

Brian

 

[guydickinson]

Thanks. Yes its GRP. I would think the prop and shaft are all connected electrically to the engine. The only other significant metal under water is the rudder - the vertical bit -(name?- connected to the tangs!) but that's stainless. Whichever way, presumably, this disconnected anode is not doing anything useful? Someone said that older props are much better alloys and don't succomb but that may be rubbish. What if the boat is now moored closer to other boats than before?

 

[SailingDoc]

It seems to me that galvanic corrosion is one of those 'dark sciences' that no-one can consistently predict. Check your propshaft-engine link; many are made of a tough plastic; the bolts are 'staggered' so that there is no direct electrical connection. The rudder and stock are not necessarily connected electrically to anything else - you need to check.

I wouldn't know about new alloys - but it was always said that props could 'dezincify' if they became an anode in a circuit - ie were connected electrically to a nobler metal also in contact with the water, but I'm not sure if this is something you can see happening, or whether the blades just suddenly crumble. What a nasty thought !!

Brian

 

[Searush]

[ QUOTE ]
(cut) What if the boat is now moored closer to other boats than before?

[/ QUOTE ]

Just don't wire your boat up to your neighbours' /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
As has already been said - it ain't magic, it's electricity! Two different metals in a saline solution close together form a cell. But they do have to be VERY close. For more info see http://inventors.about.com/library/inventors/bl_Alessandro_Volta.htm

 

[guydickinson]

Am I right in saying that if the rudder stock (stainless) is NOT electrically connected to anything, no electrolysis will take place involving that? The keel is encapsulated so that's OK. So I'm left with the propeller and shaft then as the only two pieces that either have to be the same metal or not electrically connected. If they are, then an anode needs to be connected to one them. Am I getting the principle here?!!!

 

[fireball]

Pretty much - and in that instance a shaft anode would be suitable ....

the only other explanation for wastage is where your underwater metal become part of a larger electric circuit ...

say the boat next to you has an earth leakage problem and the shortest path to ground is via the bit of metal on the post on the opposite side of your boat (so your boat is in the way). If there is going to be a current it may be easier for it to flow to your propshaft, along that to your prop and then off to the grounding post ... you could then suffer damage to your prop (and build up on your propshaft) ....
I think thats what it meant anyway!!

 

[guydickinson]

Thanks v much!!

 

[SailingDoc]

Cue for some simple experiments - physics O-level stuff (GCSEs or whatever they call them these days..). Get hold of some lumps of zinc - perhaps scrounge discarded anodes. Dangle one lump on the water (saltwater) on a string. It shouldn't corrode. Connect another by a length of copper wire to, say, a stainless bolt, or an old bronze through-hull fitting. Dangle both in the water, making sure they do not touch each other. Dangle a stainless bolt (or whatever) in the water on a string. It shouldn't corrode. Strap a lump of zinc in direct contact with stainless (or whatever) and dangle in the water. Leave them all for some weeks, inspecting at intervals. See if the theory works in practice.

Brian

 

[FullCircle]

Much more interesting if you substitute potassium or magnesium.

 

[guydickinson]

Well it comes back eventually - did actually get physics A level once....in 1968! Does an isolated zinc plate on the hull under the water (unconnected electrically to anything on the boat its fixed to) do anything at all to stop nearby boats causing problems??

 

[srm]

[ QUOTE ]
Am I right in saying that if the rudder stock (stainless) is NOT electrically connected to anything, no electrolysis will take place involving that?

[/ QUOTE ]

Sorry but no - stainless is quite capable of setting up its own corrosive cells within a single piece of metal. The problem will occur on the boundary where metal is exposed to sea water and covered as in a bearing or where the steel is fastened to another material such as grp. The inside of the s.s. rudder heel fittings on my current boat looked like pummice due to this effect, although the outside looked OK and the surveyor missed it. The new s.s. heel fittings each have a zinc anode attached, but this may not protect the area where the fitting is bolted against the GRP skeg - time will tell.

I have also seen pitting corrosion on high grade marine stainless steel used for the body of oceanographic instruments. This occurred around barnacles which sealed part of the metal from oxygen in the water.

I suspect this danger of pitting corrosion is why s.s. is generally regarded as not suitable for use underwater.

Don't have any advice regarding your rudder stock other than to partly drop the rudder while the boat is ashore and inspect the area around the bottom bearing.

Hope this helps.