Damage from no anode ?

[andrewhopkins]

We lifted our Bavaria 35 last week after a year in the water to get some slime off.

Upon doing this, we discovered that the man who had previously antifouled and also replaced the anode, had not screwed the anode to the hull and consequently the propellor had started to corode.

My understanding is that the next most vulnerable metal will deteriorate if the anode isnt working which seems to be the propellor.

Can I assume (or hope) that nothing else has corroded ? i.e. it would have to finish eating the propellor before "it " moved on to something else ?

Its a saildrive, Volvo MD2030

 

[Piers]

There is a great deal of damage that can occur - it just depends on your specific boat.

Might I suggest a chat with anode makers / specialists M G Duff, on 01243 533336, and try Chris Harris, the Sales Manager there. They are very helpful indeed, and sponsor the MBM Cruise Club.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[PaulJ]

Your propellor is probably brass or bronze, as are many through-hull fittings. You should check these very carefully which may not be easy. Clean them up so they are shiny metal, if there is any pinky tinge to the metal it means that they are becoming "dezincified" and they should be replaced.

 

[brian_neale]

Before I fitted an anode to my previous boat, the stainless steel bolts holding on the rudder suffered bad corrosion - even though there was a big bronze prop nearby! The problem was compounded by the fact that the corrosion was in the threads of the nuts and bolts and not visible on the surface, even though the nuts would have probably dropped off some time during the next season in the water. A point to check, anyway.

 

[scottie]

the saildrive should have its own anode fitted in front of the propeller
this would be of more signifigance than the hull anode i would have thought

 

[andrewhopkins]

That is the one

It was that anode, in front of the propellor that was loose. There is no hull anode.

 

[vyv_cox]

Corrosion of threads on stainless steel is due to crevice, not to galvanic corrosion. In this case there is a galvanic effect but it is due to differential aeration levels on the inside and outside of the threads. You could attach a zinc anode to stainless bolts and they would still corrode.

This is the reason that stainless fittings are not advocated beneath the hull, although it is quite unusual for rapid failure to take place.

 

[JeremyF]

I had a survey done recently on Westerly with a saildrive. As another poster has stated, the saildrive has its own anode. The hull was also fitted with an anode.

The prop was starting to be eaten, and the surveyor stated that the hull anode should be disconnected, leaving just the sail-drive one, because "anodes need to be worked hard to be efective"

Could you be in the same sitution?

Jeremy Flynn

 

[Guest]

That is not galvanic or electrolytic corrosion but crevice corrosion, If stainless steel is starved of oxygen then the oxide film which makes it rust-free breaks down and corrosion results, just as you say, where you cannot see it. Not a good material for underwater use!

 

[brian_neale]

I agree, guys - definitely crevice corrosion. That's why I suffered it for 4 years or so, replacing the bolts every year before any significant corrosion had happened. Then I fitted a hull anode, wired it to the bolts inside the hull, and they are still perfect after another 4 years. Makes no sense to me - do anodes have any significant placebo effect?

Whole point of the story in the first place was to suspect the unsuspected! Sure beats me.