Aluminum Anodes

[stownsend]

Morning

This is following on from a post of mine on the Bristol Channel sub forum pages, does anyone have a supplier for Aluminum Anodes for a Volvo Penta Sail drive MS25S ?

Cheers

Stu

 

[Quandary]

I have been searching for aluminium VP saildrive and prop anodes for several years without success so far and have tried about 4 different anode makers as well as VP. Seems that there is just not enough demand.
If you are lucky enough to find them, please post the result.

 

[Bosun Higgs]

Stuart

I've tried ally anodes for a couple of years on my boat at CBYC and found them a PITA. They still scale up just like zinc ones do in fresh water. My recommendation would be to fit a normal zinc anode and have a magnesium one on a flying lead that you can plug into the system and throw over the side when in the bay.

 

[Quandary]

Stuart

I've tried ally anodes for a couple of years on my boat at CBYC and found them a PITA. They still scale up just like zinc ones do in fresh water. My recommendation would be to fit a normal zinc anode and have a magnesium one on a flying lead that you can plug into the system and throw over the side when in the bay.

I have some slight reservations about that advice, My boat is berthed in fresh water and I do what you advise except that the hanging anode which is wired back to a bolt on the top of the leg is also zinc. My reasoning is that zinc is more long lasting and I am not mixing metals. The reason why zinc becomes ineffective is the build up of the passivated coating, so every couple of weeks the anode is burnished with coarse sandpaper. Sometimes I put a couple of bands of plastic tape around it and peel one off to expose fresh metal. My leg anodes degrade very slowly as they are only cleaned once per year, the anodes on the prop. seem to stay clean and degrade at the normal rate but I suspect it is because we do quite a bit of motoring these days. I have had this boat since 2007, the leg has never been painted or antifouled though I do coat it with Propshield renewing the coating every second winter, I no longer bother coating the prop. preferring to polish it because the wax only seems to last a few weeks on it.
With my fingers on my head I would say that so far I have not seen any signs of degradation.
BTW I have no idea whether mixing metal in anodes is good or bad but it certainly is a big part of the problem in VP saildrive/ prop combos.

 

[Tranona]

Your hanging anode is not doing anything for the prop, which is isolated from the drive by a rubber bush. That is why it has its own anode - the prop is a mixture of bronze and stainless.

 

[coreng]

You should not mix anode's materials as, if you do so, magnesium will protect zinc or aluminium anodes who won't work properly anymore. Results might then be opposite to expected effects. Be careful with drives cathodic protection experimentations as they are made of a quite fragile alloy. The best way to know if an immersed equipment is correctly protected is measuring its galvanic potential as now required by manufacturers.

 

[Quandary]

Your hanging anode is not doing anything for the prop, which is isolated from the drive by a rubber bush. That is why it has its own anode - the prop is a mixture of bronze and stainless.

I do know that, as I said in my post, the prop anodes stay clean and degrade normally, probably because of the abrasion they are exposed to as they rotate, it is the leg anodes which passivate rapidly.
To add to my post, the reason I prefer zinc for the hanging anode (as well as concern about mixing metals) is that I am convinced that there is very limited degradation in the fresh water, I am more concerned that when we go into the sea for several weeks at a time the coating that has built up on the leg anodes exposes the leg to risk there, I am particularly concerned when I connect to shore power on pontoons. The hanging anode is now five years old and despite regular cleaning is still the same shape and weight so it is probably doing very little but having seen a bronze prop. turn pink and worm eaten stainless steel pivot pins in the past I would prefer avoid any risk.
I do have a rope cutter on the prop which may compromise the theoretical isolation but since the expensive little anodes stay clean I depend on them to do the job, the fact that they would not last a whole year is a bit of a pain though. we have a six month winter lay up in the club here and with a canal jetty berth I would otherwise be happy to lift out alternate winters, I can not be bothered to dry her out against the breakwater on a spring tide which would be necessary to change the prop anodes in the water.