Anodes - zinc or aluminium

[Irish Rover]

I'm sure this has been asked before, but here goes. My powercat is in the Aegean sea, and I probably motor around 360 days per year. The longest it generally sits idle is around 2 weeks. I'm trying to stretch the period between lift outs towards 2 years. There are anodes on the propellor, shaft, and rudder on each side. Which is best, aluminium or zinc?

 

[NBs]

Isn't the water there reasonably salty?

Here, the brackish water is low in salt, and yet zinc works. Al would be better here. Your AL anode would wear out quickly. I would go with zinc.

NBs

 

[Momac]

Zinc will last longer than Aluminium.

 

[vyv_cox]

Aluminium anodes have higher electrical capacity than zinc, so size for size aluminium ones will last longer. Lots of info here Anodes

 

[Irish Rover]

Aluminium anodes have higher electrical capacity than zinc, so size for size aluminium ones will last longer. Lots of info here Anodes

That's what the Google bot told me as well, but I didn't entirely trust it. I do now. Thanks, as always.

 

[Laser310]

I am in brackish water, on a swinging mooring.

I switched from zinc to aluminum a few years ago.

The aluminum erodes more quickly.

I have a Volvo saildrive with two anodes; i understand, that one is for the prop, and one for the drive, Is that correct?

anyway, the smaller one, nearer the prop, erodes very quickly - much more quickly than the zinc did. It doesn't last a whole season.

The big one barely erodes at all.., but neither did the zinc.

 

[Tranona]

Presumably you have a folding propeller. Some are notorious for rapid anode wear, mainly because of the type of alloy used for the body and the bulk of it in relation to the bulk of the anode. A partial solution is to change to another brand of prop made of better materials.

 

[vyv_cox]

I am in brackish water, on a swinging mooring.

I switched from zinc to aluminum a few years ago.

The aluminum erodes more quickly.

I have a Volvo saildrive with two anodes; i understand, that one is for the prop, and one for the drive, Is that correct?

anyway, the smaller one, nearer the prop, erodes very quickly - much more quickly than the zinc did. It doesn't last a whole season.

The big one barely erodes at all.., but neither did the zinc.

In brackish water the zinc anode will develop a coating of white zinc hydroxide, which inhibits its action. Aluminium anodes do not do this, which is one of their plus points. The reason your zinc anodes lasts longer is that it is providing only limited protection.

 

[migs]

We have a Volvo folding prop with three little anodes around the hub, and keep our boat in the water all year (UK seawater).

* With zinc anodes and a 'polished' prop, the anodes lasted about 9 months
* With zinc anodes and a Veloxed prop, the anodes lasted about 12 months
* With aluminium anodes and a Veloxed prop, the anodes look like they will last about 15 months

The leg anode hardly wears at all (the leg is Triluxed and electrically isolated from both the prop and the engine), but an aluminium leg anode shows more 'surface activity' than zinc (which is probably a good thing...)

 

[yotter]

Thats interesting,
I have just bought a boat with VP 130 saildrive and VP 2 bladed folder (I would prefer a feathering). I have no concerns about the VP130 leg since will be well painted, but Trilux seems to be trade only now, did you use the spraycan option? I am concerned about the prop with such small 3 segment anode. How was your prop after being in the water for...how long with the Velox. I previously had a Kiwiprop (composite plastic and stainless so not much to corrode) and painted this with trilux and after 10 months afloat it was clean

 

[yotter]

In brackish water the zinc anode will develop a coating of white zinc hydroxide, which inhibits its action. Aluminium anodes do not do this, which is one of their plus points. The reason your zinc anodes lasts longer is that it is providing only limited protection.

Vyv,
That's interesting, I am no expert and you have my admiration, I would guess (I did say no expert) that if in brackish water, an anode is providing limited protection, on the other hand the structure it is protecting maybe corroding less due to the brackish water?

 

[migs]

Yotter, you're right, Trilux is now ' trade only; but they've done this by sneakily only selling it in huge quantites for hundreds of pounds...

When I run out of Trilux, I guess I'll be forced to use 'International cruiser 200', which is supposed to be the 'replacement for Trilux'. I don't expect it to be as effective (like a lot of things these days); but we shall see...

I use a paintbrush to paint the leg with Trilux, and here are some photos of our prop:

 

[Tranona]

For coating the leg I have always used either Prop-o-Drev or Ecopower spray. Perhaps not as good as Trilux but usually lasted a year. The type of coating has no effect on anode wear. I had a Flexofold 2 blade which had no anode and sprayed the hub but Veloxed the blades. The hub used to get the odd barnacle.

 

[vyv_cox]

Vyv,
That's interesting, I am no expert and you have my admiration, I would guess (I did say no expert) that if in brackish water, an anode is providing limited protection, on the other hand the structure it is protecting maybe corroding less due to the brackish water?

Yes that is usually, although not always true. But the relationship between the two anode types would be the same.

 

[yotter]

Yotter, you're right, Trilux is now ' trade only; but they've done this by sneakily only selling it in huge quantites for hundreds of pounds...

When I run out of Trilux, I guess I'll be forced to use 'International cruiser 200', which is supposed to be the 'replacement for Trilux'. I don't expect it to be as effective (like a lot of things these days); but we shall see...

I use a paintbrush to paint the leg with Trilux, and here are some photos of our prop:

Look like Velox is way to go for you. I also paint on Trilux on saildrive leg and prop and it works we. I do have some left I only have blue and green. Has anyone seen a decent non copper antifoul suitable for saildrive legs. I would rather not be buying 5 litres of trilux!