Ovnis

[gfbalduc]

Re: Aluminum and UFOs

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Surely you would still get galvanic corrosion with the props, shaft etc. Is there a specific insight that I'm missing that suggests you can leave aluminium exposed underwater without corrosion happening?

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Good question, which is not very clear in the literature.

Marine-grade aluminum alloy is supposed to protect itself by passivating superficially, and obviously galvanic protection is a job for anodes.
Interestingly, use of too much galvanic protection is advised against, as this can erode the passivated layer.

Paint, by preventing direct contact with seawater, is supposed to add some protection, but how realistic it is to preserve the antifouling completely scratch-free over a season?
Some aluminium-specific antifoulings (which happen to contain copper derivatives) claim protection only on "perfectly covered" hulls, and the yard advises against them, but the whole story is rather unclear.

 

[ovniowners]

Re: Aluminum and UFOs

Is there not some sort of undercoat underneath the antifoul? When we lifted our 385 out last February, I vaguely remember seeing some sort of hard yellow coating beneath the waterline, on top if which you put the primer and anitfouling. I thought this was some sort of factory applied coat to go between the aluminium and the antifoul. I could quite easily be wrong though!

But even so, the oxygen in the water may well be enough to oxidise the aluminium and form a protective layer, like it does over the rest of the hull. However, I am not a chemist!

In terms of practical experience, we regularly beach (or go very shallow) with our 385 during the summer months, just avoid all the over yachts stuck out at anchor at busy sites like Studland Bay. Our antifouling does not seem to suffer anything from this, but I suppose we do try to beach on softer sand rather than rocks. When we antifouled in back in February, we crawled all over the hull applying the 3 or so coats, and there were no scrapes or corrosion patches evident, and our boat must be 6 years old.

By the way, we use International Primicon primer and International Trilux antifouling, easily obtainable from any chandlery. These are suitable for aluminium craft. I believe that Jotun and AWL Grip also make coatings suited for aluminium.

Paul