Interesting article from British Marine News

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The Clean Harbours Partnership (CHP) — a UK-based partnership raising awareness of harbour pollution — has urged boaters and major boat manufacturers to replace zinc anodes with aluminium as standard to reduce environmental damage.

CHP, which is formed of local interest groups around Chichester and Langstone Harbours, has been studying the release of metals, particularly copper and zinc, by recreational vessels into the marine environment. CHP highlights that, in 2020, the Institute of Marine Sciences at Portsmouth University in the UK calculated that approximately 900 tonnes of zinc is released into UK coastal and inland waters by recreational vessels in an average year.

“The problem is metals do not degrade. They accumulate in the ocean and in sediment on the sea bed, where organisms at all levels in the food chain ingest them,” says James Collings Wells of the Clean Harbours Partnership. “Zinc anodes also contain cadmium, which is harmful to marine life, whereas aluminium does not.

“While aluminium anodes are being used by some of the UK’s major boat manufacturers, evidently, they are not being used widely enough. We urge boatbuilders and boaters across the UK to switch to using aluminium anodes.”

Aluminium alloy is more environmentally friendly and has several important advantages over zinc as an anode material. Its higher driving voltage makes it particularly effective in brackish water, and its higher electrochemical capacity means that the same-sized anode will last longer.

Aluminium anodes are more environmentally friendly, lighter, and longer-lasting says the Clean Harbours Partnership, which advises that when changing from zinc to aluminium alloy, all external anodes should be replaced at the same time. However, this does not apply to zinc anodes in the engine water cooling system if the two bodies of water are separate.

If an anode erodes more quickly than expected, it’s worth looking for an electrical problem causing stray current. Addressing this would reduce the amount of metal shed by the anodes into the environment, CHP says. Prolonged connection to shore power without a galvanic isolator can have the same effect.
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Whilst I personally use aluminium anodes on my boat, Because they are now cheaper and also work better, I think a balanced view needs to be considered. Whilst aluminium may not be harmful to marine life it is certainly harmful to humans ,who eat a lot of marine life, and has been implicated in the increase in Dementia in humans. Zinc on the other hand has health benefits to humans. I would also be interested to know if the zinc that is eroded from anodes, stays as zinc or in fact changes to zinc oxide which is not a metallic form. Copper likewise dissolves into copper salts, as does aluminium dissolve into aluminium oxide (which is still harmful to humans.)

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As an additional point, which you touched on above, regarding making sure all your anodes are changed to aluminium, at the moment, Featherstream do not make an aluminium anode for their propellors. I approached M.G.Duff in Chichester about four years back when I changed to aluminium, and managed to purchase a 500mm x 60mm diameter billet of aluminium which they cast for me out of their specific aluminium used for making anodes - it is an alloy of other ingredients to make move it even further down the nobility table, I think it cost about £60. I have since made five replacement nose cone anodes for my Featherstream propeller and have enough material to make at least 9 more. The first two were conical, which was quite hard to achieve, but now I just make them shorter but cylindrical with a small quarter round chamfer which is similar to other propellor nose anodes I have seen. Much easier to make and wastes less material. I initially trued up the billet on a 12" lathe at the Dell Quay Yacht Yard with the kind permission of Steve the Manager, but now I can just cut off a 33mm slice and turn it down to the finished 30mm deep by 58mm diameter on a small Myford lathe, then using an original zinc nose cone as a guide, I drill the 5.5mm fixing holes and then use a 10mm x 5.5 counter bore bit in a drill press to cut the recesses for the M5 x 20mm stainless cap screws which I have changed to titanium which I'm hoping are less prone to round out with a 4mm Allen key than stainless. I tried using ceramic cap screws, but one sheared off when I came to remove it hence the change to titanium.

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