Anode Selection for Freshwater Marina - Swansea/ Cardiff

Anwen

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Deep Joy is moored in Swansea Marina, which, as far as I know, is freshwater. We moved from Plymouth last August, and are about to come out for antifouling, anodes etc. This started me thinking about what anodes I should use. I have done a search on the forums, looked at the MG Duff website, and read the short article on anodes for freshwater in a recent PBO, but I'm still not sure what to do.

The MG Duff website says to use magnesium, but replace the anodes with zinc if the boat is in salt water for more than 14 days in the year. Most of our sailing is nipping out for the day along the Gower coast or similar, but we go off for the odd long weekend plus a one week cruise over the summer holidays. This adds up to more than 14 days. I'm not really that keen to start effing about with different anodes as a preliminary to going away for the week!

The boat is a Sun Odyssey 32, which has a golf ball shaft anode and a cone anode at the end of the shaft abutting the prop. MGD make a suitable shaft anode in magnesium, but not the cone anode. I'm very loath to leave the cone off, but worry that a zinc one will do nothing, and may upset the effectiveness of the magnesium one.

Given that there are a lot of similar boats in the marina, round the corner at the Yacht and Sub Aqua club, plus every boat in Cardiff Bay/ Penarth, I wonder what everyone else is doing. Any forumites with boats in S. Wales willing to share their thoughts?
 

bluedragon

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Yes...a number of comments:.

Firstly, I didn't know that Swansea Marina is fresh water? Is it fed by the river? Anyway, Cardiff Bay definitely is, and we all have this dilemma. Many of us use Mg rather than Zn anodes, but it's not universal. MG Duff do suggest that Mg is eaten away after 14 days in salt water, but mine only lost 50% in two years which included about 8-10 weeks in salt. The boat doesn't have a lot of underwater metal and it's nearly all bronze, but it is all internally bonded to each. Also, I don't have shore power earth connected to the anode via. the DC -ve and this EATS anodes if no galvanic isolator. So would a Mg anode last as long on your boat..I don't know. The Zn surface becomes passivated in fresh water, so if you can get at it (engine anode) give it a scrub-up every so often.

Another approach which works with anodes that are internally bonded is to just hang one over the side on a length of (appropriate) wire and change from Mg to Zn when you go the sea and vice-versa. More than a few boats in Cardiff do this, and that was my plan this year if the anode had been badly eaten away.

I'm not entirely convinced that those of us in this situation even need anodes. The conductivity of fresh water is so low, and days at sea are infrequent enough for many, that possibly little erosion is going to happen anyway...and maybe that's why people are getting away with Zn?? Trouble is, how do you prove this for your boat without risk?

So no magic answer, but as I see it, there are your options. Good luck.
 

Anwen

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Thanks very much, your reply is really helpful. Based on what you have said, I think it a fairly safe bet to renew the shaft anode with a magnesium one, and I I'll probably put a zinc cone anode on the end of the shaft next to the prop, simply because I can't get hold of a magnesium one. It was reassuring to hear that magnesium won't be gone after a couple of days in the sea.

I looked at the anodes earlier in the year (April) when we were anchored up in Oxwich Bay. Even with a wetsuit on, it was chuffing cold, so I didn't hang around, but simply checked that they were there, and there weren't bits missing from the prop. However, I don't want to leave it until later in the season.

See you at the South Wales Boatshow?
 
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