magnesium annode for short term

[pcatterall]

Boat in fresh water for a few months.
Concerned about prop protection.
I plan to buy a magnesium annode and dangle it over the side when stopped.
The existing hull connections ( to the zinc annode) go to the engine and thus to the shaft and prop and also to the stern gear. I would propose to replicate these connections.
Is this a sensible approach??

 

[noelex]

The main problem with zinc in fresh water is that the zinc develops a poorly conducting coating so make sure you dive and give the zincs a quick sand when you go back into salt water as the coating will continue to prevent the zinc working correctly.
The magnesium anode will only help when you are in the fresh water, but it's difficult to get a low resistance conection and the anode will be a resonable distance from the stern gear so it's effectiveness will be limited.

 

[rob2]

It may well work, but the magnesium being more reactive will also be protecting your zinc anodes and be soon exhausted. The difficult part, I believe, is finding a way to hang the anode close enough to the sterngear to be effective. The other problem you're likely to encounter is the zinc anode growing an inert coating (white compact powder) whilst in fresh water. The only way I know of putting that right is to dry out and attack the anode with a chipping hammer!

It's still worth a try, though. Please let us know how you get on!

Rob.

 

[Quandary]

I keep my boat in fresh water but spend most of the summer in the sea on cruises varying from 2 to about 4 weeks. I use a zinc hanging anode and coarse sandpaper to polish the passivation off every week or two when in the fresh water. Wire brushing does not get the coating off, I find that the saildrive leg anode does become passivated in time but so far the hanging anode which is wired back to the leg has proved effective in protecting it. The prop anode is cleaned by the motion and degrades normally.
The previous shaft driven boat was protected in a similar way but the hanging anode was an extension of the earth wiring which connected the exposed metal parts. I never mixed with magnesium because of the very short life and concern (possibly unwarranted) about the effect of mixing anode materials. The shaft anodes passivated a bit but again the rapid motion seemed to keep them clean enough to be effective and I did not experience any corrosion problems.
A friend who keeps his boat in the canal and probably goes to sea less than we do swears by aluminium anodes and I would switch to these if I could get them for my saildrive and folding prop.
I agree with the advice to clean the zinc anodes with a few weeks of returning to sea.

 

[theoldsalt]

Don't bother.

I keep my boat so far up a river it in fresh water when on it's mooring. I'm in salt water only when out sailing.

For years I used only a zinc anode with no problems. Last year I changed to magnesium upon advise and I had problems. This year I'm back to zinc.

Theory does not always work out in practice.

 

[vyv_cox]

I agree with the other posts. I kept my boat in fresh water in Holland for seven years, sailing at weekends sometimes on the sea, sometimes inland. I used a zinc shaft anode throughout that time. The zinc hydroxide that forms on the anode can be wire brushed off, underwater if you like but annually on lift out seemed perfectly OK.

 

[pcatterall]

Quote"The magnesium anode will only help when you are in the fresh water, but it's difficult to get a low resistance conection and the anode will be a resonable distance from the stern gear so it's effectiveness will be limited."
Thanks guys; The quote from noelex made me wonder if I had posed my question clearly.

I had planned to 'throw' the annode over near the stern ( as near as the existing zinc hull fastened one) and connect it internally in the same fashion as the existing one. I assume that this will act in just the same way as the protection I have but with magnesium as the 'sacraficial' metal.

My existing hull annode had developed a 'coating' as described in some responses on inspection I found that the internal contacts to the stern gear and engine/shaft were corroded so I thought that the coating had formed because the annode was not working.
In the event I used the angle grinder to polish it up, cleaned the contacts and now hope that it will work.

We are in the French canals now and plan to over winter there. I will look for a place to winter on the hard as I need to get those dammed annode bolts replaced!!
Your advice, on balance, makes me more relaxed about protection in the shart term and I can have another look this winter.

Thanks again for your responses.