Anodes

[Guest]

just brought the boat up to the river for the season, last fitted zinc anodes in May 2001whilst down
the coast.
Should I change them for mag anodes soon? Also does having the shorepower plugged in all the time
reduce corrosion via its earth?

cneighbour

 

[david_bagshaw]

Shore power & earth

I dont think having the shore power will help stop corrosion, but it could lead to more, as often the earth wire does have a small potential

On our own boat the earth from the shore power is NOT grounded to the hull

Also there is a leakage trip Circuit breaker & isolating transformer, ie 240 to 240 to separate the shore from the ship & protect from stray currents


Anodes

we use resolution in fresh & sea, but have sea anodes fitted at all times.

If you are staying in the fresh for the year change them but I have been told fresh anodes only last weeks in sea water.

David
www.yachtman.co.uk

 

[boomer]

This is second-hand but nevertheless reliable information based on the experiences of friends of ours who moor on the non-tidal Thames.

re your 'fresh water anodes only last weks in salt water' Doesn't seem to be the case - their boat was in the water ( ie between lift-outs) for a total of 20 months. New magnesium anodes were fitted at the start of that period and the boat spent a total of 5 months in salt water (S.Coast and continental Europe). When the boat was lifted last Autumn, the anodes were certainly eroded, as you would hope if they were doing their job, but there was plenty of metal left. They will of course be renewed before relaunch!

The reverse is certainly true. Too many non-tidal Thames (or other river) types are conned into fitting zinc anodes because they intend spending a couple of weeks at sea. In reality, zincs get coated in fresh water within about 6 weeks and are then totally useless in either fresh or salt water, unless of course you are handy with a wire brush and are prepared to get your flippers on and go underneath!

 

[Piers]

If the boat is to be on fresh water for the majority of its time, fit the correct anodes. Being plugged into shore power will not add to corrosion. The legislation marinas come under and the regs they have to abide by (ref electricity supplies) dmean there is no contributory factor there.

It's just down to plain old metals being in water. If in doubt, call the main makers of anodes - M G Duff (who were lecturing at MBM's Cruise Club first event this year over last weekend) on 01243 533336.

By the way, you will know if you have the wrong anodes - if you keep your zinc anodes in fresh water, they will 'pacivate' (a new word I learnt, although I may not have spelt it right!) and become covered with an off-white crust, sealing the anode from any use and preventing it even being usefull when you return to salt water! Whilst this is happening, the next most vulnerable piece of metal becomes the sacrifical anode....

Two weeks is all it will take to start the process.

So fit the right anodes for the water you are in. M G Duff have great literature on the subject. Try calling their Chris Harris who spoke to us.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk

 

[david_bagshaw]

I agree , with piers go to Duffs & take their advice, however I still would not ground a shore earth to my hull & stern gear.

What happens IF in a perfect marina set up, assuming no customer has compromised it by damage, there is a boat in a near berth with a fault causing a stray current.

Where does that go to ...... the nearest earth, YOUR BOAT!!!!!!

Passivation of anodes does occur while in the fresh water, and leads to uneven wear of the anode when returned to the salt water, where some of white compound first breaks away and errosion of the anode is more vigorous in one spot, and thus leading to an earlier renewal.

In the ideal world the boat should be slipped & anodes changes on swaps from sea to fresh & reverse.

I still say magnesium anodes will only last a few weeks in sea water, but we do have a motor yacht with a steel hull, so it might be different with other materials

David
www.yachtman.co.uk