Checking sacrificial annode circuit

[JeremyF]

My sacrificial annode seems to be erroding rather slowly - 50% after 2 years. I'm advised to check the resistance of the protection circuit. What level of resistance would be suspicious? Any other tips on checking out the circuit - do I just work my way round each stage, checking the quality of the joints and checking the resistance?

Many thanks

Jeremy Flynn

www.yacht-goldeneye.co.uk
New pics 11.02.02

 

[Piers]

Two years is wrong. Anodes are made to last one year, plus a bit.

Certainly, less than 1 ohm - remember that sacrifice takes the less path of resistance. Also, the anode should be in the 'line of sight' with the metal it is to protect.

One last thought - are you sea-water or fresh, and do you have the correct anode material?

If in doubt, try calling M G Duff, who are the experts in this field.

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

 

[brian_neale]

Anode lifetime

How does an anode know how long it is supposed to last? I have a smallish anode on my boat which is about 5" dia, about 1" thick in the centre. It protects two rudder fixing bolts, and is doing its job because it is being eaten away and not the bolts! However, I get at least a couple of years life from an anode, and would expect it to last longer if I used a bigger anode - just because there is more zinc to start with. The acid test of anode effectiveness is whether the thing it is protecting is still there the next time you look, surely

Should "Anodes are made to last one year, plus a bit" really say, "anodes should be big enough to last at least a year, but do not need to be bigger"?

 

[mtb]

Re: Anode lifetime

Two years is what I've always worked on ,if it has gone before maybe the wrong type or size has been fitted .
My tug required minimum of 4 but I put 6 ,so as to make sure of the correct amount of protection
Mick

http://homepage.ntlworld.com/boats
I want a big steel ex trawler / tug v/cheap or swap for tug

 

[Piers]

Re: Anode lifetime

You are right in saying "anodes should be big enough to last at least a year, but do not need to be bigger".

My comments were based on the M G Duff lecture I attended at the recent MBM weekend, where we were told that boat manufacturers size anodes on an application by application basis, per boat, with anodes sized to last a year, only. Hence, the need to change them annually at the annual lift out.

However, as was pointed out, other factors come into play such as the salinity or otherwise of the water in which the boat is kept.

Furthermore, some anodes are resticted in size, requiring more frequent change, such as the ring anodes on some Volvo outdrive legs.

So, yes, you are right in your statement, but boat manufacturers tend to choose anodes on a one year basis.

Interestingly, we were also told that boats built for the American market have anodes sized and placed differently from English built boats, and such a boat coming to English waters should be surveyed to ensure the protection is sufficient....

The final comment was that anodes are not a science - simply an expereince.

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

 

[brian_neale]

Re: Anode lifetime

The problem on my boat was corrosion (to the point of endangering the rudder fixings) in the threads of the bolts, affecting both bolt and nut. Crevice corrosion, I thought - not an unknown problem with stainless steel, and probably not electrolytic corrosion. Only fix is to check and replace every season. Then I fitted an anode, and the problem went away. Doesn't make any real sense to me, except as per Pier's comment - it's not a science! Still, as an engineer of sorts, I am more interested in what works than what should or should not work!

 

[sgball]

Where are you moored? Our friends are in Brighton Marina with six anodes attached to their 25ft boat. All the anodes have been eaten away by more than 50% in about 5 months!

 

[Piers]

Re: Anode lifetime

M G Duff have an excellent free leaflet (six sides of A4) entitled Cathodic Protection For Vessels In Salt Water. Their number is 01243 53 33 36. I think you'll find it covers all the points we have all made.

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