Cathodic Protection

In article , Mike D writes

Mike,

Please accept these thoughts as nothing more than suggestions for further study. I studied chemistry to a high level, but it was a long time ago, and I have no direct expertise in the field of cathodic protection.

First, I wonder if the excessive consumption of your sacrificial anode is being caused by, or exacerbated by, too much conductivity between the boat and the seawater. IOW, is it all painted adequately, and is the paint non-conductive? The electrolytic nature of anodic protection requires current to flow for the anode to be consumed, and the better the electrical connection between the boat and the sea the higher (i.e. worse) this will be.

Second, is there any metal in contact with the boat and the sea which is cathodic with respect to steel. Copper or bronze would be the most likely in a marine environment. These will increase the rate of corrosion of the steel, or of the sacrificial anode if one is present. It may be possible to reduce the effect by insulating the copper electrically from the steel, or physically from the seawater (e.g. by paint or varnish). Note I do not know if this will be a good solution, I just offer it as a thought.

Third, it is just possible that if your boat is stored in close proximity to other boats they are making the problem worse for each other. This is a bit of a shot in the dark, but it is known to be an issue at pipeline compressor stations. These are areas where there is a lot of buried metal) and for that reason are difficult to give adequate cathodic protection because the metal structures can shield each other from the protective current. Of course, I could just be getting carried away with the theory here.

Hope these thoughts may help point you to some further research. In my experience you are more likely to find good information in a book for marine engineers than on a public website. Try Amazon.

David

The usual cause of anodes wasting quickly is poor earth bonding within the boat causing a potential difference between the anode and some other metalic component on the boat or even on the shore.

Are you connected to shore power? There could be a current flowing in the mains earth connection. Try disconnecting the shore power and checking for a potential between the shore earth and the boat earth with a multimeter. (try on both ac and dc ranges.

Another possibility is that there is current flowing from your on board battery between two different metal fittings. Again check for potential differences with your meter.

The problem I have with propshaft anodes is that as they waste they become loose and work their way up the shaft.

Russell.

In article , David Littlewood writes

[snip] [snip]

Just had a look on Amazon, a search under "cathodic protection" produced

48 hits. Many highly technical, "limited availability" (i.e. they haven't got any) or duplicated, but 2 on the first page are aimed specifically at boat owners. Quite cheap, too!

David

I thought Cathodicks weren't allowed to use any protection

Regards, Tony

They use the box method. Hubby stands on box and when wifey sees twinkle in eye, kicks the box away.

.

-- Regards,

John Stevenson Nottingham, England.

Visit the new Model Engineering adverts page at:-

Back in the 1960s, I had the same problem with a 26 foot cabin cruiser. I am on the west coast of Vancouver Island in Canada, but the problem is common. I found that a good coat of anti fouling paint on the Steel parts such as the rudder, solved the problem. Make sure there is no paint on the zinc, and that it's in good electrical contact with the iron/steel parts. The zinc only had to be replace once a year after that.

Steve R.

I thought that was only for vertically challenged people and jockeys :

Alla

-- Allan Waterfal

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You must've been thinking of Viagra

Regards, Tony

A few years ago I did some work for a friend with a boat with similar problems and also shore power isolation issues. I dug up most of it off the net and followed it up with direct contact with the various system suppliers. Too long ago now to remember all the details I'm sorry to say, but I found there was no shortage of data available on-line. Start with Google and see where it takes you. Richard

Hi David, If the rudder is connected electrically to the other metal parts it may not need any protection. If not it should have it's own anode. Antifouling paint has one purpose, to slowly poison marine organisms that like to grow on the hull. It will not give adequate protection against electrolytic corrosion.

It is worth fitting a larger shaft anode forward of the prop if there is room (zinc for salt water magnesium for fresh). The fact that it wastes quicker than the other anode could just be it's closer proximity to other metals.

If there is no current from a battery or shore power I would suggest that the anode is wasting because it is doing it's job. It is cheaper to replace than the prop or propshaft. Russell.