Extra Anodes

[pks1702]

Kiki will be our of the water for 5 weeks or so giving me some chance to catch up with a few jobs (already limbering up my polishing arm!)

I would like to fit some further anodes for extra protection. She has them fitted in the normal spots but where are the best places to add additional ones? (clean answers please )

[Antares_9]

Kiki will be our of the water for 5 weeks or so giving me some chance to catch up with a few jobs (already limbering up my polishing arm!)

I would like to fit some further anodes for extra protection. She has them fitted in the normal spots but where are the best places to add additional ones? (clean answers please )

Anything made of metal plus a nice big ingot bonded to the rest and mounted on the transom, that is assuming shaft drive. Stern drives and outboards will have provision by the manufacturers, but again another ingot on the transom and bonded to the others is worth having.

A note on anode types which is of a general nature but nevertheless a good maxim.

Zinc = Saltwater use

Aluminium = Brackish use

Magnesium = Fresh water use.

The quality of anodes is an important factor also, so they should come from a reliable source, some of the cheaper ones contain impurities which reduce their effectiveness.

[Guest]

Quick question then from an outdrive boat owner - what use would aluminium anodes be on aluminium drives? They would degrade at the same rate as the drives themselves would they not?

[Antares_9]

OK, firstly there should be no aluminium exposed on the outdrive , it should be painted, you may notice blistering if water has penetrated. The other reason is drives are not "aluminium" but rather an aluminium alloy with other additives to assist in retarding corrosion. A good quality "aluminium" anode is made with an aluminium alloy that is significantly different (more anodic) than the alloy used on a sterndrive or outboard.

[pks1702]

Thanks David,

That is very useful I'll have a check when she is out of the water.

[SteveO]

I don't know anything about anodes in the context of boating on the Broads. Do all boats have them? Where are they likely to be mounted? How often should they be renewed? What are the consequences of not having them? Any information would be appreciated please.

Steve

[Antares_9]

"Galvanic corrosion (we used to call it electrolysis) can damage or destroy the under water metal parts of your boat. When two different metals touch each other or are wired together and are then immersed in an electrolytic medium (in most cases it’s water with minerals in it, such as salt) an electro-chemical reaction can/does occur. In other words, you have just created a battery that has measurable current running between the two dissimilar metals and causing galvanic corrosion.

One of the metals (the “least nobleâ€[i.e more easily dissolved] metal, called the anode) will start to corrode/dissolve much faster than it normally would do so because of this electrical current. Simultaneously, the other metal (the “most noble†[i.e. a more difficult to dissolve] metal, called the cathode) will corrode/dissolve much more slowly. For example: A stainless steel shaft is less noble than the bronze propeller attached to it. Therefore as a result of galvanic corrosion, the shaft would begin to dissolve leaving the prop intact.

So, how do we prevent this problem of galvanic corrosion? The most common way of dealing wit it is to use a simple, inexpensive device called a sacrificial anode. The sacrificial anode is a piece of metal that is less noble than any other metal found on the boat and is mounted or wired to one of the existing underwater metal parts, such as the rudder or shaft.

By bolting a sacrificial anode to the stainless steel shaft or rudder, the two metals form a galvanic couple (isn’t that romantic). With this configuration in place, the anode will dissolve allowing the shaft, rudder, etc. to remain intact and in good shape.

The effectiveness of an anode depends on a good electrical connection/bond and plenty of surface area on the anode. So as the anode corrodes/dissolves away, its surface area and its effectiveness diminishes. To ensure its effective protection, I recommend replacing an anode when it is about ½ dissolved. If you let the sacrificial anode dissolve completely, other expensive metal parts will take the place of the sacrificial anode and start to corrode away.

Sacrificial anodes are usually made out of Zinc, Aluminum or Magnesium. Zinc is considered the best metal for salt water applications, while Magnesium is considered the best for fresh water usage. For salt and brackish water, Aluminum is usually recommended.

So, there you have it, the lowdown on galvanic corrosion. Put on those sacrificial anodes and replace them as often as needed for maximum protection. If you don’t, your situation may turn out to be, “Plop, plop, fizz, fizz, oh what a disaster it is!†"

[SteveO]

Thanks, Antares 9, for your comprehensive answer. For a boat that only ever operates in the broadland rivers, with an occasional foray across Breydon Water, would you recommend aluminium or magnesium anodes?

Regards

Steve

[pks1702]

Very comprehensive answer David thank you - I learn more all the time.

Steve - to show how part dissolved anodes look see the ones on my rudder; about half gone.

If you can get to Hoveton in the week Aquafax do a comprehensive range, much cheaper than normal chandlers -another David tip

[Antares_9]

Steve, if you spend the vast majority of the time and particularly are moored well upstream the Mag are probably best for performance. It is quite important to keep an eye on them ‘till you see how they last though as if there is any stray current (electrical not water flow) in your marina they they may dissolve quicker than you may expect, in which case you may want to use Ali. Instead, or better still get the marina to sort the problem or fit a galvanic isolator, underwater bits and pieces being eaten away is not something that is too prevalent on the Broads as far as I am aware but better safe than sorry especially when it comes to through hull fittings, even though they may not be directly protected by anodes it is wise to have them boded to the rest of the system.

[SteveO]

Thanks for the advice, so does any metal that goes through the hull, i.e. prop shaft, rudder, raw water intake etc. need to be bonded to the anode electrically, or is it sufficient that they are all sitting in the same electrolyte solution as the anode e.g. the river?

Regards

Steve

[Antares_9]

Anything immersed in the solution should be protected so if it is too small for it’s own anode then it is best bonded.

[SteveO]

Thanks again Antares_9. I will check my anodes next time I have my boat out of the water.

Regards

Steve