Anodic protection

[mulligan]

I am about to launch Mystico a Robert Clark Mystery after having her re-built, I am a little concerned by the fact that the marina manager has suggested putting me in between two steel motor boats, this would not normaly bother me although I would profur being moored along with other sailing boat. The reason I am concerned is because were they plan to put me a wooden boat was moored two years prior, unfortunatly after some eight weeks the boat started to take in more water than you would expect and so to be prudent the owner had her slipped were he found that all of the galvernised bolts used on her heal pintle which was bronze had been totaly erroded, he then found that all of the bolt used when replacing the floors had be erroded but not to the same degree, this became a topic of debate, with people suggesting that there was a stray earth from one of the motor boats which might have caused the problem?. Could the problem be caused in this way and if so what can be done to prevent it and if when you replace the bolts should you use the same material for example bronze with bronze, iron with iron etc, the reason I ask this is because Mystico's cast iron floors are fastened with bronze fastenings and they looked good even after 65 years. The other thing is when wiring your skin fittings to your anodes would you also wire all of the floors ect in order to maximise protection, I may be getting worried about nothing, but when you have carried out a project such as this the last thing you want is something unfortunate to happan as a result of not asking the right questions.

Many thanks

 

[Peterduck]

You are not "worried about nothing". Your concerns are very real, and you are quite correct to be concerned. The fact that the neighbouring boats are steel is not necessarily a problem. The problem may lie in their use of shore power, which can result in stray electrical currents in the surrounding water.
Having cast iron floors fastened with bronze bolts is not good practice. One should definitley keep a uniform system of metals, at least below the waterline. Either all copper-based alloys [eg. bronze, copper, etc] or all iron [not steel!] based. As the most numerous fastenings will be those holding the planking on, everything else should be the same as them. Even though the floors are inside the boat, they will be exposed to bilgewater, and a "battery" will be set up between the floors and their bolts, and the floors will corrode. There are numerous good references to all this which you should read carefully, because you have a potentially serious problem. Because of its porosity, it is difficult to successfully galvanise cast iron, and any galvanising zinc would be quickly corroded away.
Peter.

 

[clyst]

PeterDuck is absolutely right you do have something to worry about!!! My advice to you is to move ASAP!! Stray electrical current from shore supply can be a bugger.

 

[Mirelle]

Very serious problem

Now is not too soon to start panicking!

If I might recommend a rival website, the Wooden Boat magazine forum search engine will throw up masses of data on this, because our cousins keep their boats in marinas more often than we do and are more "electrically aware" than we are.

I would NOT wire ANYTHING to an anode - that is a recipe for degradation of the wood at the cathode due to alkali build up and that can be far more expensive than replacing metal! There are numerous horror stories about this.

Under no circumstances would I put your boat in that berth.

 

[mulligan]

Re: Very serious problem

Thanks for your input, if I could ask a few more questions to put my mind at rest. I had made a support for under the mast, this was to reduce the pressure in this area, this was made in steel and galvernised, what would be in your opinion the best way to fix this through the hull?. The other thing is that Mystico was copper sheathed in the early 70's for her round the World trip, might this cause any problems which I am not aware of and if I am not to wire skin fittings ect how else am I to protect them from failure at an inopurtune moment in the future. Your help is much appreciated as always

Regards

Ross

 

[ianwright]

Re: Very serious problem

There are two views, at least, on bonding.
One (mine) holds that on wooden boats nothing should be bonded and only those parts at risk, shaft, bronze heel fitting etc should have anodes fitted.
The other holds that everything should be bonded to everything else. I suspect that this is an idea of US origin which only aplies to boats built of grp with deep freeze units microwave ovens and airconditioning that are plugged into shore power a lot.

This "Stray Current" idea,,,,,,,,, does anyone know how one would check for it?

IanW

Vertue 203, Patience

 

[Peterduck]

Re: Very serious problem

As regards the galvanised steel mast step, I would recommend replacing it with a substantial oak or other hardwood step which spreads its load over as many floors as are practicable. This can then be fastened to the keel with copper bolts.
Peter.

 

[Guest]

Re: Very serious problem

I quite agree with this and have experienced rot caused by incorrectly used anodes. An area of particular vulnerability is the deadwood at the place where the stern tube is connected usually by means of a flange and two screws. High alkalinity will result in rot and the need for serious repair in a place which is usually difficult to access.

 

[Mirelle]

Copper sheathed...!

You write as if the sheathing is intact. If it is, then all skin fittings will automatically be in contact with the copper. They are therefore electrically bonded to the copper. Anything less noble than the copper (given the immense area of exposed copper) will be anodic to the copper and will waste fast; anything more noble than copper will be preserved for ever, due to the immense area of exposed copper.

Any further electrical bonding is superfluous.

You will find that the sheathing will wear away around the waterline, espescially at the bow; this is due to the potential difference (approx 0.4v) existing between aerated water at the surface, espescially in the bow wave, and relatively un-oxygenated water lower down. The waterline copper is anodic to the unaerated copper.

It goes without saying that no iron or steel fastenings can be used anywhere in the boat below the waterline, and a stainless prop. shaft might not be the best plan.

As a matter of interest, although this is "theoretically wrong", PROVIDED that there is no stray current about the place, and in cold water, bronze and copper fastenings through galvanised wrought iron floors, etc., can work quite well - after all, English builders did this for many, many years, down to the 1950's, and many of their products have survived.

The reason why it works when it should'nt is that the iron and steel has a bigger area than the more noble fastening, and whilst it acts as an anode to the fastening, it can wear away quite slowly. This is why galvanised deck fittings, or in the case of my boat the chain plates, can be fastened with bronze fastenings quite safely.

Under the waterline, however, greater caution is indicated.

Your mast step fabrication is an interesting case; certainly do not fasten it with steel bolts. Bronze bolts may do OK. My own mast step (recently renewed, but it is made of teak) is fastened with bronze coach bolts.

 

[mickshep]

I bought a 1946 vintage, Robert Cark 5 tonner about 5 years ago. her floors are of iron, copper riveted through the hull which is of double diagonal teak. they are not connected to any anode. despite the disparity of the metals there is no sign of any degradation. Mick.