Localised galvanic action on mild steel strap floors

[Rosie1963]

Hi All,

I want to get opinions on a specific problem I have. I have 7 galvanised strap floors. All of these were removed in 2011, re-galvanised and re-bolted using galvanised bolts. They are bedded on neoprene to try and insulate against other fastenings in the frame. This was post survey where a few of the bolts had turned to nothing and I could push a biro through the holes.

Ten years on, and in preparation for another survey, I have found some localised galvanic action around bolt heads. The Iroko around the bolts looks like “shredded wheat”. This has been made good, but clearly it is an ongoing problem and it seems to have accelerated whilst being out of the water. The yard has suggested trying different types of bolts and monitor them – using either stainless or bronze and see if that improves things? At first this sound like more a metal mix, but maybe it is worth a shot? (replacing bolts is easier than replacing or scarphing planks once they have gone).

Interested in people who have dealt with similar issues and what the outcomes were.

 

[Wansworth]

No idea but seems very odd to have so much trouble have you looked a you electrics

 

[johnlilley]

Unlikely that the action increases once out of the water, more likely that the drying out highlights the problem. Possibly need a bit more information first. Are the floors also used for keelbolts or independant from the keel. What material is the keel. What material are the keelbolts. How are the floors fastened to the timber keel. Are the floors wired to anything or in contact with anything other than the insulation on the frames. What are the plank fastenings made from. Did the bolt heads still have any galvanising on them. Whenever metal coorodes, be it zinc from the galvanising or whatever this can result in some deterioration of the associated timber but this can be far worse internally as the electrolyte is diluted externally but is concentrated internally. Any close or direct contact of the bolts under water with the copper fastenings is enough to start electrochemical decay similar to what you desribe. Are all bolts similarly affected, if not then look for localised hidden electrochemical (dissimilar metal) problem. Bronze bolts through steel floors will almost certainly be a bad idea leading to major planking deterioration. Stainless bolts might be OK provided they can be checked every season for crevice corrosion. Surprisingly stainless can sometimes be fine at one location but completely fail at another only a short distance away but less likely to cause electrolytic damage with steel floors. Maybe post a photo or two of the setup. Very likely to get a few different opinions on this as there are so many variables on this type of deterioration. All that being said, exposed bolt heads of galvanised bolts below the waterline would be expected to show some corrosion after 9 years although perhaps not to cause extensive damage to planking. Even copper antifoul applied directly over exposed zinc or steel will cause corrosion as often seen on ballast keels, but a ballast keel can afford to lose a lot more structure than can a small bolt.

 

[Wansworth]

Just out of interest is that a Laurent Giles design?

 

[Rosie1963]

Yes, she is a Giles design built at Porter & Haylett Ltd., Wroxham. The Peter Ducks, also built there, all have the same steel strap floor which is original to the designs.
The only electrical wiring in contact with underwater parts are is the shaft and prop which is connected to the anode/engine as per standard.
The keel is lead and the bolts croterite. The keel bolts are not connected to the straps as the larger ones heads protrude in a slots in the frame below the straps. Other keel bolts are spaced in the floors either side of the straps.
She is copper fastened but with some bronze screws I have seen in some or the larger intermediary frames.
Not all bolts are affected - most are OK. It seems that the forward two straps are the worst affected. I am wondering whether this is because the narrowness of the bilge is tighter here and possibly bilge water rises up. I have ruled this out though as she is quite a dry ship and the bilge water runs aft through large limbers.
The galvanisiation of the heads has gone in the sample bolt, but the shafts are still in good condition. I am trying to replace one with a stainless bolt with easy access, and will monitor this. I'll try and take some photos too.
I appreciate there might be a variety of answers, but everything helps when trying new approaches and ruling out ideas.

 

[Wansworth]

Stainless is in my experience with use bolting can suffer from shielding,good quality galvanized bolts are the best,imvhog

 

[johnlilley]

As only selected bolts are affected, this does suggest unseen electrolytic damage emanating from the individual bolt somehow in close contact with a bronze or copper fastening. The wood deterioration is certainly indicative of electrolytic damage. Could it be that a screw or rivet is very close to a through floor ferrous bolt as it passes through the timber but cannot be seen. Try connecting an ohmmeter to the new bolt and an adjacent plank fastening or screw. There should be a very high resistance (compare with others that are no problem) If the resistance is markedly lower then that would possibly indicate a current path that is enough for gradual deterioration. It is very common for hood ends to become soft at the point of the hood end fastening because the screw end is just glancing one of the commonly used ferrous scarf bolts for the centre line structure on many vessels as it is impossible to see how close the fastening comes to the scarf bolt. The scarf bolt corrodes excessively & the timber around the hood end fastening decays. Additionally the galvanising acts similarly to an anode and sacrifices rapidly in the event there is close contact.

 

[Rosie1963]

Thanks for that. I think that you must be right that there could be some “hidden“ fastenings nearby. I also seem to recall that it is in this area of the hull that the Biro went through years ago. I’m wondering if this could have been the very spot and that this problem has occurred all along. What it clear is that I need to map all the fastenings through those straps and monitor the suspect ones!

 

[Rosie1963]

This is an example where the stopping compound was bulging out. On remomval I could see that the two different types of fasteners were alongside each other (as suggested). Luckily the wood here is not so bad as in other areas. It seems to be more of a problem at the tuck and towards the bow. I am assuming that this is because in these areas it is harder to avoid the plank fasteners.

 

[Wansworth]

What about burning off the whole paint system,coat of primer and see what there overall condition of the hull is

 

[johnlilley]

The other giveaway is that the putty used as a stopper, as you have discovered, becomes useless & soft & pushes into a soft bulge on top of the fastening easily seen through moderate thicknesses of antifoul and paint