Plywood woes

[steveh]

We are selling our plwood built boat.
She was professionally built circa 1985.
After a prospective buyer had a survey done, it seems
that she was built using steel nails to hold the ply sheeting
to the frames. These are now rusting and causing bubbles
to the fibre glass roving. This in turn has allowed water
into the wood and given very high moisture readings.
The buyer of course has not proceeded and the broker has advised
that we drop the price by 40% and sell her as a project.
My question is, does anyone know why steel nails would have
been used on what is a very well built boat using Iroko and marine
ply and no signs of short cuts?

None of this was picked up by our own surveyor, 3 years ago,
when we brought her even though it is not a recent problem.

Any explanations welcome!

 

[Wansworth]

What is ther to say ,gripfast should have been used and who ever built her was either an idiot or not worth the name as boatbuilder.This does not help you.

 

[Mirelle]

That is very odd.

I am not surprised that a surveyor did not pick it up, as with the nail heads being covered by the sheathing there would have been nothing to go on, and one would expect bronze ring nails to be used.

However, I suppose one starts by working out how and why moisture has got to the nail heads, to cause them to rust and lift the roving in the first place.

After all, there are plenty of ex fishing boat hulls built using steel boat nails to hold the carvel planking to the frames, and some of them are rather elderly, now.

 

[Seanick]

Re. steel nails, its quite common to use steel nails for this type of job. Many production strip planked boats were pinned together with steel nails whilst the glue dried. They are cheaper, and have lasted long enough.
The ply is still hopefully glued to the frames and stringers, so there is no real lossof strength issue. If you are worrried about that, then add extra fastenings where possible. Grind back all fastening lumps in the sheathing and resheath locally. Ventilate the boat to help drying from the inside. You need to know exactly what the moisture content in the damp areas is before worrying to much. You may need to strip some paint from the inside to help drying too.

(Most old fishing boats are fastened with iron, which lasts much longer than steel)

 

[lexi]

On a smaller scale I have punched nails through plywood rather than draw them out but the hardness of Iroko frames may not help you in doing this. The hole could be filled with a cartridge of resin in a gun and another stainless or similar one put in near it. Surely galvanised nails could have been used at a minimum. Preservatives could have attacked the nails.
Alex

 

[Wansworth]

seanick...Iwould agre with you using galvanized nails in stripplanking but in the case of fastening plywood surly good practise would be gripfast or simular as the nail is not so burid...By e the way is Coombs still a boatyard ?

 

[oldharry]

I would guess the nails were used to hold the ply sheets in place while the glue set, and have no further structural function. As the boat was to be sheathed, the builder did not expect them to come into contact with seawater, so the material did not matter.

I would want to know how the water\has actually got to them now. First off, what was used to sheath her? If it was standard Polyester resin and glass, then failure is inevitable. The only surprise is that she has actually lasted this long!

If it was done properly she either had a Cascover sheath applied (as mine has), or was epoxy sheathed. Either way, water should not be now be penetrating either the sheathing to ply bond, or the ply itself. Apart from the bilge area where water collects occasionally, the hull sheeting on mine remains substantially dry.

I rather fear that you may have a bigger problem than just rusting nails - the Boatbuilder clearly did not expect the nails to cause problems, so the conclusion must be that something has gone wrong with the sheathing or it's bond to the ply.

The problem with sheathed ply is that if it gets even slightly damaged the water gets in and destroys the bonding. Even the smallest nick must be resealed promptly - its a bit like preventing rust on a steel boat - the protective paint must be kept perfect if it is to do its' work.

The last sheathed boat I had, had similar problems - it had a persistent and unidentifiable leak. When I started to investigate in earnest, I discovered almost the entire sheath had detached from the timber, and the whole lot just came away...!

 

[steveh]

Thanks for the information.
The heads of the fixings have rusted and caused
bubbles in the covering. I think the water has entered
here. She spent about 10 years in a boatyard which is
why she has done so well.
If the nails are only temporary, then will just replacing
the covering, once she has dried, be sufficient?

 

[oldharry]

Hope so!

The worry from what you have told us is the report of a 'high moisture content' in the ply. This may mean lots, or may simply mean the surveyor who reported doesn't know how to use a moisture meter. I am a bit inclined to think the surveyor did not know the specifics of testing ply. Wood immersed in water gets wet and behaves very differently to GRP. Moisture meter readings are not easy to interpret. A high reading may simply mean there was a heavy dew last night! A Surveyor unused to ply is unlikely to place the right interpretation on the readings and I am not at all sure that using a moisture meter actually tells you anything about the condition of the veneers!

However, as far as stripping and replacing the covering is concerned, a little care is needed. IF the ply really does have a high moisture content, then it needs to be dried out before it will accept a new covering. New Ply 'from the store' should have a moisture content of around 15% and this is fine for sheathing. Once it has been immersed the moisture content shoots up, and it has to be dried out again before even epoxy will bond properly, and you certainly do not want to be sealing a lot of water into the sheeting, as it provides ideal conditions for rot to set in!