Cold moulded ply boats

alienzdive

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I presume from the name (correct me if I am wrong). That a cold moulded ply boat is ply, laminated up inside a mould, this removes the need for as much frame work as the epoxied ply laminates holds the shape of the vessel.

If the hull rots to dust, how can it be replaced? Does it need to be re-built inside a mould again?

Are there any good links for this type of construction or repair?
 

drawp

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I built a 35ft cold moulded plywood boat 20 year ago. The hull was 4 layers of 3mm marine ply laminated outside a temporary jig. Mahogany frames were then laminated inside the hull after the jig was removed. Rot has not been a problem and the boat is still going strong but I have to keep a very close eye on it to make sure rain water doesn't get into the edges of the ply anywhere. At the time I used a resorcinal phenol glue which is not a gap filling glue. If you used epoxy and sealed the ends of the ply as well as filling any gaps, rot would be even less of a problem. Fortunately I have not had to do anything apart from cosmetic repairs to the hull but major hull repairs would be extremely difficult.
 

john_morris_uk

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Its not usually the whole hull that rots. If it did, you are correct in your assumption!

If a section of hull rots then it is possible (with a moderate amount of skill, and a lot of time and patience) to cut away the rot and laminate a new section of hull into place. The best repairs cut layers of laminate away in a stepped configuration and then areas of laminate are epoxied into position over a temporary mould.

Try "The Boat Repair Manual" by George Buchanan which is excellent on all boat repairs on all hull types.

I think there is a book called "Wooden Boat Repair" by the Gougeon brothers which feautures West Epoxy system and descibes the techniques used.
 

alienzdive

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Yep, I have taken it over as a project. I will get great satisfaction from seeing the end result. I have had numerous fibreglass and chined ply project yachtys previously, but for me this is a new construction technique that I am unfamiliar with.
 

LORDNELSON

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Years ago I was given a Fairy Swordfish (cold moulded 15 foot racing dinghy) with two damaged sections on the bilge. I cut the laminates in a "stepped" manner, lined the inside with pieces of waxed soft flexible plastic (like milk bottle stuff) and then did a fibreglass repair. The boat was sucessfully sailed and raced for several years after that; if your boat is riddled with rot, this may not be the answer! Good luck.
 

alienzdive

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I am not wanting to know how long a piece of string is.

I am nearly confident enough to undertake the entire rebuild of the hull structure. There are just a few areas which I am uncertain about but I am sure that they will become apparent when I get to these satges.

I am undecided as to wether I am going to strip the original hull in stages of about 3 metres and re-build each stage with the fist and second skin. or strip the entire hull back and re-build. I am worried of losing some of the shape with the latter, as it has beutiful lines especially for a 1950 design.

I was also wondering how long a project like this will take? I know the exact costs and time for the interior, painting, electrics, engine etc. But can not work out (as this is the first time I have seen this type of construction) how long the hull will take to re-build. It is 9 metres long and I estimate about 60 sheets of 6mm marine ply to get 1" finished hull thickness. As well as a few large drums of resin, filling powder and plastic mixing buckets etc. I can get good bulk prices from the local suppliers. I think 100 Litres of epoxy would be about right?
 

oldharry

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You are a brave man Alienzdive! I have never undertaken a cold moulded hull rebuild, although I have done some patch repairs. My guess will be that your difficulty will be in preserving a true shape to the hull, unless you build some sort of a mould to work to?

The original build system for producing these hulls varied somewhat, but always involved either building into or over a mould, rather like GRP. The Fairey Atalanta for example was built by "tacking thin strips of mahogany veneer in criss cross layers on a mould, with glue coats in between. Then the whole things is enclosed in a rubber bag from which the air is pumped out so that the atmospheric pressure presses the bag down onto the veneers and squeezes them against the mould. ... the whole thing is pushed into an oven where it is baked until.... (it has)... hardened into a complete, homogenous shell. Cold moulding is a similar process...." (Quote: Denny Desoutter, Small Boat Cruising, 1965).

Not easy without the kit? But perhaps the key to it is in using thin veneer strips tacked to a mould formed from the original hull, rather than using plywood. I would guess that modern epoxies will make the job a great deal easier than in 1965! Certainly the thinner the material you use, the easier it will be to get a fair hull, but equally the longer it will take, but I would certainly not expect to be able to force anything as heavy as 6mm ply to the compound curves you almost certainly have. It would certainly be impossible on an Atalanta which has some quite steep curves, that plys will certainly not take without splitting.
 

kds

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I have done extensive repairs on deck and much less extensive repairs on hull of a 26 ft. Atalanta. These were hot moulded and the veneer was about 3 mm only. I eventually developed a technique of taking rotten veneers off with a router as this prevented splitting the next layer. The whole job is not very skilled but is immensely time consuming. If you have more than two layers to replace, I tackled 2 from the outside, one at a time and only did the others from the inside after glue had set. This retains the shape.
Cold moulded hulls I have seen were often built on a light frame and stringer framework.
 
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