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We are planning the construction of a Pahi 63 starting this year. We have run the numbers and are satisfied with the prospect. The first input I would like is plywood type. I am considering Meranti Hydrotek  for the hulls and other areas of the build that require better rot resistance and strength. Moving to Okoume and Douglas fir for areas that will allow for there strengths and lightness. Any thoughts and experiences? We also was considering Port Orford Cedar for our lumber. Any thoughts and/ or experiences on this?

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Meranti and keruing are horrible stuff to use as timber, less so as ply I suppose. I shouldn't think that the sources of this ply are sustainably grown, but then it's hard to find any that is. Both timbers like giving you long splinters in your thumb which infect pretty quickly if you don't get it all out. On that basis you would think that they are inimical to rot, but they are average on that score and quite heavy. I would guess the wood dust needs keeping out of your nose.

Ref fir ply I have used quite a lot for basic carpentry, mostly jigs, supports, staging etc. Personally I would find it annoying to use for accurate work on a boat in the thinner thicknesses - the sheets are often kinked. Even 18mm is like this sometimes.  It reacts quickly to changes in humidity, so if you are building in an open shed or in the open you are always up against that. There are a lot of core gaps unless you can find a top quality one which is supposed to be consistently clear of them. However it is light and many boats have been built with it.

Core gaps in any but the most expensive ply can be a problem as interstitial condensation can occur in them, over long period of time being possible future pockets of rot. Fir ply is fairly durable, but over a long period of building in non-optimal conditions, protecting the edges from damp can be difficult until they are properly finished.

The only way to counter some problems like this is to build very quickly with a large team in properly heated and dust and fume-extracted factory conditions. Hardly DIY building on a 63ft catamaran.

Most people I have met over the years who have worked much with epoxies have (like me) had a serious reaction to them, and this also bears out the desirability of the kind of programme alluded to above.

Port Orford Cedar is an American timber I'm not familiar with. I have an interesting U.S.  wood ship- building manual from the U.S. Navy Dept (1948) "Wood, a manual for its use in wooden vessels"  which describes the timber as follows:

"White -Cedar, Port Orford (Chamaecyparis lawsonia) - Port Orford white-cedar, which is the heaviest and strongest of the cedars, grows naturally only in a narrow belt bordering the coast of Southern Oregon and Northern California. It is classed as moderately light in weight, of moderate strength and hardness, and its heartwood is highly resistant to decay. The wood is of a fine and unusually uniform texture. It is used in boatbuilding principally for planking, but sometimes has a tendency to split in the way of fastenings. It has a spicy odour different from the other cedars."

(I am not a professional boatbuilder. I am a professional woodworker who has built a couple of boats.)

Thank you very much for your input. Since posting this discussion, we have moved our thinking to resin infusion with foam coring.There seems to be too many materials quality issues that have to be dealt with foreign manufacturers in using plywood. I love wood, that is the problem. We will have to work it into the interior. 

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