Using double layers of marine ply

I am considering building a 28 ft. open deck sailing catamaran and the two original options for the hulls ( with a dory shape )are:

1. 1/4 inch marine ply (I would use 1088 Okoume)
6 oz.( per sq. yard) glass on the outside with epoxy
stringers and frames
coated with epoxy on the inside

2. 5/8 inch foam (probably corecell-5lb. density)
36 oz. ( per sq.yd) glass and vynelester (sp?) on the outside
28 oz. (per sq.yd) glass and vynelester on the inside

I would like to avoid the stringers and frames and am thinking about a third option:

3. two layers of 1/4 inch ply bonded together with epoxy or Purbond with the ply layers at 90 degrees to each other. An additional thought with this bonding process was to staple the two layers ( after coating them) together with plastic staples that would hold the panels tightly together ( especially for the Purbond). This would help me to avoid the internal framing, and use only a light fabric on the outside. It appears though that this third option would add 250 lbs. per hull

Of course I have thought about the possible higher resale value of the foam.
Any help will be appreciated.

 

Staples may not hold the plywood tight enough. I use hex washerhead screws.

 

Cat,

How about foam-core with plywood skins? You could skin one side of the foam on the bench, then fasten the panel up and add the second skin. Suppose you could use balsa too, (lower cost) but I suspect you'd have to do both bonding-operations in place.

 

Cold Molding

The third option is called cold molding, I do it all time. No need to install the plywood at 90 degrees to each other. It would be best to scarf the plywood for full length panels on the first layer and you can butt joint the second layer and dish out the joint a bit and glass it well. The construction method you are thinking of using is called stitch and glue, go to http://www.boatbuilder.org/ for more info on it. In stitch and glue there are no or few ribs or stringers. The bulkheads are glassed in and the hull joints are all glassed as well, this is what makes the hull strong. Epoxy is used as the glue and as the wood sealer. When cold molding plywood I find it helpful to screw the perimeter and staples can be used in the field and just pull the screws after the epoxy has cured. It is also a good idea to drill 1/8" holes everywhere in the outer layer about 10" apart. As you install the holes should all weep epoxy and add more staples or screws as needed. This will help you avoid voids between the two layers.
---Joel---

 

Yokebutt,Gonzo, and Boatbuilder.org,

Thanks for your quick responses. I have only worked with female molding so I really appreciate your getting me straight on the one off questions.

 

How about stitch and glue? The Epoxy fillets with glass tape are in effect stringers.