COST AND NOTES ON GLUEING TWO SHEETS OF PLYWOOD

[bob mitchell]

        In building the transom three sheets of 3/8 marine plywood were used. Originally the outside sheet was mahogany, in case one wanted to show wood, but I did not do that. Cost of a sheet of 3/8" marine plywood in Chattanooga, a few pieces at a time, is $45.00 dollars. I have gotten a quote for $42.95 if I order 30 sheets - which I will probably do because I am planking the hull in two layers of 3/8". My reasons for doing this are that I generally work by myself, or at best with one helper, and secondly I do not like the stress that the 3/4" plywood introduces. So when you are talking about 30 sheets the cost of gluing becomes significant. 

           The transom lamination gave me a chance to see just how much glue was required.The cost of gluing, which I had not considered, are significant, and obviouly varied depending on the type of glue selected. I tried one lamination with PL PREMIUM POLYESTER ADHESIVE, at $7 dollars a 28 oz. big tube, and it took 4 tubes. I have had extensive experience with this adhesive, and love it, BUT IT IS NOT THE RIGHT ADHESIVE FOR LAMINATING PLYWOOD, because when you spread it thin it becomes tacky to curing (semihard, not squishy) on a hot day almost instaneously. While the tube says a joint can repositioned for up to 30 minutes, I will not use this for laminating dry, absorbent plywood again. It took that many tubes because it took a thick layer for the adhesive to stay wet.

      The next lamination I used Raka Epoxy with a slow hardner, and everything went really well. Glued it up in early morning (85 degrees) before it got too hot. I applied 15 oz. (10 resin, 5 slow hardener) to each sheet, and then an 12 oz. (8 resin, 4slow hardener) ketchup level thickened mix, but feel like I could do fine with 9 oz. (6/3) to each sheet, and then the 9oz.(6/3) paste, so that gives me 42oz per 4x8' lamination of two sheets of plywood. Raka Epoxy in the 6 gallon kit, with hardner, runs $318, so just roughing in 42 oz per pair of sheets  I should get 3 laminations per gallon of resin, or basically, roughed including postage, to $17 for epoxy per lamination. . So ball park 40 sheets divided by 2, times $17 and we get $340 for epoxy to laminate the plywood for hull. (I would, after further study, double that figure)We are not talking about the outside fiberglassing here, just laminating the 3/8 double layer planking. This is a relatively small amount of money in the bigger scheme of things, so I will probably stick with my original amounts.

         One excellent piece of equipment you can make in a minute is a wide squiggy - use the rubber or vinyl strips that you get at home depot for cabinet bases - just add a couple of strips of wood screwed together for a handle. Another useful addition is a paddle bit with sharp ends cut or ground off. It makes a great mixer in a battery powered drill on slow speed - saves a great deal of time that you might need. Raka, with their slow hardner, gave plenty of working time even in the hot weather, but clamps need to stay on for quite a while when laminating a curve because of the inherent stress. While I have significant experience with adhesive in constuction and woodworking, this is the first time I have laminated plywood for a boat, so maybe what I say should be taken with a grain of salt. It would be great if someone else on the site might share their experience - but we seem to have a rather silent ship.