Composite plywood is really Coosa board. It is used for transomes, stringers, floors, or anywhere else you would use plywood. Coosa comes in several densities with the Bluewater 26 being the top end used for structural loads. It is 30% lighter than plywood and will not absorb water or rot. But is is pricy. One can use lighter but less strong Coosa board for seat frames, decking etc. It comes in thickness ranges from 1/4" to (I think) 1 1/2" and can be had in up to 4 x 12' sheets. Many suppliers will sell it to you in 48 x 32" sheets because shipping large sheets is expensive. When I did my research 2 years ago I found the factory in Pellham, AL and called them but they could only sell it to me in quantities of 25 sheets or more - but they did give me a distributer just across the freeway - advanced-plastics.com. When I asked them for a quote I was blown away by their low price of $200 per 1/2" 4' x 8' sheets. The best I had found up to this point was around $275 per sheet. And the best part is that Advanced Plastics is just South of Birmingham, AL which is right on my way from my house to the cabin/shop where the boat is - no shipping costs. Pictured is my Coosa transom and stringers. I bought 7 4 x 8 sheets of 1/2" and laminated it for 1 1/2" wide stringers and transom. The best way to do this is to cut the pieces larger than needed and use resin and cloth as a glue to bind the laminations together and put heavy things on it until it cures. This will be far stronger than a 1 1/2" thick piece and will also allow you to stagger the laminations if you are using smaller pieces to build a large piece such as a stringer. I would not stagger laminations for a transom. Use full sized pieces and laminate. Coosa cuts just like plywood and can be worked with any common woodworking tools. It is, however very messy so wear clothes and maybe a breathing mask like you would for a serious fiberglass sanding job. Also Coosa will not hold screws so you will have to glass in some plywood to do that as this next pic showing the underside of the splashwell shows. I plan on cutting holes there for rigging hoses. The 2 squares are 3/16" thin plywood beveled on the edges so fiberglass cloth will seal it to the coosa board with no air voids. One more thing - Advanced Plastics has about 8 locations - one hopefully close to you.
