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Plywood notes: Sheet sizes: Plywood comes as standard in 1220x2440 (48” x 96”) sheets. French-made Okoume marine ply sheet is a little longer and is 1220x2500 (48”x98 7/8”). If it is available it is worth to get this slightly larger sheet, as the extra length allows more space between parts. Plywood grades: All plywood to be at least exterior grade, water/boil proof WBP (sometimes referred to as commercial) grade. The glue is usually the same as marine grades, but the core is often made from lower grade veneers and may have voids. Check different board sources for the following: 6mm (1/4") plywood sheets must have outer plies of 1.5mm if of 3-ply construction and at least 0.5mm for 5-ply construction. Try to get 5-ply, the multiple laminations have less of a bias during bending than the 3-ply variety. I do not recommend the use of it, but if 3-ply exterior grade is used make sure the core has no voids as this will weaken the boat and may actually break during building if it occurs in a highly stressed place. If you find voids in hull panels after cutting out, play it safe and discard. You can probably use it for smaller panels and parts by cutting around the defect. I have analysed the costs of a typical dinghy project and found that the cost saving realised by using WBP (exterior grade) instead of marine ply is less than 10% of the overall project, i.e. material costs and includes a sail rig, but does not factor your time in. To my mind this is a clear indication that one should use proper marine grade plywood to BS 1088 spec or other equivalent national specs. You ought to also consider your time spent on the project. Lower grade ply may not last as well, lead to breakages even while building and all your effort, which is the same whether you build with cheaper or more expensive ply, could be wasted. Weight: Generally, Okoume/Gabon marine ply will be 25% lighter than the plywood made from denser grades of wood. The weight saving alone is worth the extra cost of Okoume marine grade ply! You may have considered using fir plywood but it has the disadvantage that it always develops surface checks and becomes a never-ending headache in terms of re-finishing. In my opinion fir plywood should not be used in dinghy building. Usually you are advised to sheath it using glassfiber and epoxy to overcome the checking problem! - if you add the cost of glass and epoxy it is cheaper and less effort to go for marine grade plywood. There is also a weight penalty in that the fir plywood is heavier to start with and the weight of glass and epoxy ups the weight once more. Unless you are building a work boat, use marine plywood. On the other hand on larger boats or boats that need the protection i.e. if regular pulling up on a beach is part of its service life, then sheathing of the bottom is either going to be specified or can be added by the builder. Preparation of plywood joints prior to glassing: Anywhere that an epoxy/glass joint is made, sand the ply surface to remove any finishing waxes, greasiness from handling etc. so there is no problem with adhesion. Applying fillets and glass on inside joints needs no other preparation. For a flush outside finish recess the glass into the plywood. Recesses can be made as follows: Grind a recess with #60 grit soft disc (on small angle grinder), careful sanding removes approximately one ply thickness or 0.3-0.5mm (12-20 thou), 50mm (2”) wide. Alternatively, if you have a router you can make yourself a simple base with an adjustable stop and using the largest straight router bit run it around the edges that need recessing. I use a 16mm (5/8”) bit and need to take three passes, each time moving the stop so another band of recess is cut. For a detailed description of the jig read: Hints and tips for the use of routers The recess will ensure the glassfiber is flush with the surface of the plywood once finished by filling and sanding. This is desirable for a good class of finish. If it does not bother you that the glass tape is sitting proud on the surface, the recess can be omitted. Cutting plywood: Use a jigsaw / sabersaw with a blade that has fine teeth. 18 tpi (teeth per inch) is a good pitch. For a Black & Decker or Ryobi saw you can make your own blades from 18 tpi hacksaw blades. Use a shop-bought blade as a sample and grind the foot on a bench grinder to match. The hacksaw blade is too wide, so to make it narrower, you also have to cut away from the back of it. Remember that a jigsaw blade cuts on the upstroke so the teeth have to point upwards and grind the foot so the blade angles slightly forward (in the direction of cut). Hacksaw blades are very hard and will out-perform any shop bought jigsaw blade – they just last and last. Using fine tooth blades has the advantage that it does not lead to splintering of the ply surfaces. Finishing the cut: It is possible to cut quite close to the line with the 18 tpi blades so finishing won’t take long. Best method is to use a hand plane with a sharp blade. If it is not really sharp get out the whetstone, in fact keep it handy throughout the project. After sharpening, run the back of the blade lightly over the stone to take the burr off. Make sure to hold the blade very flat while de-burring. Test the sharpness of the blade by resting it on your thumbnail, trying to drag at right angles to the blade. If it is sharp, it won’t drag at any point of the blade. For scraping of epoxy and epoxy glass sharpen the blade the usual way but don’t remove the burr. Sharpen regularly. Pre-coating: Once the hull panels are butt jointed and cut out, it is best to seal the surfaces with a coat of epoxy. Epoxy can be applied by brush or roller, but a squeegee speeds things up. You can just pour some epoxy onto the ply after which the squeegee is used to spread it out evenly. Find a detailed description in: Epoxy application techniques A great time saver is to do your sealing already at the time you join the hull panels. Once the joint is assembled and weighted you just carry on mixing epoxy and spread it on all exposed ply surfaces. Later you will only have to deal with small areas around the joint. The inside surface you can seal once you have transferred bulkhead and longitudinal lines for interior fitment. Updated: 05 June 2011 Back to Tips and Hints menu

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