Saturday, July 07, 2012

Hull sheathing

The new hull has now been sheathed in 6mm marine plywood.  This entailed making eight scarf joints.  A 9/1 scarf slope was used with all panels being beveled using a hand plane.  For me at least, the hardest part of beveling the scarfs seems to be to hold the plywood absolutely flat.  Thin plywood has a tendency to bow slightly either up or down, so I built an 8-foot jig and used multiple clamps to hold the plywood straight and level.  Four of the scarf joints (the short pieces) were bonded on the jig and four were bonded on the hull itself with equally successful results.  I did not want to try to position, epoxy bond, and clamp 19-foot panels by myself on the hull frame.  The hull was designed with flat cross-sectional panel sections located eight feet from the stern with backing plates (3/4" lumber) for the future joining of fore and aft panels.  That way I was able to split the sheathing task into forward 11-foot panels and aft 8-foot panels.   Each piece needs to be positioned precisely after being pre-coated with fresh epoxy; it needs to be done somewhat quickly so that the epoxy is still in a easily flowable and penetrating state, and then cleanup to remove squeeze-out and drips needs to be completed.  Especially at the hull forefoot and topside stern tumblehome, the panels need to easily bend to significant curvatures.  Thus, these panels were soaked in a shallow basin of water for two hours and then temporarily clamped in place until they dried out.  In this way they adopted a pre-warp to more easily conform to the curvature and required much less clamping force when permanently bonded.  The plywood sheathing will provide the foundation for the next layer of hull coating, 5mm thick planking.  This will be followed by fiberglass cloth and more epoxy resin.