Saturday, August 29, 2009

initial hull sheathing

Once all the frames and the anterior keel are mounted and aligned on the strongback, the entire framework was then faired so that panels would lay flat instead of merely touching one edge of each frame. This provides more panel support and more bonding/nailing surface. Once faired, rosin paper patterns were made for each proposed panel of the hull sheathing. I was planning to make thin ply patterns but found that stiff paper was accurate enough in this case. I had to decide the order of panel placement and the position of junctions between panels. I wanted to place the panels which required the most bending first, and I wanted to join separate panels in places where hull curvature was at a minimum. On my previous boat I scarfed all the panels together before placement, but found that handling an 18+' long, narrow panel wet with epoxy resin, precisely placing it, and getting it securely fastened in place quickly was a difficult task. Fortunately, this hull has convenient panel juncture points where either a butt plate could be placed or a nearby frame could back an in-place scarf joint.

Those plywood panels which required significant bending, the bottom forefoot and stern side tumblehome, were submerged in a shallow basin of water for several hours and then quickly clamped on all edges into place of the frame and left to dry for a couple days. The panels were then removed, trimmed for a more exact fit, and bonded into place. As more panels were placed with adjoining edges, clamps could no longer be used on all edges. For these locations I used a nail gun with a 3/4" or 1" 18 gauge nail and scrap 1/4" ply placed under the head to facilitate later removal.

At this point, the entire hull is sheathed, and we can begin to see the full shape of the hull. I now want to cover the entire hull with a second layer; 6 mm. just isn't enough thickness for me. Sure, I could used multiple layers of resin and cloth, but that stuff is heavy and expensive. The strength-to-weight ratio for wood is excellent, so I will add about 4 mm. of thin wood planks; sand it as needed, and finish with a layer of resin and cloth. I've had to research band saws and blades to get the right setup for "resawing", cutting a normal plank into 3-5 mm. thicknesses. The band saw I bought was missing an important set screw, and I was unable to properly adjust it until I figured out that the mechanism was not functioning properly. Looks like it is now properly set, and producing thin planks is my next task. I have picked cypress wood for the planks- available, relatively cheap, fairly light, and rot resistant.