LOFTING & CUTTING OUT HULL PANELS
January 24, 2021
With our bulkheads cut out it's now time to turn those unwieldy scarf-joined plywood sheets into the pieces that will form the first layer of Burnett's hull - hull panels. The hull panels of a stitch-and-glue boat have curved edges. To get the curved edge of one panel to meet up with the curved edge of an adjacent panel, the two panels have to be bent. The hull panels are designed so that, for the edges to meet, they have to be bent into the shape of the boat you are building. Getting the edges of Burnett's eight gigantic hull panels lined up and joined therefore should naturally make them take the shape of a 39' sailboat. Nesting the bulkheads inside the hull panels adds the support needed to take the sag and twist out of the assembled hull panels, which are fairly flexible over their length. However, as with the bulkheads, the hull panels need to be precisely the right shape to result in the correct hull form.
Four of Burnett's eight hull panels, each ~40' long, screwed to the posts of our shop for storage until we are ready for assembly. It can be hard to imagine how the seemingly abstract shapes will take the form of a boat.
Planks of a traditional wooden boat are analogous to the hull panels of a stitch-and-glue boat. However, unlike a stitch-and-glue boat where the shape of the hull panels defines the shape of the boat, the planks of a traditional wooden boat are a patchwork of pieces used to put a skin on a shape predefined by a set of temporary molds (these molds don't end up being a part of the final boat). One of the reasons stitch-and-glue boat building is quicker than traditional wooden boat construction is that temporary molds are not required. Instead, the shape of the hull comes from the hull panels, which are a permanent component of the boat.
Lofting Hull Panels
The process of lofting (drawing to scale) a hull panel is similar to that for lofting a bulkhead: a suite of points are measured out on a sheet of plywood from a baseline and joined together with lines to generate the shape of each piece. However, in contrast to the more equidimensional bulkheads, the hull panels are long (up to 43 feet) and slender (maximum 5 feet wide). We therefore had to use a string line for our baseline instead of our twenty-foot-long straight piece of aluminum. Also, because the curved edges of the hull panels need to be precisely the right shape to match up with adjacent panel edges, the suite of points used to generate the lines is densely spaced for more control. We drove nails into the points, and in the same way that we produced the curved lines on the bulkheads where they will support Burnett's cambered deck, drew out the panel edges by springing a batten (long strip of flexible material) to the nails and tracing the batten.
Our 3/4" square Douglas fir batten seen above worked well for most of the panel edges. For sharper curves, such as the leading edge of this hull panel where it will come together at the bow, we used a metal ruler standing up on its edge.
Cutting Out Hull Panels
We used the same tool to cut out the hull panels as we did the bulkheads: a skill saw equipped with a sharp blade. Depending on the sharpness of the curve to be cut, the cutting depth of the blade can be adjusted to give more control. If the blade depth is set just slightly deeper than the thickness of the plywood, the minimum amount of blade required to make the cut is in the wood, and it is possible to cut sharp curves. If set to full-depth, nearly the entire diameter of the blade is in the wood, which minimizes wandering of the blade when cutting near-straight portions of the lines. Often, builders will cut out the panels slightly oversized and pair them down to the line with a hand plane because it’s a low-risk way to complete the task and allows some room for error. However, we have gained quite a bit of experience making precise skill saw cuts when building previous boats and timber framing our shop, and were comfortable cutting right to the line to shave some time off of this step.
The closeup photo above shows the skill saw blade depth set as deep as possible to cut a near-straight portion of a line. Above right we are cutting a gentle curve that is easiest to cut with the blade depth set about halfway between minimum and full-depth.
Making Symmetrical Pairs
Once the first panel was lofted and cut out we used it as a pattern to make a copy for the other side of the boat. After tracing the panel onto the next sheet of scarf-joined plywood, we cut out the second panel, leaving approximately 1/8" of material outside of the line. We then mated the surfaces of the two panels together and used a flush trim bit on a router to bring the larger of the two panels down to the exact shape of the first panel. This is an easy way to get two identical panels, which should lead us towards a symmetrical boat.
A flush trim router bit has a bearing that sits below its blades. The blades are set so that they cut the same diameter as the bearing. Whatever sits above the bearing will therefore be trimmed down to match whatever the bearing is riding along.
The final step in dressing up a hull panel is putting a 45° chamfer on its edges where it will abut an adjacent panel. The chamfer is a sloped surface that gets rid of the sharp 90° corner of the panel. Getting rid of the sharp corners makes it easier to fine-tune the alignment of the panels when stitching them together (this will be easier to visualize when we post about that step).
The depth of the 45° chamfer router bit is set so that the bearing rides along the panel edge and the blades cut a chamfer for half the thickness of the plywood. The above photos show the chamfering router bit we used and an example of how the edges look once cut.
Routers are amazing tools that allow the user to very quickly put nice neat edges on a work piece or make copies of a pattern. One downside to how fast they are is that it's pretty easy to get carried away when you are using one! When you see the sawdust flying off the blades and the perfect edges appearing behind the router, as in the above photo, it makes you want to just keep going. I was on the router when we were putting chamfers on the sheer panels, liked what I was seeing, and was about to put a chamfer all the way down its top edge - an edge that doesn't abut another panel and therefore doesn't need a chamfer (the deck will sit on top of this panel edge). Fortunately, Spencer was on the ball and tapped me on the shoulder to point out I was about to shave off a portion of the panel that we wanted to keep. There's a reason he's the Shop Foreman.
With all our hull panels and bulkheads cut out we are now ready to start the assembly stage and finally put some of the pieces together. Exciting stuff! Next we will be stitching the two bottom panels together and raising them towards the ceiling where they will be out of the way for us to start erecting the bulkheads beneath them.