LOFTING AND CUTTING OUT BULKHEADS
January 10, 2021
Stitch-and-glue boat building is similar to timber framing – the building method we used to construct our shop – in that many individual pieces are cut out and set aside before putting them all together in one big assembly stage. Both construction techniques therefore lead the builder down a similar emotional path. First, there is excitement and energy with starting a new project. This first stage is memorable but spans an infinitesimal portion of time in the marathon that is building a 39' sailboat or a shop big enough to build it in. For the second stage, the builder enters a two-dimensional world for a prolonged period, toiling away on individual pieces, many of which the untrained eye wouldn't even recognize as a component of the three-dimensional object they will eventually become. Anxiety accumulates in this stage of the project because even though each completed piece means you are one piece closer to that glorious assembly, it also means you have invested that much more time and effort into a stack of wood that doesn't look much like your end goal. Are all those pieces going to fit together? Or have we made some critical mismeasurements along the way? In the case of Burnett, there is the added uncertainty of building a boat design that has never been built before. Even though we have confidence in her designer, whose numerous successful designs speak for themselves, I'd be lying if I told you I haven't wondered how easy it would be for some computer glitch to result in errors when converting the digital model of the boat into paper plans from which to build it. With any luck, the anxiety that has accumulated is released in the third stage, the assembly stage, when the pieces quickly transition into an easily identifiable three-dimensional entity; the time and effort put into making each individual piece pays off when you finally get the satisfaction of really seeing what it is that you are building.
Basking in the post-assembly stage glory. It took us three months to cut out the 131 pieces that make up the timber frame of our shop, but only three days to erect it.
Lofting Bulkheads
For the assembly of a stitch-and-glue hull to go smoothly – and therefore yield the satisfaction that should come with that step – it is crucial that all the pieces being put together (bulkheads and hull panels) are the right shape. Harder than accurately cutting out the pieces is drawing them on the wood out of which you are cutting them - "lofting". When building a smaller vessel, it is typical to print full-size paper patterns of the bulkheads. The builder can use these patterns to trace the parts onto the wood and then cut them out. But this is impractical for a boat the size of Burnett; even if there was a printer that could economically print out a pattern for a 13' by 8' bulkhead, changes in humidity from when and where the paper pattern was printed to when and where the builder uses the pattern could easily change its shape to the point where it is no longer accurate enough to build from.
This World map on one of our shop walls was tight and snug to the wall when it was pinned up. Now it is loose and wavy, analogous to how a paper pattern for a large boat part might significantly change shape when shipped from one climate to another. In this example it is likely the tongue and groove fir wall that is changing size, not the laminated paper map, but you get the idea.
Without full-size paper patterns, we had to loft the shape of the bulkheads onto the plywood using dimensioned drawings provided by the designer. These drawings consist of a suite of points that, when connected with lines, define the shape of a bulkhead. Each point is measured with reference to two very important lines: the center line (CL) and the design water line (DWL). To visualize the center line, imagine a vertical plane stretching from the tip of the bow to the middle of the stern. This plane would bisect the boat into two symmetrical halves, port and starboard. The intersection of this plane with a bulkhead oriented perpendicular to it defines a line on the bulkhead, and that line is the center line. To visualize the design water line, imagine a boat sitting perfectly still in a dead calm ocean. This imaginary boat is loaded up with what the designer thinks will be a typical weight of passengers, supplies, gear etc. You can imagine the surface of the water as a plane that slices the boat into a portion that resides below the water, and a portion sitting above it. Where a bulkhead intersects this plane defines the design water line on that bulkhead.
DWL
CL
Drawing of Burnett's bulkhead #10. Dimensions to the points needed to draw the full-size bulkhead are referenced from the center line (red) and design water line (blue).
Transferring the points from the dimensioned drawing to the plywood from which the bulkhead will be cut is where a simple-sounding task gets more challenging than one might expect. The difficulty is in accurately positioning the points with the desired precision of 1/16”. If the CL and DWL are not square to each other, all the reference points will be off and the bulkhead will be asymmetrical. Asymmetrical bulkheads would result in an asymmetrical boat - undesirable for both hydrodynamics and aesthetics! Drawing the CL and DWL perpendicular to within 1/16” over their length (roughly eight feet on Burnett's bulkheads) is challenging. We avoided using what seemed like a more elegant but time-consuming solution, trigonometry, by purchasing an exceptionally straight, twenty-foot-long aluminum square tube. $70 well spent, as we were able to clamp it down to the plywood wherever we wanted the design water line to be and use its sharp, straight edge to butt our square against and measure out all the points.
The top of the bulkheads are curved where they will support Burnett's cambered deck. To draw the curved lines between the points that define them, we used a batten. In boat building, a batten is a strip of a material that bends evenly to produce smooth, uniform curves. Battens are made from a variety of materials (wood, plastic, metal) and are needed in a range of stiffness to accommodate different curves (stiffer batten for a gradual curve, more flexible batten for a sharp curve). After locating the points using the dimensioned drawings, we drove nails into them and bent a wooden batten around the nails, then traced the curve.
Cutting out Bulkheads
After double- and triple-checking our measurements it was time to cut out the bulkheads, which is the easy step. A skill saw equipped with a sharp blade is the tool for the job; it can be plunged into the plywood so that the offcuts (scraps) are as large as possible, and with the blade depth set just deeper than the thickness of the plywood (3/4"), it has no issues cutting the gentle curve for the cambered deck.
Plunging into cuts results in the largest offcuts possible. The bigger the offcut, the more likely it is to become a part of the boat down the road.
In addition to her twelve athwartship (side to side) bulkheads, Burnett has two longitudinal bulkheads that add some of the support needed to form her fantail stern (more on her curvy rear end to come). These longitudinal bulkheads are oriented perpendicular to the athwartship bulkheads and have slots cut into them so that they interlock. This will be easier to visualize in a future blog post about standing the bulkheads up on the building frame. The long-awaited assembly stage!
Cutting slots in a longitudinal bulkhead where it will interlock with athwartship bulkheads.
