SPLINING PLYWOOD FOR BULKHEADS
May 15, 2020
Why do we need bulkheads?
At 39 feet long, Burnett will neither be a big boat nor a small boat compared to the average cruising vessel. A 39-foot boat is still a very small living space for two people, however. With family and friends aboard to share adventures, the space will feel even smaller. Accommodations will be similar in size to a tiny home, but in addition to people, those accommodations will have to house enough food, fluids (water and diesel), and equipment to be self-sustaining for months. On top of that, Spencer and I have a penchant for Hawkins cheesies and Old Style Pilsner, neither of which are particularly compact items. The challenge of limited space on cruising boats has fostered creative solutions from boat designers over the years, and the result is that several components of a boat serve more than one purpose. That is the case for the bulkheads in Burnett.
Bulkheads, which are vertical walls that can run athwartship (side to side) or fore and aft (bow to stern) in the boat, are necessary structural components of a stitch-and-glue boat the size of Burnett; without them the hull would not have the strength or stiffness required for its intended use. But in addition to their structural function, our clever boat designer (Sam Devlin) has strategically located the twelve bulkheads in Burnett so that they also compartmentalize the boat in a desirable way. For instance, our private cabins are sectioned off from the rest of the boat by bulkheads, and one of those same bulkheads that defines the cabins acts as the aft (back) wall of the pilothouse. Space is therefore saved because, due to their structural necessity, all the walls within the vessel that divide it into its distinct functional areas would be there regardless.
Herbert's forward-most bulkhead makes the bow of the boat strong and rigid enough to punch into an oncoming sea, but also doubles as the aft wall of a dry storage locker.
Bulkheads in a stitch-and-glue boat serve the same structural purpose as the more densely-spaced frames of a traditional wooden boat (see photos below). The absence of these frames in a stitch-and-glue design is one of the factors that make them quicker to build.
This traditional lapstrake boat does not have any bulkheads but has oak frames inside of its cedar planking.
Unlike the hull panels, which are bent into curves, the bulkheads are straight and can therefore be constructed out of thicker plywood (¾"). The thicker plywood is stronger and, because of its stiffness, easier to keep true when lining up the bulkheads on the building frame. Burnett will have a beam (maximum width) of 13 feet, so we needed to join multiple sheets of plywood edge-to-edge to get large enough panels on which we could "loft" (we'll get there) the bulkheads and cut them out of. Because the bulkheads don't need to bend, we could use the quicker and easier spline joint instead of scarf joints to join the plywood. In a spline joint, slots are cut into the edges of the two pieces to be joined, and a third piece (the spline) is inserted into the the slots and glued in place.
Cutting Slots for Splines
We used a router to cut slots in the edges of the plywood sheets we wanted to join. The router bit we used has a bearing that rides along the edge of the plywood to keep the depth of the slot consistent.
Gluing Spline Joints
We used epoxy to glue the splines in place. All surfaces of the slots and the splines were coated with unthickened epoxy before assembling the joints.
Our scarfing table worked well as a flat surface on which to assemble the joints. We splined three pieces of plywood together and used pipe clamps to pull the joints tight before holding them in place by nailing them to the table. The pipe clamps were then removed and another three sheets of plywood joined on top of the previous sheets. We put wax paper in between each set of three to prevent it from being glued to the sets above and below it. Our first attempt at spline joints is captured in the time-lapse video below. It turns out that time-lapse can make a hellish nightmare look like a fairly smooth operation. We made several mistakes on this attempt, one of which was cutting our splines ever-so-slightly too thick so that an excessive amount of clamping pressure was required to snug up the joints. Another mistake was, in what seemed like a way of saving time, applying epoxy to too many splines and sheets at once. We therefore didn't have the option to reduce the thickness of our splines on the fly (they were covered in wet epoxy), and were in a race against time to get everything joined before the epoxy started to set. We made the splines ever-so-slightly thinner for our second attempt at gluing, and with classical music playing in the shop, had a relaxing glue session that resulted in a superior product in comparison to the first go-around. What a difference a thirty-second of an inch can make!
Cross section through a finished spline joint.
As seen in the slow-motion video below, our spline joints are a point of weakness in the plywood when it is bent. But that's okay with us because when we need the bulkheads to be strong, they will be being compressed, not bent. The bulkheads are oriented approximately perpendicular to the hull panels. Burnett will be most stressed when she is crashing over and through big waves, or when the outside of the hull is impacted by solid objects (coral, rocks, ice, flotsam and jetsam, deadheads, other vessels, etc). All of these stressors push in from the outside of the hull, putting a compressive load on the bulkheads. The spline joints will do a fine job of keeping the pieces of plywood that make up each bulkhead aligned, and are as strong as the rest of the plywood when under compression.