BREASTHOOKS AND SHEER CLAMP INSTALL
March, 2021
The carefully positioned bulkheads on the building frame gave us most of the control points we needed to guide the hull panels into their correct shape when stitching them together. Burnett’s bulkheads, however, are spaced two to four feet apart, and the edges of the sheer panels (the highest hull panels above the water) need continuous support from bow to stern to establish smooth, visually appealing curves. That’s where those sheer clamps that we scarfed up near the beginning of the project come in. The sheer clamps are a lamination of two ¾” by 2 ½” pieces of Douglas fir that are bent around the bulkheads and glued in-place, with their position determined by the notches we cut into the bulkheads before standing them up.
Our hardest working employee, sealing the end grain of the plywood where a notch was cut into the bulkhead to receive the starboard-side intermediate sheer clamp.
Breasthooks
The port and starboard sheer clamps get tied together at the bow with breasthooks. A breasthook is a triangular piece fit into the bow to buttress the hull panels, giving them a fighting chance in a head on collision and adding strength and stiffness for beating into an oncoming sea. Burnett’s breasthooks are a lamination of four ¾” pieces of marine plywood, and have notches cut into them to establish a strong joint with the sheer clamps.
Burnett’s two breasthooks, ready to receive the sheer clamps and strengthen the bow of the boat.
Why Laminate?
The sheer clamps are a lamination of two pieces of wood instead of one solid piece because a solid piece would not have been flexible enough to bend into the curve we needed. One can make a thicker piece more flexible by steaming it, but a piece of wood loses some of its strength when steam-bent. All else the same, our 1 ½” by 2 ½” laminated sheer clamps are stronger than a steam-bent sheer clamp of the same size and, not needing to build a 40’ steam box, were easier to construct.
Dry-fitting a shear clamp to check that everything is as it should be prior to getting things covered in glue.
Sealing the Wood
In a stitch-and-glue boat where most of the components are held together by seams of fiberglass set in epoxy, the goal is to seal up all wood with epoxy so that it never sees any water. Adding water to wood causes swelling, which would put stress on the joints holding everything together. In addition, water is an ingredient in the recipe for rot, so keeping all of Burnett’s wood dry will prevent it from becoming a host to microorganisms that would degrade its structural properties. We therefore sealed any faces of wood that would become inaccessible after the sheer clamps were installed by painting them with epoxy. This was also an opportunity to incorporate a glue joint between the pieces, which meant the addition of some wood-flour-thickened epoxy on top of the unthickened epoxy once it soaked into the wood.
Once sealed off with unthickened epoxy, thickened epoxy was applied to the surfaces that would contact the sheer clamps. The thickened epoxy acts as a bridge between pieces in our imperfect joinery.
Some Glue to Hold Things Together
The next step was to prepare the two pieces to be laminated and become each sheer clamp by rolling unthickened epoxy onto the faces that would be mated together. A layer of epoxy thickened with fumed silica was also added to one piece of each lamination to ensure the glue joints are continuous without any voids.
With the epoxy applied and the two pieces mated, this shear clamp is ready for installation.
The sheer clamps were held in place until the epoxy cured by a combination of permanent stainless-steel screws and temporary clamps.
The need to hold the two-piece lamination together as well as in the right spot fostered creative clamping strategies, like clamping clamps to clamps.
Short screws were used to suck the two pieces of fir together (screw to the left) and long screws used to fasten the sheer clamp to the bulkheads (location being piloted with the drill).
Even though the sheer clamps are a two-piece lamination, a lot of torque was required to bend their ends into the breasthooks. Where the sheer clamps sweep up and meet the breasthooks is where curvature and twist are greatest over their length.
This was the best we could do where the shear clamp meets the breasthook, which meant some sculpting of the wood before we could hang the sheer panel.
A fallen soldier. Every hair on the back of your neck stands straight up with the explosive energy release of a broken clamp; it happens so quickly that you aren’t sure what broke, the clamp or the piece you were clamping?
It’s true that you can never have too many clamps of different shapes and sizes when building a boat. I kick myself remembering times gone by when I have passed up a good-looking clamp at a garage sale, at those times thinking I already owned what seemed like a ridiculous number of clamps. Alas, the only clamps left hanging on the rack after the sheer clamp install were the daintiest of C-clamps.
Our armada of clamps, holding the sheer clamps in place until the epoxy cures.
Only two cute little C-clamps didn’t make it into the mix.
Rolling Bevels
After the epoxy cured, one step remained to get the sheer clamps ready for panel install. The intermediate sheer clamp, which is the one further off the floor in the photos, runs along the boat where the lower edge of the sheer panel abuts the edge of the panel below it (in a right-side-up vessel frame of reference). It supports the edges of both these panels and guides them into a nice fair curve. Being a hard chined boat (we’ll save chines for another post), these two panels come into this meeting point at different angles. We cut the notches in the bulkheads so that the outer face of the intermediate sheer clamps would be oriented parallel to the sheer panel. This meant they would need a rolling bevel cut into their lower halves (still in right-side-up vessel frame of reference) to be flush with the lower panel. A rolling bevel is one where the angle changes along its length. This was necessary for us because, unless you are building a really ugly boat, it will have compound curves (curved in multiple directions). For Burnett (not an ugly boat), the sheer panel and the hull panel below it are near-parallel to one another at the bow, but the sharpness of the angle between them increases aft (towards the stern) along the sheer clamp.
Towards the bow the rolling bevel is subtle. Further aft the angle between the sheer panel and the panel below it is sharper and more material needed to be removed for the bevel to be in-plane with the edges of bulkheads.
The goal was to get the face of the sheer clamp parallel to the bulkhead edge at each one, and to have a gradual transition along the sheer clamp between bulkheads.
A pencil line down the middle of the sheer clamp and frequent checks with a straight edge provided guidance.
Cutting a rolling bevel in clear, straight-grain Douglas fir with a sharp plane is truly a multi-sensory joy; a smooth sound comes off the blade as it cuts, and a pleasing aroma is released from the freshly exposed wood as you watch shavings curl up behind the tool. As the plane dulls, the experience degrades, and one is left doing the cost-benefit analysis of taking the time to stop and reinject the task with pleasure by sharpening the plane. Below you can see I have opted to forego sharpening (and therefore some pleasure) to achieve the end goal faster by removing my boat building shoes (Birkenstocks) to get the great grip afforded by slightly sweaty bare feet on a concrete slab. This allowed me to push harder on the plane. I then passed the dull tool off to the guy responsible for the rolling bevel on the other side of the boat (sorry Spence) and grabbed a beer to keep me company while admiring my work.
The long, thin curls of wood hanging from the sheer clamp were cut early in the process before the plane was dull.
With continuous, smooth curves running from the breasthooks at the bow down the sheer clamps and harpins to the stern, we were ready for the excitement of wrapping the hull panels around the inner framework of the boat and stitching them together.
Sweep up the shavings and we’re ready to install the hull panels!
