Interior structural timber • Tobias Lunt

Next set of structural boat work: frames, hanging knees, and floor timbers. These are pieces that tie everything together — frames tie planks to each other, floors tie planks to the backbone, and hanging knees tie the deck to the hull.

This post covers the interior structural work in Ariadne’s cockpit area. We fabricated a new set of laminated frames, hanging knees to support the side decks, and cockpit floor timbers that had to be fitted around existing keel bolts with their tops co-planar to carry the cockpit sole.

Laminated frames

We’d laminated frames before for the forward sections, so the process was familiar. Thin strips of white oak are glued up over a form with epoxy, clamped until cured, then removed and beveled to fit. The form is a critical piece — it defines the hull shape at each station, and the shape differs for every frame. By screwing blocks to a plywood strongback with all station lines marked, we could create form shapes as needed and glue up the frame blanks in batches.

Lamination form with station lines — The lamination form, with station lines and curve offsets drawn directly on the plywood. Each frame gets laminated over this form, which defines the hull shape at that station.

Frame lamination clamped on form — Two frame blanks clamped up on the form. The black plastic keeps the epoxy from bonding to the plywood. You can never have too many clamps for this! Voids between laminations eventually lead to frame failures.

Laminated frames on forms — Bonding the laminated curves to the body of the hanging knees. The next step in the installation process is beveling the outside edges to match the hull camber.

Picking up bevels

With laminted frames like these, the outside faces also had to be beveled to sit flush against the inside of the planking. The hull is a compound curve, so the bevel angle changes continuously along the length of each frame. Getting this wrong means a frame that only contacts the planking at a few high points instead of bearing evenly along its full length.

WoodenBoat article on beveling frames — From WoodenBoat. A string stretched from the underside of the deck, in plane with the frame station, can be used to pick up bevel angles when stretched to the inside faces of the planking where the frame will eventually go.

Beveling laminated frames in the shop — Cleaning up frames in the shop. A power planer does the rough work, taking the frame blank down to the right dimensions. Final fitting is done after taking bevels in the boat.

Hanging knees

Hanging knees are the brackets that connect the deck beams to the hull sides. They transfer loads between the deck and the hull structure — everything from the weight of someone standing on deck to the shock of slamming into a wave to the torsional loads produced by heeling and righting moment. A knee that’s too small or poorly fitted is a structural weak point.

In the old days (and to some extent now, when timber is available) boatbuilders would saw knees out of “grown crooks” - brace roots or branches with a sharp 90 degree curve that matches the hull to deck joint. You need to be careful to minimize grain runout so the knee doesn’t split at the fasteners. We laminated two pieces of oak with perpendicular grain, then finished up with a laminated “ring” around the exterior of the knee.

Door skin template for hanging knee 33P — A thin plywood template ("door skin") for hanging knee 33P. The template is scribed to the hull shape and notched around the sheer clamp, then used to transfer the shape to the oak stock. Each knee is a custom piece — there's no way to batch these out.

Hanging knee installed — A new hanging knee in place. The vertical face is fastened to the hull planking and frames; the horizontal face bolts to the underside of the deck.

Hanging knee wider view — A wider view of the same area. The knee ties the deck beam to the hull side, keeping the deck-to-hull joint rigid.

Hanging knee and frames inside hull — Looking forward in the interior of the house. Two lighter-colored hanging knees are visible nearest, with new laminated ring frames visible further forward.

Cockpit floor timbers

Floor timbers run athwartships across the top of the keel, tying the two halves of the hull together at the garboards. In the cockpit, they also serve as the structural support for the cockpit sole, which means their top faces have to be perfectly co-planar. Any error would show up as a rocking or uneven sole.

The complication in this location is that each floor timber had to be fitted around existing keel bolts and their welded-on nuts that protruded through the top of the keel. Some floors are built in halves and glued up around the bolts with G-flex epoxy, with horizontal channels cut to clear the nuts. Every one is a custom piece: scribed to the hull shape on the bottom, beveled on the faces that bear against the planking, and leveled across the top.

Floor timber glue-up around keel bolt — A floor timber being glued up in halves with G-flex epoxy. The timber is built in two pieces to fit around an existing keel bolt with a welded-on nut.

Cockpit floor timbers installed — Floor timbers installed in the cockpit. The tops of each timber are co-planar — this is the surface that will carry the cockpit sole.

Wider view of floor timbers and frames — A wider view showing the floor timbers, which we epoxied to new laminated frames running up the topsides. The structural skeleton of the cockpit is now complete.

None of this work will be visible when the boat is finished. The floor timbers will be hidden under the cockpit sole, the knees deep under the cabin house, the frames behind joinery. But it’s the structure that everything else depends on.