Frame Joinery Redux

Although most of the problems with hull structure have already been solved, there remained one problem that stood in the way of completing the design: how to join together the frame. It consists of 4x4 softwood (fir) timbers (3.5x3.5 finished size) combined into a box structure that reinforces the bottom the deck, the bow and the transom and provides support for mast steps. After working out a design that included a dozen different steel brackets that had to be custom-fabricated at considerable expense, I realized that I don’t like it at all: too complicated and too expensive. And so, as usual, I sat back and waited for some new ideas to filter in from the ether.

Eventually this happened. An unrelated project required me to build a rectangular frame by joining together some square cross-section sticks using L-brackets. To avoid the wood at the ends of the sticks splitting as I drove in the screws, I wrapped the ends using several turns of fiberglass packing tape. It worked just fine. The tape took up all the force that would have gone into splitting the sticks along the grain, and the resulting joins were impressively strong.

Transferred to Quidnon’s frame design, this technique will make it possible to assemble the frame using just 3-inch-wide perforated steel strips cut to two or three different lengths and bent to various angles. Some of the brackets will need to be bent to specific angles other than 90º, but this is not a complicated procedure.

The procedure for fabricating the frame now consists of the following steps:

1. Using a chop saw cut 8-foot 4x4 timbers to required lengths.
2. For each timber, shave down the first 6 inches off each end on all four sides using a planer.
3. For each timber, router off the corners on the first 6 inches on all four sides using a router.
4. Roll on a layer of epoxy to the prepared ends, wait until it “tacks up.”
5. Wrap each end in three layers of 6-inch-wide fibergass tape.
6. Saturate the tape with epoxy; let it cure.

Frame assembly then consists of matching up the timbers and the brackets and connecting them together by driving in a lot of self-tapping screws using a cordless drill. There is no need to worry about the wood splitting, and the resulting joints are strengthened by the fact that they are pre-stressed: the wood is compressed between the screws and the fiberglass. In a humid marine environment the timbers will gradually absorb moisture and swell, increasing the pressure on the fiberglass and the screws, holding them in place securely. (The choice of softwood for the timbers is critical: when hardwood swells, it generates enough force to burst fiberglass.) The strength of the joint is determined by the force needed to crush the wood fibers, which is somewhere around 6 times greater than the force it takes to split them along the grain.

There are still several more complicated pieces that will need to be fabricated: engine bracket, mast tabernacles, masthead fittings, tiller and keelboard hardware and bow rollers. These are all key elements of the design and there is no way to simplify them. But the frame joinery is now very well in hand and can be done cheaply using components that can be locally sourced in many places around the world.