Crossbeams, little by little

Trimarans are finicky to build. There really aren't too many ways to simplify the highly loaded crossbeam structure, particularly when (as in our case) it has to be able to fold for trailering.

Our Starwind 860's aft crossbeams are now sitting partially assembled in the garage. The webs, the strut blocks and the centre blocks are in place, and it is now time to start closing them up.

The highest load on a beam like this occurs at the support strut attachment near the middle of the beam. Towards the outrigger and the centreline, the stresses decrease dramatically.

At the strut attachment block, therefore, we add a substantial structure to spread the loads evenly throughout this part of the box beam and prevent a stress concentration that would cause the beam to break. The first part of this structure is a set of sawn cedar frames to spread the loads from the strut block into the box beam walls.

The oscillating saw comes in handy for trimming everything flush at the right height.

Up next is a series of thin cedar planks, laminated together with epoxy and bent to shape. All of this cedar, taken as a whole, is tapered to match the expected stress on the beam; it's thickest (and strongest) at the strut block and can safely be thinned down in the lower stress regions.

Next, we laminate four layers of 6 mm marine plywood to form the bottom flange of the box beam. This is one area where it does not pay to skimp on epoxy.

With the lamination complete, everything gets screwed together to hold it in position while the epoxy cures. Later, we'll remove as many of the screws as possible, as they serve no long-term purpose.

Once the beams are completely boxed in on top and bottom, the edges will be rounded off and taped with fibreglass. A final coat of fairing compound for cosmetics, plus a few finishing touches, and they'll be ready to paint.

We have another pair of these crossbeams to make for the front, and then we'll move on to other parts of the boat.



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