Crossbeam details

The crossbeams of the Starwind 860 are its most complicated parts, from a construction standpoint. Today I'll show you a few internal details from the aft pair.

The two aft crossbeams, joined by the centreline hinge, span 4.6 metres. At the moment, we're still sharing a single-car garage with an existing boat, so there isn't much room to lay them out.

The fore and aft faces of the crossbeam, the webs, are laminated from plywood to a thickness of 25 mm. A complete box section is formed once the top and bottom flanges are in place. The box beam shape is a fantastically efficient structure for the combination of loads that these crossbeams will see, and is relatively straightforward (if a bit tedious) to build.

The crossbeam is loaded at the end, by the outrigger, and is supported by pin joints at two places: the centreline, and the strut (which attaches just below the kink in the middle of the beam). The forces in a beam configured like this are highest at the strut attachment, and fall off dramatically as we move away from that point. We can therefore use a lighter section near the outrigger, and reinforce the middle of the beam to handle the greater loads there.

In the photograph at left, you can see some of the extra wood that's been added along the direction of the crossbeam at the high-stress point. The total cross-section of all the wood, combined, has been tapered to approximately match the expected loads on this part of the beam. It looks a bit rough, but that's OK – this area will be completely sealed off; all that really matters is that the right amount of wood is in there and it's all laminated together securely.

The odd-shaped block of cedar in the next two photographs is another such reinforcement; it will handle some of the stresses in the top flange of the beam near the kink.

Above that block comes another layer of wood strips in the high-stress area. Here, they're being bent to shape with hot water; it's much easier than steaming them for this relatively gentle curve. By the time I have time to work on it again, they'll have taken enough of a set, and dried enough, to be epoxied in place.

The middle compartment of the crossbeam will be sealed airtight when the top flange is glued on. To allow for the wiring runs that will eventually be routed to the outriggers, we've embedded these 50 mm conduits in the crossbeams. Wires from the main hull will run through a flexible conduit beside the strut, then enter the crossbeam and pass through this conduit to the outrigger. An access hatch in the end of the crossbeam will let us get inside to fish the wires and to work on the outrigger's junction boxes.

The centreline joint still needs a bit of cleaning up, but we can now confirm that it's properly aligned and rotates as it should.



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