The next logical step after bulkhead 3 was to do bulkhead 4. I deviated from plan again, making it from a single piece of 9mm ply that goes the full width of the boat. I wanted the area around the front of the centerboard case and back of the mast step as strong as possible. To do that I knocked up a couple of doublers for either side of the spine between bulkheads 3 and 4, and added tabs to the front of them that pass through bulkhead 4 and into the front of the centerboard case. This will stop the centerboard case pivoting with respect to the mast step.
Here's the detail of the join. It holds together nicely even without epoxy. I'm hoping once it's epoxied and filleted it'll be nice and strong. I'm thinking perhaps adding some unidirectional carbon tape along bulkhead 4 once things are together could also be helpful, as bulkhead 4 transfers loads from the shrouds to the centerboard and mast step, so we want it to be as stiff and strong as possible.
I cut the side pieces for the centerboard case from 6mm ply. The rest of the centerboard case will be made from "Tasmanisn oak", which I'm told is neither Tasmanian nor oak. Indeed I believe it's also known as Victorian alpine ash, which is Victorian but isn't ash, being a eucalyptus. Anyway, it's a good middle-of-the road hardwood. reasonably strong and durable, but without the weight of jarrah, and with nice straight grain. It's readily available locally in lengths to 3m. I'm thinking this will be my go-to hardwood for the boat. I'll use it for stringers, keel plank, keelson, etc.
So here's what the spine is looking like now, assembled as best I can without actually gluing anything together (I'm waiting on my first shipment of epoxy - what is it with boat supply places taking forever to fill orders?). The astute can see my first go at bulkhead 3 leaning against the wall in the background, and the rapidly growing pile of offcuts.