Chain plate / bulkhead structure

[Tony Allport]

The boat is a 1969 Santana 37, it has seen better days but seems to benefit from the heavy layup typical of boats of that vintage. The chain plates have leaked where they penetrate the deck and the plywood bulkheads that they attach to are deteriorated in way of the tabbing that bonds the bulkheads to the hull. Three out of four bulkheads have been compromised. I typically consider this a serious failure but in this case there are also heavy diagonal cables tying the chain plates to the mast step. The mast step is a massive assembly of I-beams. My read on this structure is that the deck and bulkheads are acting more like spreaders in compression rather than anchoring points on the hull, and that the mast step is carrying the rigging load. This leaves me wondering if the dis-bonded bulkheads are less serious in this case.

Tony Allport

[Brion Toss]

Hello,
Great question. First, all bulkheads athwart the mast act as spreaders; it’s just that with the cables on this boat the vertical load component is largely removed from the bulkheads. But the bulkheads still have to deal with the consequences of horizontal loads, and this is where the tabbing comes in, keeping the hull/rig system intact, while the weather chainplates try to pull the bulkhead inboard (with some up, too), and the keel tries to pull the weather side of the hull down. Perhaps the deformation is less with the cables you mention; it was developed to allow for lighter hull structures without loss of stiffness. But the tabbing is far from decorative.
Overall, if the hull scantlings reflect the advantages of this configuration, I’d say the tabbing is just as significant as with a conventional setup.
Fair leads,
Brion Toss