Again, the materials used in late-model "Katie Marie" prove to be different than that used in earlier boats...
Of course, she had no diagonal braces, so I am sure that that had an effect on what I see with her OEM strongback. Although it is *not* rotted, it *has* cracked (see pics, cracks are evident). Without diagonals, it was under more stress, I'm sure. Additionally, it is a solid 1/2" less in the vertical dimension than what ebb measured on his strongback (3.75" vs ebb's 4.25"), and only 1 3/4" wide (1" less than ebb's!) - so it is a significantly smaller beam. Yet she still sailed on for 40 years almost...
One thing I noticed, and mentioned before IIRC - the flexation (is that a word? it is now...) of the side decks, forward of the bulkhead, which resulted in some cracking of the inner deck skin there. I see no evidence of this cracking on the after side of the bulkhead. It is because of this evidence of force transmittal that I decided to go with the wider beam. I think it was point loading which was responsible for the cracking - the deck was trying to bend around the bulkhead. (!!!) So, to clarify what I think may be a misconception because I didn't clearly state it before: The width of my upcoming beam is only in the smaller part an attempt to make up for its reduced thickness; primarily, in my addled mind, the increased width is to spread loads across the structure while stiffening it to alleviate/cancel any torsional loads. Do dat make sense?
By way of explanation: I know that the beam will flex downward, even if only incrementally, when the boat is under strain of full sail and pounding into seas. Said bending will transfer the load out to the cabin trunk sides. Visualization: hold your hands, halfway cupped, fingertips touching at the top of a small arc, to simulate the arch shape of the inside of the cabin trunk. The mast sits on your fingertips. Imagine it pushing down, simulate it with your hands, you can see that it transfers that force out to the sides. End result: the cracking I see. A wide beam will spread that load across more surface area, which translates into more strength, and less of a load at any one point. Right? It will serve in a similar manner to do what the diagonals Katie never had would have done, had they been there.
*That's* why I want the width. Spreading the love...
I'm not counting on the entirety of the strongback to support the mast, just a small part of it. I'll be supporting the mast in the vertical with 2 metal poles I will put roughly in place of the former doorway frame. The poles will contact the strongback beam and cabin sole on wide bases. Up top, under the mast, center-to-center of these bases will only be about 16", so the span which needs to be strong enough to resist the crushing force of the mast will be unsupported for less than 14".
I am still designing it in my mind, haven't yet settled on that which feels "right". I've been thinking 2" thick on the beam because I think that done properly it will be strong enough, as well as a being visually nice. I haven't ruled out a thicker spot in the beam under this area, tapered into the rest of the beam for visual flow and load-spreading. I suppose I also could go with a metal plate in this span area, and go even thinner on the beam, since it will be poles doing most of the work.
Last, I'd assemble the beam of vertically-aligned plywood pieces with resin *and* glass cloth between the layers of ply, like in my "boxes" drawing, not just resin or glue. By using plywood instead of foam for the core material, I think it should add strength/stiffness.
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