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It turns out that some aluminum alloys have about the same shear strength as
nylon bolts.
It appears that a 5/16 bolt is easy to get through the hole for the keel
lock down on the P-19.
An aluminum dowel/rod of 5/16 diameter used as a shear pin to yield at 1000
lbs of shear would have to have a shear rating of 13,000 psi. This yield is
2.5x the weight of the keel, and less than the weight of the boat.
The following alloys have the required shear:
Aluminum alloy 6061-0 shear of 12,000 psi
Aluminum alloy 6063-T1 shear of 14,000 psi.
Buy aluminum rod of this diameter from local metal supply (these alloys are
common). Cut pieces to appropriate lengths allowing for holes for small
cotter/retaining pins on ends. The pins hold the aluminum dowel in place in
the keel trunk. The pins go on the outside of the keel trunk, where the
dowel sticks through it.
I think the aluminum is a better way to go over nylon because it is more
controlled. The variability of nylon bolts is a bit scary. Also aluminum is
easier to get than special nylon bolts. Aluminum should be cheaper too.
Make sure you get the proper alloy because some alloys of aluminum are super
strong. The dowels are better than bolts because the threads on a bolt put
additional stress on the bolt, so the bolt may yield at a lower weight. The
dowel has less uncertainty.
Jim Nolan P-19 #426 Panache
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