by which I mean very incomplete engineering (even if they turn out to be strong enough). As you say, the actual stresses in the bolts is dependent more on the stiffness of the materials and sections involved, and none of this is typically considered in boat design. An extreme fin can be very strong - but no good structural engineer would design one as pictured if given a choice ( I note that most yacht designers are not good structural engineers). The better ones have the fin fitted into a socket in the hull, which allows spreading the load over a larger area than just the 6 inches or so pictured. Keels like this don't fall off very often, so the guesswork and safety factors are frequently adequate. But they do fall off sometimes, and for me "frequently adequate" is an insufficient standard for the keel on an offshore boat. Additional engineering need not be costly in construction. Simply flaring the top of the keel section by a couple of inches would have no measurable impact on the performance of the boat, while reducing the stresses by a large percentage.