I did some stress analysis on a 24" bracket with (importantly) a center rib in the flotation volume.
The center rib helps a lot.
The loading is for 5 Gs. (not 10) Plus full thrust from a 100 hp outboard. Something sort of like the loading you might see landing after skipping off the top of a wave.
The deformed shape is magnified 50 fold so you can see what happens.
A rib that goes from the real transom to about halfway between the lower two outboard mounting holes seems to be an important structural feature.
Yield strength for this aluminum is ~180 MPa, but repeated stresses above 50 MPa would be a bad thing, in very general terms. I expect the loading from the outboard holes to be much lower, as the stress is distributed along the casting, not just inside the bolt holes.
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