Those bolts don't actually shear, but break by bending to and fro due to engine torque variations. The threaded bolt part that protrudes from the prop drive washer is very prone to this failure mode.

First, use high tensile steel bolts. Any old bolt just won't work.
Second, drill them with as little play as possible; drill from the prop back side
Third, tighten the bolts very carefully, untul all is seated well, without crushing the wood.
Fourth, check and tighten at regular intervals.