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Originally Posted by oab_au
That 6 cylinder crank, slows a classic harmonic fatigue break. Middle of the crank, starting at the oil hole drilling, to spread out to the webs. I guess it would have had the flywheel/damper combination replaced with the drive gear.
In a boat the engine runs at a constant speed, that in this case, looks like it was right on the cranks undamped resonance.
Have they checked the other engines crank? 
Harvey.  |
This is most certainly not a classic harmonic fatigue break. This type of failure involves torsional forces, which usually cause a break at, or close to a main journal. Alternatively, If the break is in a web, it will occur close to the crankshaft centre line.
In this case we have an off centre failure, which would indicate flexing due to an out of balance component, rather than simple torsional vibrations. There is no indication that the crack has started at the oil hole, but rather that it commenced at the inner edge of the bearing surface, where there is a sudden difference in structure. The outer looks torn away as the last part to break off, and shows a sudden occurrence rather than fatigue. Overall this evidence does not indicate a torsional component in the forces involved.