Jimbo,

That is not a stress crack but is a crack that developed through galvanic corrosion. This corrosion is caused by minute electrical currents setup by two disimilar metals that rob the 'anode' material of stucture at the molecular level...the aluminum in this case. It surprises me that I haven't seen any manufacturer put magnesium anodes on our aluminum parts. My monohull has anodes on it's mast as do most power boat motors and such.