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Re: Propeller shaft, coupler and struts
Bronze contains about 15% zinc. When the piece suffers from electrolytic corrosion (either galvanic or stray current) the zinc dissolves leaving the bronze with a pinkish hue and visible pitting on surface. A strut in this condition is seriously weakened. I have seen a strut break under power and stay on the shaft getting whipped around 800 (shaft) RPM and digging a hole in the hull before the shaft fell out.
Not enough information provided to say if the cause is galvanic or stray current but it is certainly not "electrolysis" which is a commonly misused term by those who do not understand electrolytic corrosion. Galvanic corrosion is a fairly slow process while stray current corrosion can be a shockingly (pun) fast process. If stray current is the culprit you may have an electrically unsafe boat.
Can't tell from the photo but your propeller must also be suspect. Very few propellers are bronze, most are actually manganese bronze which is actually a brass which contains around 35% zinc. If the strut is compromised the propeller is in likely worse condition.
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Last edited by boatpoker; 07-15-2014 at 06:33 AM.