The stray a/c can be difficult to find.
Did the propeller/shaft have anodes when you purchased the boat?
Your propeller installation probably has one (or more) sacrificial zinc anodes. Make sure that you keep an eye on their decay once you have installed the new prop. This may require a dive once a month or so. If the a/c near your dock is suspected to be high - move the boat to anchorage before diving on the prop/shaft. If the decay rate is such that the anode(s) will be less than 40-50 percent before your next scheduled haul, you will have to move the boat, or find the stray current. Talk to the marina operator - they may be helpful.
Propeller replacement is expensive enough that if the decay continues at a rapid pace, you should hire a marine electrical technician to help you find the source, or at least verify that it is not coming from your own equipment.
Last edited by wwilson; 03-06-2006 at 11:37 AM.