I've got a 17½' Alumaweld Stryker with a Merc 65 Jet. I've used it in the salt a few times, then dipped it in a freshwater lake to flush the salt out.

What I've got happening is little pits in the hull that I'm sure is caused by electrolysis. I've disconnected all the ground wires from the battery and then reconnected them one by one checking for a ground between the hull and positive post. I found that the main ground wire going to the Merc is grounding to the hull. Is it possible to isolate the motor ground from the hull? What kind of options do I have?

Also, how can I add more zinc? I've only got one piece affixed to the lower unit. I saw on another BB about welding directly to the hull. Is that the most recommended method? Is there a formula, or rule of thumb, for the amount of zinc one should have?

Thanks.