..., or not to zinc.
Use Sealloy bars from Marinette Yachts (http://www.marinette.com/
Here, upriver on the Potomac, the water is fresh.
At Colonial Beach, perhaps a bit of salt, no?
In any case, sealloy is actually lo-grade aluminum: just "noble" enuff to sacrifice itself to the hull; especially in region where the Nibral props & low-grade Iron rudders play a role.
See them here: http://www.marinette.com/rparts08a.htm
Bolted them to the "keel" with SS fasteners in that area, and up front where something strange was happening.
Then, cut small sections and atttached them to the rudders & trim tabs as well. Really work great.
Recommend SS rudders if the wallet allows (my next upgrade): otherwise, go to http://www.chrisparts.com/rudders_struts_etc.htm
- ask for Jim Wyck -- and go from there.
Marinesalesparts.com is also a good (expensive) choice.
Hope all goes well.
PS - Also know a very (very) good mobile welder who redid my rudder posts; if you'd like to know just private message me.