...if underwater components are not galvanically identical, bonding them completes a circuit and causes corrosion where otherwise there would be none...
Any discussion about galvanic or stray current corrosion requires some genuine adherence to correct terminology or we will get lost
in the woods.
There are three legs on the galvanic corrosion milk stool:
1. Dissimilar metal components,
2. Submerged in the same electrolyte,
3. Electrically connected.
By the way, and for the record
is defined by the Merriam-Webster Dictionary as: "a. The producing of chemical changes by passage
of an electric
current through an electrolyte." Electrolysis - Definition and More from the Free Merriam-Webster Dictionary Electrolysis
is a misnomer when applied to galvanic corrosion as electrolysis
taking part in the galvanic corrosion process.
For the sake of this discussion, I am going to use a portion of the ABYC's E-2 description of a cathodic protection bonding system: A green insulated, tinned, stranded conductor of at least AWG 8 connected to each underwater metal component and with a resistance to the sacrificial anode of <= 1 ohm. (There are other conditions and caveats, but this is the simplest working definition.)
Now that we have that out of the way...From a galvanic corrosion standpoint, unbonded underwater fittings manufactured of high quality bronze will literally out last most owners. If these same fittings are bonded, as defined above, then they will be electrically at the same potential, and galvanically, less negative (more noble) than the anode that is the third component of this system.
..removing the bonding wires could be allowing your zincs to corrode now where before the thru-hulls were corroding instead.
As stated, this is fundamentally impossible. The galvanic series precludes the "...thru-hulls were corroding instead." from happening. And by removing the bonding wire, the OP has removed one of the three conditions for galvanic corrosion to occur.
Two more factoids:
Galvanic corrosion is a slow process.
Stray current (DC) is a very, very rapid process.
The OP now has a cathodic protection system comprised of three zinc anodes, a bronze composition propeller
, a stainless steel
shaft, and an electrolyte. His anodes have wasted twice as fast this season than last. Same mooring, etc. The existing problem is within that system or a "sneaker" that is attached to the system that wasn't before.
When the OP found a less than optimum connection of his wasting hull
fitting to the cathodic protection system, he could have stopped the wastage by providing the <= 1 ohm connection of that thru hull
to the CP system.
I will not venture to try and troubleshoot what the root cause of this problem is. Corrosion issues can only be solved
by understanding the processes involved and crawling around the vessel with a very good voltmeter and a Ag-Ag Cl half cell. So, once again, I agree with Wayne: hire a professional.
Hope this helps.