I would describe myself as a diligent J 109 owner when it comes to maintaining my boat. That said, I rely on a professional boat yard to make sure that all of the mechanical issues on the boat are addressed - including complete maintenance in and out of season. I also use a diver to make sure that the bottom stays clean - report any issues - and change the zincs.
I have hull #250 - I bought new in 2005.
Last spring it was discovered that the inside of my sail drive was completed corroded and the drive needed to be replaced. I know anything below the waterline is not meant to last forever - but I go out of my way to make sure that everything is checked over closely and a continuous basis. I even dropped a zinc grouper overboard for safety sake.
What I did not know - is that for at least over five years or more - the zinc anode located at the the top of the saildrive (not the one by the folding prop) had not been changed. When I discovered in a later SD-20 manual that there was an anode at the top of the drive that needed to be changed while the boat was out of the water - I was told that my older version of the SD-20 only had one anode near the prop.
Thanks to the miracle of the internet, I was of course able to locate the old manual for the original SD-20 installed on my boat. Not surprisingly, there was also another zinc anode located on top of the old drive.
Whether or not who this would have completely protected the internal portion of the drive - who knows. But it was frustrating to know that this had gone unchecked by professional Yanmar mechanics for years.
Needless to say replacing the drive was not an inexpensive exercise.
This is for those of you, who like me, are not that mechanically inclined and are reliant on making sure the yard covers the boat from stem to stern every winter - and maintained during the summer.
The lower split zinc is replaced every year. For added protection at the dock (I'm at a slip with shore power) I also used one of these Martyr "Grouper" Zincs clipped to the port side stern rail. When the boat was on the hard I verified a low resistance connection from that to the prop to make sure it provides protection. The ground system has connections to the stainless on the aft section, and is also shown in the owners manual.
ps: most of the manuals for the J/109 including the sail drive are on the class website. On the menu mouse over "Info" then click on "Drawings & Manuals".
You may also want to check for leakage currents both AC and DC to determine if there is an electrical path causing stray currents eating your sail drive.