my boat is slipped 3-4 months a year in FW. With magnesium anodes, and properly functioning Mercathodes...I have all but eliminated corrosion.

One problem area remains. The top caps. With all that stainless high on the drive...the electrolitic effect is substantial, and the majority of sacraficial anode (magnesium in this case...FW) is at the cavitation plate level or below.

tired of this, I knew I needed anode to sacrafice up high in proximity to all that stainless steel bolted to the aluminum case...

after alot of thought and looking at available magnesium anode shapes and sizes...
and a good look at a Bravo top cap...
I found this mag anode for OMC/Johnson/Evinrude
Canada Metal #CM123009M
West Marine #1258623

comes with 2 threaded holes. Drilled them out, put a little chamfer...drilled and tapped 10-32 holes into the "void" area at the back of the Bravo top cap. Used allen head s/s cap screws and s/s lock washers. put a little pipe dope on the threads for the hell of it.

both sides of both caps...now there's something up there to sacrafice to electrolosis other than the caps themselves.

check it out. Everything is a mess from the Corsa muffler install ect. forgive me...

I have an article saved that touches upon this. Ran across this while doing a search for a buddy that was considering one of those anode fish to hang in the water while in his slip. Not sure if true or not.

A cut and paste from that article:

There is a common misconception that you can overprotect your drive by using
too many zinc or sacrificial aluminum anodes. This is not true. The
corrosion potential of any metal is a voltage that can be measured by a
reference electrode. Such measurements in water commonly are made with a
silver/silver chloride reference electrode. The corrosion potential of a
sacrificial anode is a characteristic value for that metal, and it does not
matter if you have one piece of the metal or 100 pieces. The corrosion
potential stays the same. Of course, 100 anodes would be expensive, heavy,
and a considerable drag under water. Only by increasing the corrosion
potential by using a different anode material (such as magnesium in
seawater) can you overprotect your drive.

Manesium is only for freshwater--zinc for salt. Otherwise very accurate ond necessary--critical in fact. I lost pair of outdrives because a homemade boat next to me was "hot"--his 12V cables were hanging in water.