The pictures below show asymmetric (galvanic) corrosion on the port trim tab vs. the starboard trim tab.
Poor DC ground connections between these two new Yamaha outboards is causing the Port engine to operate at a higher voltage potential than the Starboard engine. As a result, the Port zinc anode has deteriorated faster than the Starboard zinc anode.
The solution was to install a high-quality battery ground cable between the Port and Starboard engine’s starter batteries. This keeps both engines (i.e., propellers) at the exact same voltage potential and eliminates electrolytic plating/corrosion between these underwater metals.