As soon as your boat is placed into any saltwater environment, electrolysis and galvanic corrosion will take effect instantly. 

If not addressed it can corrode Propellers, Rudders, Shafts, Prop Nuts, Trim Tabs, and any other metal part.

Zincs are installed on every saltwater vessel because they will sacrifice themselves in order to protect other more valuable underwater metal components on your boat. Over time, regular replacement of zinc is required in order to properly protect your vessel.