Why we need zinc’s
Zinc anodes are used to protect your boats metal parts from galvanic corrosion

How zinc work
Zincs are made of less noble material that way they sacrifice them self before other metal parts are affected.

When to replace
If lost more than 50% of its original size
The bolts tying them to the boat are close to be loose
The zinc is loose (causing it to be contaminated and stop its activity)
Zinc not active
There are times zinc have to be replaced even you may think they still look good

Galvanic Corrosion
when two metals are at the same proximity and having a liquid conductive medium they produce electric current, one metal losses electrons to the other causing  pitting and damage to the less noble metal
To avoid the damage to less noble metals sacrificial anodes are used, more known as zincs