In boating and/or marine terminology, what are ‘zincs’?

The question, as written, is answered only by the option relating to sacrificial anodes based on its reference to usage in boating or marine terminology.

‘Zincs’ are used when dissimilar metals are in close proximity to one another in order to counteract the corrosive effect of the resulting electrolysis effect. Dissimilar metals are frequently used in the propellers, propeller shafts, rudders and the struts supporting the propeller shaft; often they are brass, stainless steel and bronze, respectively

Chemically, an electrical current is generated between the dissimilar metals and the softer of the two is the one that suffers corrosive damage as a result. The solution is to introduce a third metal into the equation, that metal being softer and more willing to give up its ‘free metal’ to protect the others. Generally, the greater the salinity of the enveloping water, the greater the corrosive effect of electrolysis.

In use, the electrical current and its corrosive effect erodes the zincs which are sacrificed in lieu of the other metallic parts. Aside from the risk losing a shaft, propeller, strut or rudder and damaging or disabling the vessel, the decision is largely a financial considering the zincs are quite inexpensive.