A zinc block (or zinc anode) is a sacrificial piece of zinc metal attached to a boat's underwater metal components, such as props, shafts, and rudders. It protects against galvanic corrosion, which occurs when dissimilar metals (like bronze and stainless steel) are in seawater. The zinc corrodes ("sacrifices" itself) to prevent damage to expensive metal parts.

Key details about zinc blocks:
Purpose: They prevent electrolysis and galvanic corrosion on underwater metal components.

Operation: They are a softer metal than the boat's components, causing them to degrade over time and require inspection.

Maintenance: Should be replaced when they are about 50% worn.

Installation: Installed on rudders, propeller shafts, and engine components. They should not be painted to remain effective.

Alternatives: While zinc is traditional for saltwater, aluminum (salt/brackish) and magnesium (freshwater) are common alternatives based on water type.