An old and a new zinc on Sea Gypsy's stainless steel prop shaft.

Zinc is an element, and a metal, but when we say zincs, plural, we’re referring to casts of dull, gray metal attached to the underwater parts of a boat to prevent galvanic corrosion. They come in different shapes. Collars that clamp on a propeller shaft. Plates that bolt to the side of a hull. Low profile discs screw onto a rudder. A rod shape that screws into an opening in an engine. Zinc can be cast as a fish and hung by a wire over the side of a boat. In some situations a boat's metal fixtures might be wired together to a remote zinc for protection in a system known as bonding. 

Galvanic Corrosion is an electrochemical process that occurs in metals in contact with seawater. I’ve been around electricity and chemistry for 76 years and so far have learned both involve electrons moving from one place to another. This is the subatomic environment of atoms and molecules and quarks, etc., and a good reminder we don’t see most of what’s going on in the world. For example, the width of a strand of your hair equals a million molecules of water. A cubic inch of water contains 500,000,000,000,000,000,000,000 molecules. And molecules are huge compared to electrons, which are indivisible, and the prime mover in electricity. 

Specifically, S/V Sea Gypsy has two metals in proximity under water. A bronze propeller and bronze castings that support the rudder, and a stainless steel propeller shaft connected to the engine. When these two dissimilar metals are in contact via seawater, an eclectic current flows between them. The less noble metal, that is, the metal less tolerant of oxidation will shed electrons to feed the current. The bronze being the less noble metal, electrons will leave the bronze and result in wasting. Unchecked, galvanic corrosion will compromise the structural integrity of the bronze and ultimately lead to failure. 

Enter zincs, or sacrificial anodes. The zincs interfere with this bronze /stainless relationship by shedding electrons faster than the bronze. Zinc is the least noble metal, so when bronze, stainless, and zinc are connected by seawater the zinc wastes and the bronze and stainless are not compromised. Timely replacement of zincs on a boat is an essential maintenance issue.

While here, I should mention the seriousness of electric currents in seawater. In short, never, never, swim in a marina. Marinas with their extensive electrical systems, and sometimes boats with faulty wiring, can give off stray currents powerful enough to paralyze muscles and cause cardiac arrest resulting in electric shock drowning. There is a tragic story around here involving a group of children who went swimming off a dock near a boat with a faulty electric system. One boy drowned.  The surviving children described the sensation of struggling to move their arms and legs, but fortunately were far enough away from the source.

Finally, risen as we are from the planet’s crust, it makes sense zinc is an important trace element in human bodies and essential to health. Our immune systems, metabolism, wound healing, skin health, and sensations of taste and smell depend on an intake of zinc. Fortunately there are many sources of zinc in food: oysters, beef, poultry, pumpkin seeds and cashews, and lentils are a few. With a good diet we should ingest plenty of zinc without having to bolt a zinc casting onto a wrist or an ankle a couple of times a year.