Is Your Boat Zinc-ing ?

April 3, 2013   So what is the big deal about putting zincs on my boat, and why should I care ?   Galvanic Corrosion When two different metals (like your bronze propeller and your stainless steel prop shaft) are put together Corroded Prop 1underwater, they basically create a battery and electric voltage can flow between them. As in a battery, one material is more noble (the SS shaft) and the other less noble (the bronze prop) and current will flow from the less noble to the more noble. As the current flows, it brings elements of the less noble material with it, which over time will result in the ultimate disintegration of the propeller.       Electrolytic Corrosion The other kind of corrosion we must battle is called electrolytic corrosion. It is created when a stray electric current is introduced into the water surrounding your boat by another boat or the dock, often caused by faulty wiring. This kind of corrosion is more accelerated than galvanic corrosion, especially in salt water, where the salt in the water acts like an electrolyte.  Zinc on Rudder Where Does Zinc fit Into This ? Because zinc is further down the noble metal scale (see Order of Activity in Metals chart) it will corrode first and give up parts of itself to protect the important (and more noble) underwater metal parts of your boat.  Thus the term “sacrificial zinc” or the brand name “Martyr”.  The size of the zinc(s) should be about one percent of the metal surface area of your prop shafts, propellers, metal rudders, struts and thruhulls.  Zinc Chart Though a related but separate topic, you should know that all metal through hull fittings, shafts, struts, rudders, engines, etc. should be wired together inside the boat to create a uniform “circuit” that the zincs will sacrificially protect. This wiring is called “Bonding”. Corroded Prop 2 Check with the Rex and Cove store staff to select the correct Mil. Spec. zincs needed for your boat. They are definitely not something you want to ignore or skimp on ! 4/3/13     By Capt. Rick Delfosse   203-216-7800

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