Don’t get me wrong; we love Jo Beth. We love her dearly. Perhaps that’s why we put up with her issues. That’s why we continue to nurture her and support her, and look after her. After all, we expect the same from her.
There is a whole laundry list of tasks, large and small, cheap and expensive, that need doing. However, the issue of the utmost importance to resolve is one of electrolytic corrosion.
Our shaft zincs – protective collars made of zinc that fit snugly on the propeller shaft – are degrading fast. Very fast. Faster than they should.
A new shaft collar zinc is shown on the right. The zinc on the left is what remains of a new one installed on Jo Beth's propeller shaft after less than two months in the water.
For the non-sailor, or non-boat owner, I’ll explain.
Seawater is an electrolyte, or an element capable of carrying an electric current. When two dissimilar metals are immersed in an electrolyte – seawater – an electric current will flow between them. This creates a very elemental battery. Now these currents are very small; mere millivolts, but the greater and damaging effect is that one metal will sacrifice it’s electrons to the other. This is known as electrolytic corrosion. Every metal and metal alloy has a place on the Galvanic Series Chart, from the most active (anodic, or least-noble) to the least active (cathodic, or most-noble), and a metal’s position on this chart determines how active or inactive it will be when immersed in an electrolyte or otherwise subjected to electric current.
Metals commonly used on boats below the waterline are aluminum, stainless steel, and bronze. In the case of Jo Beth, the only metals in contact with the seawater are stainless steel (the propeller shaft) and bronze (the propeller, through hull fittings, and rudder fittings). Because bronze is lower than stainless steel on the Galvanic Series Chart that means that Jo Beth’s bronze propeller is sacrificing itself to the stainless steel propeller shaft. Left unchecked, the propeller would eventually, given enough time, disappear.
Because bronze propellers are expensive, and we don’t want to have to replace ours again and again and again, we place fittings made of zinc in the proximity of the two active metals. Zinc is very low on the Galvanic Series Chart and will sacrifice it’s electrons to nearly every other metal when immersed in an electrolyte. So a small collar made of zinc resides on our propeller shaft solely to be sacrificed.
The problem is our propeller shaft zinc is giving itself up too rapidly. It’s important that we find out why, as rapidly deteriorating zincs are often symptomatic of other problems. Typically, those problems are within the on-board electrical system, but they can sometimes be brought about by other boats in close proximity, or the dockside electrical system.
Electrical grounding on a boat can be tricky. Mostly, this is because there are two electrical systems aboard: DC and AC. DC, or Direct Current is the electricity supplied by batteries. AC, or Alternating Current, is the stuff you buy from your local electric company.
Often, electrolytic corrosion is referred to as ‘stray current corrosion.’ This is because a faulty ground on the AC system aboard a boat can ‘leak’ current to the DC system. The AC current will attempt to seek a path to ground and the resulting stray current will cause electrolytic corrosion. Boats in close proximity can also be the source of a stray current.
The real rub is finding the leak, or the problem within the system. Every grounded circuit on the boat, of which there can be hundreds, is suspect. Improperly grounded radios have destroyed engines because of electrolytic corrosion. Testing must be done, and I’m off to get some basic tests made. I’ll discuss what I find in the next entry.
