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2014-12-13

I zinc, therefore I exam


Not all zincs are created equally, and not all zincs are even zinc


There is clearly always more to learn about how even strong materials react to stress imposed under laboratory conditions versus in the real world, just as there is much to learn about galvanic action on  metals when they are used in a boat hull in the weak battery fluid that is salt water. I have decided that I will take a little chance with deformation with the more pliable grades of mild steel, for instance, in order to avoid a catastrophic failure with stainless steel (SS) or other comparatively brittle grades of steel. It's among the things I'm learning as I'm teaching myself to weld. 

Even a simpleton such as I am should come out of this with an even number of digits
I will also mix metals with proper isolation if the potential for galvanic interplay is low or can be mitigated. An example would be my aluminum pilothouse roof, which sits on an inward flange of the mild steel sides of the pilothouse. It was originally through-bolted in 40 spots and sealed with a bead of the dreaded 5200, which I think is great stuff for gluing on fibreglass keels, but should come otherwise with dire warnings. particularly for those end-user who tend to idly scratch themselves.. It took many hours and many reciprocating blades to pry off the roof of Alchemy in 2011 in order to swap out the old engine via a Polecat crane. Now that the engine installation is more or less completed, and, short of installing a series of water tanks that will fit through the companionway, the reinstallation goes thusly: Grind the mild steel flange back to bare metal. Coat with a couple of layers of high-zinc galvanizing primer. Top with Endura two-part epoxy to keep it unexposed to the various elements for the foreseeable future.
Not necessarily a brand endorsement, but among a group of similar products required.

Here comes the science: I want to put nylon bushings in every steel flange hole (40 of ‘em, remember?).  Then I want to lay down 1/16th strips of Delrin or HDPE atop the flange with appropriate holes for the bolts. Beside this plastic strip, which performs, along with the bushing, the role of electrically isolating the aluminum roof from the steel hull structure, I want to also lay down a line of butyl tape. This can be a fairly narrow line of tape, as it will be compressed in place as a barrier to water ingress, on the outside of the HDPE strip, which itself can be bedded with something that will given without severe difficulty in the future. Fit the 40 SS bolts and 40 new nylon washers and Nylock nuts to fit through the bushings, and put dielectric goo on the threads. Dog down as is sufficient to squish out the butyl, and trim to desired neatness.

This method (SS bolts, “other metal deck” and “other metal backing plate”) also applies for any deck gear, as making backing plates from SS is a royal pain, whereas power tools shape 1/4″ aluminum plate easily. In practice, this means attention to paint service is critical, as "rust" is a lesser worry than voltages, in some senses. EPDM rubber strips, able to withstand compression and UV exposure, seem like a good bet here when (for instance) bolting the typical sort of SS clamps to support awnings or biminis to the extant, painted pipe rails. The likelihood of paint plus rubber failure is low. And, of course, when one is making electricity, the need for effective insulation/isolation is commensurately greater.

"What do you mean, 'you forgot the gasketing'"?

Even on a plastic boat, I’ve sometimes chosen encapsulated ply over aluminum as a backing plate choice because of the number of SS bolts involved and the unlikelihood of keeping everything both isolated, in strong compression, and yet fully dry.

My impression is that in materials such as chain, plate or in gear such as failed stays and bolts, it is the prep and awareness of the role electricity plays in the weak electrolyte we call the ocean that defers or at least delays the majority of unpleasant surprises. Certainly the process of “ground tackle” is not commonly thought of in an electrical sense, but dissimilar metals in anchoring, even in a galvanically benign (presumably) open roadstead can have a negative effect on top of the differing material characteristics, such as ductility, of their respective parts.
Two very helpful volumes that are destined for the onboard library.

I have found Bruce Roberts’ “Metal Boats” and Nigel Warren’s “Metal Corrosion in Boats” excellent resources in understanding, at least for starters, a complex subject with many possible reactions and solutions. But even if you think you know this topic, and have taken all necessary measures, one should never neglect to actually examine your vessel, metal or not, for signs of wastage or corrosion due to galvanic issues. Part of dealing acceptably with this involves a little gadget called an isolator,and is a topic I will return to in the future.
Meet my little friend.