Building Odyssey
Sacrificial Anodes
Sunday, August 29, 2010
Zinc has a relatively high electrical potential, and thus, Is eager to give up its electrons when in an electrolyte like seawater, and in close proximity to other types of metals with lower potentials. This makes it very useful for protecting these other metals such as stainless steel prop shafts, bronze props and aluminum hulls, from galvanic corrosion. Even though our hull will be coated in a thick dielectric epoxy coating, it is still possible that damage to this coating will expose bare aluminum. The zinc anodes on the outside of the hull will corrode away before any damage occurs to the other metals. Or that is the theory. The reality is that some metal boats don’t have anodes, and on others, the anodes get eaten at a high rate. It all depends... on the temperature, salinity, stray current in the marina, proper interior electrical connections and good karma. The recommendation for the surface area of our hull was for four 9” diameter x 1” thick zinc anodes. Early in the project I had purchased some anodes of this description, but the maker’s method of attaching them to the hull left much to be desired as they consisted merely of a 1/2” steel plumbing nipple cast into the center of the disc. This size was way too big for a 1/2” bolt and too small for a 5/8” , and had no internal structure other than the thread to hold it together.
With anodes, it is vital to have a good electrical connection between the zinc and the hull material. It is also important that the anode be firmly held to the hull, and not create undue drag. So we decided to make some recessed “ pockets “ for the anodes to reside within, so that their outer surface would be flush with the hull. I also decided to cast my own anodes, with proper internal hubs.
Sacrifices have been made.
99.95% zinc melts at about 750 F. so is quite easy to melt and cast. I found that it poured best when heated to 1000F. I made a simple rolled steel perimeter mold, clamped to a piece of 3/4” steel plate. This plate had a tapped 1/2 hole to accommodate the hub, which also had a removable top and bottom plug to create a recess so that for the bolt head. I plasma cut the 4” diameter by 3/16” thick hubs after drilling, as this made handling under the drill press much easier. I have several digital electric kilns and used an assaying crucible for the melt. Each anode weighs 17 lbs. They poured fairly cleanly, although I did flame off the tops of several to melt in the edges. Later I beveled the lower outer edge with the die grinder so that it would fit the pocket, which has a fillet weld, better. I made a total of 8, 2 sets, so hopefully these will last a long time. I can just picture myself on a beach somewhere, sandcasting zinc anodes...
Determining the position of the anode pockets was a matter of locating the general neighborhood, then finding where they would fit between the stringers and frames, allowing enough room to properly weld them on the inside. It is an inexact science anyway, due to the previously mentioned variables, so I feel like these two forward anodes, although a bit close together, will help protect both the anchor chain, and the area of the keel most likely to lose its epoxy coating. The aft anodes are just abaft the keel on either side of the prop shaft. I will also install 2 smaller discs in the rudder. The gigantic pre-turning- the -hull hit list is growing shorter by the day. Note n the picture above that I have removed the angle stiffeners from adjacent to the bottom seam. She looks even sleeker now, although my tools have a tendency to roll off now.
Cutting large holes in the bottom of our newly welded hull requires careful planning, and a large dose of gumption.Fresh consumables in the plasma canon make for a much neater cut, and the hardboard circular template helps make the cut the right size. Don’t try this trick on your wood or plastic boat.
