How To Install A Zinc (or any sacrificial) Anode
Sacrificial anodes are vital to controlling corrosion below the waterline. Anodes, most commonly made of zinc and thus referred to simply as “zincs”, provide corrosion protection to the metal parts to which they are installed or connected. However, anodes can only provide this vital protection if they are installed correctly. Here is a list of things that I do for each anode installation to ensure proper function throughout the life of the anode. I use the term “zinc” frequently, but these rules apply to any type of sacrificial anode.
Replace zincs when they reach 50% of original size. Most zinc types will begin to lose effectiveness or integrity as they corrode past 50% of their original size, particularly shaft zincs, so it’s a good idea to replace before you lose protection.
Select the proper location and anode size. Some zincs like rudder and trim tab zincs come in different sizes, several of which may fit the same application. If your rudder zinc is corroding very quickly, you may want to go up to the next size. For shaft zincs, you have freedom with where to place the zinc(s). I recommend placing the shaft zincs as close as possible to the prop, especially for props without a dedicated prop zinc. This will afford better protection to the prop, which generally corrodes more easily than the shaft. For especially long shafts, put one zinc toward the prop and one zinc toward the boat, spreading out the protection. Remember to leave space between the shaft zinc and the front and back of the cutlass (strut) bearing, so that water can freely lubricate the bearing. A good rule of thumb is between 1-2 times the shaft diameter in the front, and at least ¼’’ behind the bearing.
Clean the mounting area. This is extremely important. Scour the mounting area completely clean with a stainless steel scrubber or bronze wool before you install a new zinc. Make sure all of the material from the previous zinc is cleaned off. This will ensure a tight, quality electrical connection between the zinc and the metal part. The electrical connection between the metals is the thing that allows the zinc to provide protection to the metal part, so this not to be overlooked.
Clean the mating surface of the zinc. Unless the anode is brand new, it may have some oxidation or slight corrosion beginning to form on the surface that will mate to the metal part it’s being installed on. Give the zinc itself a quick scrub with the same steel or bronze wool you used on the mounting surface to make sure the zinc is as clean as possible for installation.
Tighten the zinc properly. For a shaft zinc, this also means tapping it with a hammer to make sure it is completely seated on the shaft. Tighten the screw(s) to make sure the anode stays in place, especially on parts like props, shafts, and rudders, which encounter lots of movement, vibration, and water flow. I find that slotted screws work fine for shaft zincs, but Allen (hex) head screws are a must for props and rudder / trim tab zincs. For rudders in particular, it is necessary to be able to tighten the screw down hard, and a hex wrench allows this much better than a screw driver. Without that tightness, rudder zincs tend to work loose and may cause a rattling sound. Prop zincs also need to be very snug, but be careful not to tighten too much. Prop nuts are generally brass and the screws are stainless, so the harder stainless will strip the brass if too much force is used.
Neat trick: For prop zincs that have a tendency to come loose, try coating the screw in 3M 5200 sealant, then immediately install the zinc. The 5200 works to sort of glue the screw in place and to the zinc itself, while probably absorbing come vibration. I use this trick all the time on boats that had previously lost zincs repeatedly, and have seen a huge reduction in zincs coming loose. Best of all, 5200 cures underwater, so a diver can use this trick too. When it’s time for a new zinc, the 5200 comes apart quite easily when the bolt is unscrewed.
For rudder zincs on larger boats, I will put a second nut on the bolt after I have tightened the bolt down. This acts like a jam nut to prevent the bolt from loosening. I have found this to be helpful in keeping zincs tight even in the face of strong prop wash from large props, which had previously been able to work the zincs loose.
Check your zincs. Don’t trust your anodes to last until the next time you haul your boat out. If you do nothing else to your bottom between haul outs, at least have a diver check your zincs every 3-6 months. The cost to do this is small, and you might just save yourself a prop or set of trim tabs down the line.