I normally place two zinc anodes on the exposed section of my prop shaft and haul the boat every two years for new zincs and anti-fouling. On this schedule the zincs are usually pretty shot when I get to see them again around their second anniversary. Last year, instead of a haulout, I had a diver scrub the bottom and replace the zincs. When I hauled the boat today the zincs had totally gone after 12 months. Assuming the diver did what he was paid to do, should I be considering putting three or even four zincs on the shaft or........?
Some thoughts...
*Two years on zincs, which are supposed to be "sacrificial", is really on the side of abnormally slow wear. This could be indicative of poor contact between shaft and zinc and they are not sacrificing the way they should. 90-150 days is fairly "normal" wear but it can go either side of this, up to 360 days, and be dependent upon many factors such as water temp, use of the boat, salt content of your marina's water, how much underwater metal you are protecting, the electrical potentials of the metals you are protecting etc. etc...
For example, if you have a manganese bronze shaft and manganese bronze prop the zinc erosion would be somewhat slower if the boat was not also "bonded" ....
If you had an AQ-22 shaft and manganese bronze prop the wear would be faster. If the boat was bonded, the wear would be faster because you are protecting more metals.
If you added a graphite impregnated packing to the existing AQ-22 shaft and manganese bronze prop the erosion would be even faster. It all depends upon electrical potential.
*"Gone after 12 months" I would not consider rapid zinc depletion at all. 12 months can still be considered beyond what normal erosion could be in a "by the book" protected boat in high salinity water. Long lasting zincs, beyond 12 months, can often be a bad sign, not a good one but this is not to say zincs can't last beyond 12 months, just that it is out on the fringes of "normal"....
*You ideally need to get someone there with a silver/silver chloride reference cell to determine your level of active "protection"... On numerous occasions I have measured boats with intact zincs that are showing almost no protection. This is often due to improper installation technique of the zinc or the contact over time has been compromised.. In these situations the zincs last for a long time and the owner thinks this is a good thing even though they are doing very little. When they are doing little to no protection they don't tend to erode... When the zincs are installed properly, and making good contact, they now erode at 90-150 days or so (ball park)....
*Thousands of zincs are installed by divers every year and it can be done just as well in the water as out of the water. The shaft and zincs just need to be clean and free of oxides or contaminates that can reduce continuity.
Really the only true way to know the level of protection for your boat, and what is "normal" for your boat, is to use a reference cell to determine a baseline for protection then measure it over time to see when (time wise) you drop below the required protection threshold. My guess is that beyond about 8 months, if you used a reference cell, you'd find you were likely under-protected (this is a shoot from the hip based on lots of actual measurements but your water, current etc. may be different than around here). Zincs can also lose good electrical contact with the shaft as they erode. When they have poor contact, they seem to last a long time.
