Zinc wastage needing replacement every 90-150 days is way too fast. Properly set up you should see no more than 25% wastage per year.
This is dependent on many, many factors including current, salinity of the water you are in, the electrical contact between shaft and zinc, electrical potentials of the submersed metals, weight,volume of the protected metals. etc..
The reason zincs get wasted and are there in the first place is to protect metal as electrons flow from 1 metal to another. Electrons will only flow if there is an imbalance between metals. If your bonding system is properly set up with a bonding wire connecting the metals there should be no imbalance as the metals will be at the same electrical potential, therefore no electrical flow.
They are there to be the sacrificial metal, why they are named a sacrificial anode.. Any time there are dissimilar metals submersed in an electrolyte there is a "galvanic circuit" and the resulting electron flow, bonded or not. Each metal has is own electrical signature or "potential" + or -.... If bonding worked, as described, (unless I misinterpreted the statement) then theoretically we would not need a sacrificial metal to protect the more noble metals. But when you submerse dissimilar metals in an electrolyte you have created a galvanic circuit which now needs protection beyond bonding. A bonded boat will also require more zinc content to protect the added weigh/volume of the underwater metals and maintain good protection levels..
If all your underwater metals were identical and you were not plugged into your neighbors boat then, in theory, you may not need a sacrificial anode because with identical metals, electrically, there is no galvanic cell created.. Unfortunately we have numerous dissimilar metals on our boats, each with their own electrical potential, and all submersed in an electrolyte.
When dissimilar metals are immersed in an electrolyte the electrical potentials of the different metals cause the least noble metal to corrode and shed into the electrolyte. We attempt to control this by adding a metal that is not critical to the system and that becomes the new least noble metal. It sacrifices itself to save the more noble and more critical metals from erosion. Usually these are zinc in salt water but more boaters are now using Mil Spec aluminum anodes.
The key with zincs is that they are more negative in electrical potential than the metal we are trying to protect but not too negative, like magnesium, as that can cause other issues. These metals are termed "sacrificial" because that is what they are, expendable.
Some anodes in salt water can last longer than others, such as MIL Spec MIL-A-24779 aluminum anodes.
Salinity, current, how much you use the boat, mass/weight of the metals being protected, electrical contact and the electrical potentials of those metals being protected all contribute to the zinc erosion speed. Because of these factors what is "normal" for one boat may not be "normal" for another boat in another area and the best we can hope for is a range of "normal" for a certain area.
Assuming your bonding system is intact if you are plugged into shore power you could get an imbalance from the shore ground that will cause DC currant (AC does not cause galvanic corrosion in most cases) to flow from your boat to the dock or a boat nearby. Installing a galvanic isolator will prevent this.
Exactly, without some form of isolation when you "plug in" you now are physically connected to all the underwater metals of your neighbors boats. If their zincs have eroded your zincs are now trying to protect their underwater metals. If you have a bronze shaft/bronze prop and have normally had 12 months service in your water, and plug into a boat with a lot more metal, and AQ-22 shafts and graphite packing the speed at which your anodes erode will increase.
If you are having to add more zincs or worse yet hang those awful fish zincs from your boat then you have an issue and the best thing to do is fix it.
If you have been taken out of your normal range and your normal range is within what is considered normal then yes you must address why the zincs are eroding so fast. I often find little protection on boats with fully intact slow eroding zincs.
For the OP he should be able to get by with 1 zinc once a year at most.
I don't know how that can be said without taking actual measurements of the boat in question using an Ag/AgCl reference cell to first determine how much zinc he actually needs to hit the desired level of protection. We would ideally then need to know what is "normal" erosion for the salinity, current etc. at that marina on perfect boats known to have sufficient zinc protection and zero on-board or foreign electrical issues.
It is important to remember that anodes should be replaced when about 50% depleted, not when they are gone.
Just yesterday I was working on a boat where the steaming and deck light circuit to the spar had "shorted" to the spar.. Without a thorough corrosion inspection, testing every part of the on-board electrical circuits, it is hard to know why the zincs are eroding so quickly or lasting so long...
Most people just slap on more zinc than they technically need and call it a day. In most cases they have decent results...
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