Below I copied my old post from the 33 site, on how to test your zinc levels. I went through this business myself a few years ago. Good luck, it can be quite a process to figure out what is wrong.
In my case with a 1987 B33, I found that I needed bonding system work, a new galvanic isolator, and (finally) that the sailboat next to me was throwing stray currents.
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You need a high-quality digital multimeter, and very long ~10' leads. Use the resistance setting, which requires that there is a battery in the multimeter. Touch the leads together, and it should read resistance as zero Ohms (use knob or whatever it has to adjust to zero if it not). Then touch the lead to any 2 metal parts on the boat, and you should read zero or near-zero resistance between them.
If not, the grounding is no good on between whatever 2 things you touched the leads to. That's the only way to know the bonding system is okay. I had plenty of wires that looked just fine, but when tested they weren't grounded, or worse, were only partially grounded with significant resistance, due to the wire getting crunchy and old inside the original factory crimp connectors, or due to corrosion at the contact lug. As I understand it, it's better for them to be completely ungrounded than to have any sort of partial connection.
Start at the zincs, if there is a contact point inside, and check connection to the copper grounding strips on each side of the boat, on the inside faces of the large stringers. Check the connection between the shaft or engine to the copper strips. After establishing that your zincs are connected to your grounding strips, check that each thru-hull, sea strainer, rudders, struts, et cetera are all connected to the copper bonding strips with zero resistance.
Basically check anything metal that is in contact with water. In my boat some non seawater-wet things are grounded too, like the rub rails, stainless rails, outriggers and fresh water tank. Not sure why but I don't know everything either. Maybe for lightning or static charges.
If you have everything already well grounded then consider yourself lucky, and it would be a simple matter to add a transom zinc if you want extra protection from corrosion. I added one. I used to throw over a zinc fish clipped to a strut bolt before that, before my bonding system work began, and the thing disintegrated noticably over a period of weeks. I also had a stray current near me and a bad galvanic isolator (more on this later).
Interesting thing about the ship's bonding system is that if in good shape the zincs last longer, maybe for years. Bad grounding means stray currents that eat up your zincs, then go to work on whetever other metals you have (like your rudders and props).
This sounds like a lot of work but it reallys isn't... unless your bonding system needs to be completely re-worked like mine did, in which case YES you have a big job on your hands. To do it right the genny has to come out too, more bad news.
Once everything is grounded okay, then you want to take it to the next step: determine if you have enough zincs, or if you have too much zinc (yes it is possible). Then find out if you have stray currents lurking about that come from leaky electrical sources - either your boat, or a neighbor's boat, or from your marina. Only after eliminating all these things can you be sure you are okay.
This procedure to test it is below. Note that this can be done at any time, for any piece of metal on your boat... but if your grounding system is not right then you are only checking that one piece of ungrounded metal, not the whole boat.
You'll need a corrosion electrode, I got mine here:
http://www.boatzincs.com/corrosion-refe ... specs.html, a good-quality digital multimeter, and a few spare hours.
You should also get and read
Nigel Calder's Boatowner's Mechanical & Electrical Manual , it is the absolute bible for this stuff.
Turn everything off and disconnect the shore power cord. A vessel’s bonding system should have with a DC voltage between -900 mV and -1100 mV (relative to a silver/silver-chloride electrode) to properly protect underwater metals from galvanic corrosion.
Readings less than -900 mV indicate the cathodic protection system is weak and/or failing. Your bonding system is not working, or/and you do not have enough zincs.
Readings higher than -1150 mV indicate overprotection, a situation that could damage underwater steel, aluminum alloys and wooden thru-hull backer plates. If you get this condition then you have too many zincs - remove some.
After taking an un-hooked reading, start by hooking up the shore power and take another reading. Then start turning on things on the boat one by one, all while watching the readings as you go. Readings that change dramatically as you turn electrical circuits on or off indicate a problem with that particular circuit or component. Do this again with the genny running. Check everything, DC and AC, and actually turn on your radios and chartplotters too.
If the voltage reading changes when you hook up the shore power cord then you are getting stray currents from the marina's leaky power grid via. your shore power cord ground wire, through your boat's underwater metals, to the seawater and finally to the ground. This is a very common condition and cause for quick loss of zincs and for corrosion in general. In this case you need a new galvanic isloator, unless the stray current is over 1.5V in which case any isolator will be overpowered and something else is very wrong. If you have over 1.5V don't swim in that water or you could get paralyzed by it and drown, which is suspected to be the hidden and underlying cause of most marina drownings. This is why I won't dive on my boat in marinas anymore.
There is more, but this is the grand summary of what you need to do to check out your bonding and cathodic corrosion protection (zincs).
On my 20 year old boat I needed all new bonding wires, a new galvanic isloator, and in the slip I was in at the time I still had something going on. I was loosing zincs too fast. It wasn't until I got the corrosion reference electrode was I able to find the problem: my neighbor's shitbox sailboat. I decided to be bold and turned off his shore power breifly.... the current level on my boat changed dramatically (bingo). I moved to another slip and finally the problem went away.
