Heat Exchanger Zinc

[TBOT422]

I'm preparing to replace the zinc in the HE on the M35B. I see the new zinc is threaded into a brass plug. Should I apply any sort of thread sealant or teflon tape to either the zinc/brass plug connection or the brass plug/heat exchanger threads? It's my general approach for all plumbing jobs, but I'm assuming it would negate the electrical function of the zinc. If sealants/tape are not acceptable is there any solution to a leaking zinc other than just tightening it further?

Thanks
Gary (Hull #422)

[KenKrawford]

Gary, I think you are correct that it would be best not to use teflon tape. I'm sure as the zinc is tightened it would cut throught the tape so there would still be contact between the zinc and the heat exchanger. It it leaks without tape, it would probably be better to use a conductive anti-seize compound. Loctite makes one - http://www.amazon.com/Loctite-Copper-An ... B0006Q7H36

[AynB]

FWIW: I use nothing between the brass and the brazed on fitting for the HE anode. I check it often enough that I don't have a freeze up happening, and I have found only one of the brass plugs to not have been machined correctly and allowed a leak.
I have found that if I replace the actual zinc often enough, I can reuse the brass plug and just insert a new zinc. I carry three brass plugs and 5 zincs for a season.

[TBOT422]

5 zincs per season? That means you're replacing the zinc about every 2 months. Is that normal?

I have no idea how long our HE zincs last. We've only owned our C350 for about 4 months now. I was replacing a water pump hose that required draining the coolant. Since the engine had about 460 hours on it, and the manual suggested replacing the zinc at the 500 hour check, I decided to just go ahead and do it at the same time.

When I removed the zinc, it was nearly the same size as the new ones with very little erosion in the 4 months we have owned the boat. I'm not sure when it was last replaced, but the previous owner's notes seem to imply it was around the 250 hour range. That would be about 2 years ago based on the total engine hours. I was just going to clean it and put it back in, but when I tried to clean it, it just fell apart, so I put in a new one.

I bought the zincs (with the brass plug) on-line from Torrensen Marine for $3.13 plus the HE gaskets ($2.20) and O-Rings ($0.66). I opened up both ends of the HE to inspect it and clean it out, and replaced both gaskets and o-rings as well. I've read other threads that seem to imply you can buy the zinc separate from the brass plug ans save money. From my perspective, any boat item that just has a single digit left of the decimal point is not something to worry about what it costs.

I've also read other threads about the zincs being too long. On our HE (M35B built in May of 2006), the new zincs from Torrensen were no where near too long to jam on the far side of the heat exchanger. The port the zinc screws into is hex shaped lug about an inch long welded to the HE tube and offset slightly from centerline of the HE. So when the new zinc was installed, the end of it barely reached the centerline of the HE. Absolutely no possibility of it jamming into the HE.

Now, I'm just crossing my fingers and hoping that the water pump hose was the actually source of the leak I was trying to fix. I ran the engine at idle for about 1/2 at the dock, then 2000 RPM for another 15-20 minutes, and a couple of minutes close to 3000 RPM. No signs of coolant drips or other leakage yet. I'll keep an eye on it.

Thanks to everyone for all the inputs. This board is immensely useful to us new C350 owners.
Gary (Hull#422)

[AynB]

You will get very different longevity if you are in fresh water than you will in salt.
I have over a thousand hours on the engine and yes, I go through about 4 to 5 HE zincs a year.
I check 'em every first of the month and when about 1/2 to 2/3 gone, they get replaced.
One other note; we live aboard and we don't let the boat just sit - might make a difference too.
al

[TBOT422]

Al,

Our boat is in salt water, so I would expect that to be the worst environment for the zincs. We don't live aboard but do use the boat pretty frequently. We don't have to run the engine much since we can be in the open Gulf of Mexico in about 30 minutes. But, I suspect the real difference is that the previous owner was an electrical engineer, and added a DC power supply to the shore power. When we're on shore power, the DC battery banks (all 4 of them) are completely disconnected from the circuits. The DC power supply powers all the DC components like the fridge, etc. without using the batteries and constantly re-charging them. Maybe that's the difference. I checked the previous owner's maintenance log, and as I suspected, he replaced the zinc on 5/1/09, or a little more than 2 years ago. When I removed it yesterday, it was nearly as large as a new one. That's a huge difference between once every 2 years and 4 or 5 times a year.

The guy who cleans the bottom of our boat also cleans a 27 foot Island Packet about 4 slips away from us. They go through a shaft zinc about every 2-3 months, while he's never mentioned a word about our shaft zinc. Clearly something is different.

Thanks
Gary

[AynB]

The DC system is unlikely to have any effect at all. The Zinc is there to protect from galvanic corrosion - that which takes place anytime you have two ( or more ) dissimilar metals in an electrolyte. In your case ( and mine ) we have copper, brass, aluminum as well as steel and a few other alloys, all in contact with the same seawater in the HE unit. The zinc is least noble of the bunch, thus gets sacrificed to protect the rest. You can have no battery, no shore power, no ground , no galvanic isolator ( e.g. a small wooden daysailer with an olde one lung gas motor ) and if you have metals and seawater, you will have galvanic corrosion unless you have a zinc. Count your self lucky with the longevity of your zincs.

Also, you may or may not have a "shaft zinc" , if you don't you probably just have a prop zinc like the rest of us. There's damn little room for a shaft zinc on the C350's , thus the need to put something on the end of the prop. My prop zinc lasts about 9 months unless I add a "fish zinc" when in harbor.

al

[TBOT422]

Al,

Although you are correct that 2 dissimilar metals in salt water will have galvanic corrosion, it is my understanding that shore power connections can make it worse. The shore power ground essentially connects you to all the other boats at the marina, so if they don't have adequate protection, your zincs can erode faster than normal because your zincs are protecting the other metals on the "circuit".

I started thumbing trhough the paperwork from the previous owner and found a spec sheet for a ProMariner Galvanic Isolator that is supposed to galvanic current flow between neighboring boats. So maybe that's an issue too. At this time, I have no idea whether the boat actually has this installed or where it is. I'll need to go on a scavenger hunt the next time I'm on the boat to see if I can find it.

It's my experience that 'luck' has nothing to do with the the laws of physics.

Thanks
Gary

[AynB]

Do not confuse galvanic isolation (GI) with galvanic corrosion (GC).

Most marinas are wired to have a common ground - it's code. That common ground in the shore circuit connects to the boats common ground and because the engine is grounded, that dang prop and shaft have the potential of being sacrificed because your next door neighbor has no zincs and has no isolation. That is where the GC comes into play.
What the GI does is breaks the shore side ground to the boat side ground continuity in such a way as to allow such items as Ground Fault Interrupter to work, but provide a galvanic disconnect between your prop and your neighbors. No circuit, no damage... unless your two pieces of dissimilar metals ( steel shaft and bronze prop ) are not protected by a zinc. I just went out and dipped my fish zinc and read my test voltage between the zinc wire and my engine: 1.05 Volts DC and 0.24 milliamps. And that is with the GI working and the shore power unplugged! That is just current produced by the shaft, prop versus zinc.

During our research phase, we learned some rather interesting things about "marine" GI's and modes of failures - most all of them "fail" in a short mode. This means that the lovely expensive hunk of metal in your cabinet , when it fails, is not doing anything - the ground is shorted and there is no longer any protection. We no longer have a "marine" GI, opting for a model ( UL listed even ) that is intended for the dairy industry - when it fails, it fails to open and has an alert feature - would not want a hundred cows hooked to mechano-electric milkers to get shocked now would we.

The HE anode is within the common neutral / ground of the boat, thus the only electrical currents it has to deal with is those created by the dissimilar metals. You also have a zinc built into your head (toilet) - you have steel, brass, and copper in the electrolyte ( incoming flush water ) and in contact with the human waste electrolyte.