Zinc Anodes in DDs?

[richardoren]

Hi Guys,

Sure didn't know what to reply. In the process of hand scraping, dry sanding repainting, varnishing and generally fully refitting the abandonned HATT prior to her Mediterannean launch... and a shipyard neighbor told me to be sure to replace the zinc anodes mounted "inside" the engines. Any comments are welcome.

Cheers,

Richard

 

[Passages]

Yes, DD's have engine zinc anodes. Yes, they need to be replaced periodically. If you were not aware of this, let me direct your attention to the square or hex bolt heads located in the following locations in the raw water circuit:

Top of the transmission cooler
Raw water pump
Top of the fuel cooler
Heat exchanger (raw water side)

Pull them off, They should be 50% intact or better. If not, replace. If yes, lay them on a hard surface and give them a good rap with a hammer. If it does not shatter and you still have better than 50%, you can reinstall it.

Don't use teflon tap to seal them to the engine and be sure to make certain that the zinc in firmly seated in the brass head.

 

[MikeP]

We do not run zincs in the RW pump or the HE because they gunk up the HE/intercoolers with zinc particles caused by the erosion action of the water. I quit using them two years ago when I pulled the HEs and found that most of the junk in them was zinc flakes, etc. Additionally, at least on our 8V71TIs, there is no water in the HE when the engs are not running since it is "downhill" from there to the exhaust outlets.

I have considered installing a zinc in the downstream side of the HE since any zinc will be flushed straight out of the engine but I don't want any zincs between the intake and the HE.

I have seen no evidence of any problem with this and since the engines are bonded to the external zincs, there is protection present. Per the PO, our boat has never had a zinc in the RW pump - at least since 1992, though he did use zincs in the HEs.

I will certainly admit that not running zincs is based on my personal observations on my engines only. It is MY OPINION that they are not necessary in the water pump/HEs. Others will disagree completely.

 

[Passages]

I'm sure the engineers at Covington and J&T recommended engine zincs for some reason. If your engines raw water system naturally drains when not running, then I agree, engine zincs are not necessary.

Also, If you operate in fresh water AND away from shore power (a mooring) zincs are probably less critical.

 

[REBrueckner]

My experience on 8V71TI's is exactly as Mike posts. A few years ago I removed the pencil zinc on the raw water input side of my HE's and kept only the zinc on the output side.

 

[Passages]

You only need 1 zinc per device. Discharge side is preferred. Covington and J&T just gave a choice of multple ports to facilitate easy access.

 

[krush]

Just because water could possibly drain out doesn't mean there isn't plenty of electrolyte left over to cause you pain.

 

[REBrueckner]

Krush makes a valid point if the zincs remain immersed. On the other hand, when I have removed either of my HE zincs (8V71TI's), NO water has ever come out...I stick a finger in and can't feel any water in there. Hence the zinc is out of water and useless unless the engine is running and raw water present. Incidentally, corrosion on large metal ships increases when underway;I'm unsure if that occurs in much smaller recreational fiberglass boats.

These engine pencil zincs would offer protection against local engine component galvanic corrosion, when immersed. Galvanic corrosion is inversly proportional to the cross section area of water between different metals and to the distance between them; in other words, even if open piping full of raw water existed directly to external ocean raw water , there is so little area present, any corrosion with external underwater component metals is very unlikely. With DD, the water pump impellers almost cuts off all conductivity. Stray current corrosion (leaking dc currents especially) might theoretically be a problem, but is also unlikely because the transmission to shaft connection provides the usual electrical flow route....

Bottom line: When you extract a pencil zinc and no raw water comes out, and the zinc deterioriation/flakes might cause cooling water blockage in the engine, you are much better off leaving it out.

 

[Passages]

FWIW, When I pulled the zinc off the Hx on my 6-71, there was always plenty of raw water in the chamber. From my thinking, zincs are cheap insurance against damage. Flakes of zinc oxide (or is it zinc chloride?) were taken care of by acid boiling the raw water circuit every couple years.

 

[Fanfare]

Unfortunately, I am more likely to then forget to put them back in before the next startup. That would be exciting!

 

[Nonchalant1]

I don't get it.

I have zincs n my raw water pump and the heat exchanger. When I run in salt water, the zincs corrode and get replaced avery 4 months or so. So if the zincs are not there, wouldn't that amount of corrosion go to the next metal on the periodic table?

However, I have zincs on the input and output side of my heat exchanger so I could certainly take the one off of the input side. The gear cooler on my 8V71TI is integral to the heat exchanger and protected by that zinc. The oil cooler is cooled by coolant and no raw water.

Doug Shuman

 

[MikeP]

The issue is, are the zincs being eaten by doing their job as sacrificial metal OR by the erosion effect of the rapidly moving water? What I noticed in our engines was that the wear pattern on the zincs looked totally different than the wear pattern on the zincs on the prop-shafts/rudders. The ones in the engines just looked like you had worked them over with an abrasive as opposed to the "eaten" appearance of the others.

Also, since, as noted earlier, the zincs in the HE on our 8V71TIs are not in the water at all when the engs are not running, they serve absolutely no galvanic purpose (if needed) 95 percent of the time (engs not running) anyway.

 

[REBrueckner]

Nonchalant....How many hours do you run every four months?

At 100 to 150 hours annually, my engine zincs last two to three years...sometimes one is gone, usually it's still half there and I'm afraid it will break and flake off...so I change it....Even shaft and rudder zincs lose maybe half their volume from abrasion, not galvanic protection....

 

[Walter P]

My boat has an isolation transformer. Does that mean I don't need engine zincs at all? Since I have 6/92s and they are quite sensitive to heat, I figure any little bit helps.

By the way I would really appreciate your responses, however when it comes to electrical issues I am as dumb as a door so please keep it simple. Thanks..

Walt

 

[krush]

Also, since, as noted earlier, the zincs in the HE on our 8V71TIs are not in the water at all when the engs are not running, they serve absolutely no galvanic purpose (if needed) 95 percent of the time (engs not running) anyway.

It is not prudent to extrapolate your experience to every other engine--even if it is a 8v71ti.

 

[MikeP]

"It is not prudent to extrapolate your experience to every other engine--even if it is a 8v71ti."

Absolutely true!

I thought I was very clear that I was talking about the engines in our boat, NOT the engines in any other boat. If I was not clear that I was referring only to my own experience/observation of this issue on MY boat, I apologize. I completely agree that engines are not the same - I have many years of experience working on engines and many examples of settings/components for one engine that worked perfectly while on another identical engine they worked rather poorly.

 

[krush]

Just makn' sure

 

[Nonchalant1]

Nonchalant....How many hours do you run every four months?

At 100 to 150 hours annually, my engine zincs last two to three years...sometimes one is gone, usually it's still half there and I'm afraid it will break and flake off...so I change it....Even shaft and rudder zincs lose maybe half their volume from abrasion, not galvanic protection....

We went 6,000 miles last year and ran 660 hours, which was an long trip. My quote was for when I was in salt water running a lot. We were also at dozens of strange marinas where stray current might have been an unknown issue. In fresh water, my zincs last years. In salt water, my shaft zincs will get eaten away in 3 months, while my engine zincs last 6 months with a lot of running. Even the engine zincs will have that hard white "eaten zinc" stuff, not just abraded. Therefore, I think they're protecting SOMETHING, and they're easy to replace. The Hatt and DD engineers knew more than I do. I take them out and rap them on something hard and if they break, I replace them. We'll be in fresh water this year so I won't even have to replace them at all. After I installed carbon brushes to ground my shafts when running, the shaft zincs lasted longer.

BTW - my 8V71TI heat exchanger zinc holes also do not gush water if the engine's not running. They're just wet.

Doug

 

[REBrueckner]

Walter P: Zincs protect primarily against galvanic corrosion, the corrosion caused by different metals....as copper heat exchangers and iron engine blocks....so you should use engine zincs even if you have an isolation transformer. An isolation transformer helps assure your zincs will be used for their intended purpose...and not used up by stray electrical currents from your neighbors boat in the marina....

 

[REBrueckner]

NonChalant, Doug...not surprising your zincs would be used up faster with all that time running....more running, more abrasion...some boats use more zinc that others for no apparent reason....I sometimes get almost two years from rudder and shaft zincs by hanging an extra zinc over the stern at the dock...a guppy type extra zinc....and sometimes I don't get that much...And you can always add an additional shaft zinc, two in total, near your props, if they tend to disappear faster than you'd like...