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Author Topic:   Zinc Sacrificial Anodes--When to Replace
jimh posted 08-10-2003 09:47 AM ET (US)   Profile for jimh   Send Email to jimh  
My otherwise fresh-water-only boat and motors will be spending ten days in salt water. As a fresh water boater, I tend to consider salt water as dilute sulfuric acid.

My outboard engine zinc anodes are older. Some minor erosion of the steering tab has occurred. The large bar on the bottom of the transom mount is in good condition, but the surface is pitted and shows some yellow residue--maybe some dead marine life?

Is there any gain to be had by cleaning the zinc surface with a wire brush?

Do I need to replace these before salt water use?

OutrageMan posted 08-10-2003 10:24 AM ET (US)     Profile for OutrageMan  Send Email to OutrageMan     

For that short of a time, I would not worry about it. When you get back, you may want to change them.


raygun posted 08-10-2003 10:42 AM ET (US)     Profile for raygun  Send Email to raygun     
I've always been told to replace them when they are half gone. It's a question I've asked several dealers, the sharp guy who owns The Chandlery here on the island and even Bombardier customer service, as my boat is tied up at our dock all summer here in Puget sound. They are "sacrificial" anodes I was told, and cleaning them with a wire brush(I asked about that as well) isn't necessary. Pitting and minor corrosion are normal wear. I'd say they sound like they still have plenty of service life left.
Morocco posted 08-10-2003 01:35 PM ET (US)     Profile for Morocco  Send Email to Morocco     
Jim --

Have fun and stop worrying about those anodes. From the way you describe them, I'd think you have a couple of months of use in salt water before you have to replace them. My 25 Revenge is in a salt water slip year round. With the motors tilted out of the water most of the time, I replace zincs once a season. On my trim tabs, which are always submerged, I have to replace about every 60 days -- they are 3 inch disks, and I wait until they are down to just past the size of an 'silver dollar.'

For the younger guys...well, ummm...a poker chip?

As long as they are not painted or mounted over paint they should be doing the job -- which results in a rough, dull silver patina. I've been told that if they look bright, they are being eaten by an abnormal current leak and will go really fast.

Don't clean them, you would be just wasting time.

newt posted 08-11-2003 08:16 AM ET (US)     Profile for newt  Send Email to newt     
Jim, are you sure that your anodes are zinc? I thought that for fresh water the recomended anode material was either aluminum or magnesium. At least that is what the guy on Ship Shape TV was telling viewers this past Saturday. Might be worth verifying that you actually have zincs on your boat before splashing in the acid (salt) bath.
knothead posted 08-11-2003 08:32 AM ET (US)     Profile for knothead  Send Email to knothead     

I take mine out and wire brush them when growth appears on them. I also want to make sure that the bolts will still work. I've replaced all the zincs once in three years.

My Montauk sits in salt water 2 months out of the summer and so far I've had no problems with the zincs.

Good luck on your trip!!


Peter posted 08-11-2003 09:01 AM ET (US)     Profile for Peter  Send Email to Peter     
Sounds just like the one I changed out on my 70 Yamaha this season. It has a little bit of a yellowish tint to it. The OEM Yamaha original bracket anodes are about 16-17 dollars at my dealer, cheaper than the knock offs sold at West marine.

The engine bracket anode is the only anode always in the water when the boat is in the water and so I deem it to be more important than the steering tab anode. The steering tab anode only spends part of its time in the water when the boat is underway. When the engine is tilted up (normal position for extended periods of non-use such as overnight) the steering tab anode does almost nothing since it is not immersed in the "dilute sulfuric acid" electrolyte.

kglinz posted 08-11-2003 10:51 AM ET (US)     Profile for kglinz  Send Email to kglinz     
Peter brings up a point I have wondered about. If you tilt your engine up, but it doesn't clear the water, should you lower it back down until the zinz is submerged?
DaveH posted 08-11-2003 11:49 AM ET (US)     Profile for DaveH  Send Email to DaveH     

Newt is correct about the choice of sacrificial metal anodes in fresh vs. salt water.

Jim, I focused on corrosion and structures in college. Do not worry about a few days in salt water. The anodes should be changed as stated earlier when metal loss is 1/2 or every season. Do not brush them. The most important aspect is to ensure good metal to metal contact with the engine part it's mounted to (creating a circuit).

Actually your statement about sea water being dillute sulfuric acid made me chuckle. As far as a trace constituents in order of magnitude: chloride (Cl), sodium (Na), Sulfate (SO4), magnesium (Mg), calcium (Ca), and potassium (K) consitute 99% of all sea salts. But in terms of conductivity the order is Cl, Na, Mg, followed by SO4. Therefore, I would be more concerned about magnesium than sulfates causing problems (can you tell I'm bored today?)!

Ref: [i] Oceanography[/] 1982, by M. Grant Gross

Peter posted 08-11-2003 12:24 PM ET (US)     Profile for Peter  Send Email to Peter     
To get the steering zinc in the water means that the water pump intakes are in the water and thus prone to marine growth fouling if you leave your boat in the water for the season like I do. Intake fouling would be a far greater problem than any corrosion caused by a little bit of the foot touching the water.
lhg posted 08-11-2003 03:22 PM ET (US)     Profile for lhg    
Make that 12 days, Jim.

Newt brings up an interesting point. My problem is I don't know about Yamaha's anode material.

In about 1989, Mercury was the first to switch to the lighter weight aluminum anodes, abandoning zinc. These are much less "noble" and sacrifice more quickly, hence giving greatly increased protection. I have found them to be excellent. Mercury only makes them for Mercury engines, and they are not available aftermarket, period. Hence I am wondering if Mercury has a patent here. I know that Mercury has them made by Canada Metals. But with Yamaha's close relationship with Mercury, it's possible they have made a deal to get this aluminum alloy.

If Yamaha has switched to aluminum, I would get them, both for the trim tab and the engine bracket. But if Yamaha's anodes are still zinc, I would not bother. My 1985 Mercury 115's, which originally came with zinc, have been replaced with the aluminum anodes. Mercury no longer even offers the zinc versions.

For freshwater, I have tried the after market magnesium anodes, but not found them to be worth the bother, nor to be any more active than the Mercury aluminum. However, if your only other alternative is zinc, they would be worth it.

Peter posted 08-18-2003 09:57 AM ET (US)     Profile for Peter  Send Email to Peter     
My 15 SuperSport/Yamaha 70 was moored in saltwater for approximately 3 weeks with the engine tilted up overnight when not in use. I just hauled it out and took a look at the sacrificial zinc anodes. The new zinc anode spanning the bottom of the engine bracket showed signs of early pitting. I believe some of this is caused in part by marine growth (I find that barnacles love to attach themselves to the anodes). The steering tab anode still looks new with no erosion or pitting.

Perhaps too late here but the owner's manual recommends brushing the anode clean and replacing it when it is half expended.

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