Anode Replacement

Spyderweb

Planned to replace anodes on 330.  1st time to try such a project.  Always heard they should be replaced when 50% used up by weight.  Never knew what 50% looked like.  So I bought 2 sets of Magnesium anodes (freshwater boater).  Figured I'd be smart and weigh each piece , then compare to weight of what I take off to get an idea.  Haven't done project yet (boat is 100 miles away still covered) but I remembered the broker replaced the prop anodes last Spring when I bought the boat since he reconditioned props and said they would look better with new anodes.  Just today I remembered I kept the old ones, so I weighed an old one and compared to weight of new one.  New is 130 grams, old is 158 grams. WHAT???  That's backwards.

So, some questions.  New ones are from boat zinc.com.  Are they apt tp be significantly lighter than new ones form Merc?

Is it possible original anode was a different metal which is significantly heavier than magnesium?  Boat was freshwater only, bought from original owner. 

Paul

Liberty44140

Could be the old ones are zinc which is heavy

Ian

Aluminum is around 1.5 times the weight of magnesium so the old are likely Al. I was told that Al is best to use for both, especially if there is a chance of getting into salt and Mg should never be used in salt. I have Al from boatzincs that covers me for trips into salt as needed and last 2 seasons. Last year I added the prop anode as an add on.

Spyderweb

If Al is 1.5 times heavier that would explain it.  No chance I'll ever be in salt water with it so I guess I'm good with Mg.   Judging from the photo below which is how 2008 Rinker looked when I bought it (Fall 2017) I'm not impressed with the protection of the drive Al did.  I had a 2007 Bayliner 285 bought new that I sold for this boat and the drives looked close to new and the anodes were never changed.  Of course, I guess I don't know if it had Al or Mg anodes.

So I guess my little experiment doesn't tell me much about the difference in weight from a used Mg and a new one UNLESS someone out there has some pics of used Mg and weighs them and posts their weight and photos here.

Thanks guys.

Willhound

Here you go @Spyderweb. I just happened to change mine the other day and haven't tossed them yet. Both trim cylinder anodes came out the same, although one wavers at 79 grams before hitting 80. These are magnesium and only a year old. Lots of current in our marina. Sorry, no prop anode on my 2000 model. 

Spyderweb

WOW! This is terrific.  Here's the results. 1st photo is 93% of original, 2nd is 88%, and 3rd and 4th are 90%.  So, does this mean you could safely go another 2-3 years?    

Willhound

I dunno. They looked pretty bad to me so I just changed them. So you're saying a brand new cylinder anode only weighs about 90 grams? Another consideration is that the way these were deforming I had a hard time getting a socket on the bolts as it was. Couldn't imagine in another year.

Liberty44140

So here is my question. Do these really not have another season on them? They say to replace at 50% and those are maybe 20% eaten, probably less. I get the theory that its cheap insurance so might as well do it, I typically do these every year too, just looking for thoughts as I'd think that if you went another year you would be just fine. 

Ian

I have found every 2 years, by second year they are down, maybe could last a season (5 months worth here) but as you say, cheap insurance.

Dream_Inn

I've let mine dwindle down pretty good, probably less than 50% easily.  No problems.  Yes, I do keep mine on a lift, so they're really on in use when in the water.  But, I can watch them on my drives (& tabs) every weekend when I pull her out.  I went 5 summers on my last set (now, remember, it's out of the water the majority of the time, but I'm throwing it out there as a reference).  

Spyderweb

Here's the anode the broker replaced on my 330 when I bought it.  No idea how old it is.  But working the math that Al is 49% heavier than Mg, this turns out to be 18% used up.  Definitely more uniformly pitted than anodes of @Willhound.  

Liberty44140

That is pretty interesting Spyder. I would have guessed that to be at least 30% used up. So while I would absolutely change an anode that looks like that, assuming it really is just 18% gone it could have gone another season. Interesting....

Willhound

Got me wondering too. Have never heard of the 50% thing. Another thought though, I'm thinking that as mass drops off the rate of disintegration will not be linear but rather exponential. Same external forces with electrical and water current and impurities in the water but less mass to handle the load. So at say 25% gone I'd think you don't have another full year left??

Spyderweb

@icoultha made the comment above that Al is about 1.5 times the weight of Mg so it got me thinking.  I looked it up and found Mg is 33% lighter than Al.  Working the math, then, Al is 49% heavier than Mg.  Close enough for what we're doing.

I kinda agree with @Willhound that it may not be a linear rate of disintegration.  When I get the rest of mine changed (next week?) I'll bring them home, weigh and report.  At least it will give me somewhat of an idea in my situation about how fast they're eroding.

Kaptain_Ken

Kaptain_Ken

Here, even better since posts like this usually start debates:  https://www.boatzincs.com/pdfs/Chart_A-Which_Anodes_Should_I_Use.pdf

Spyderweb

Finally got up to boat to replace the anodes.  After looking at them, my theory of weighing old and new to determine how much the old are used up is on a bit shaky ground.  My previous assumption was that different manufacturers of anodes would be same dimensions and weight, hence a good before and after comparison.  A look at the 2 prop anodes below shows the original that came with the boat (right) and the one I replaced after 1 year (left) are differing heights.  So I don't know if they (or the other ones) actually started out same mass when new.

New ones are Mg, old are Al.  I used the conversion that the Al would be 149% heavier than Mg when new.  

But, assuming all started life equal, here's my results.  
Flat trim tab is 89% of original
Plate is 93% of original
Old prop anode is 82% original and 1 year prop anode is 96%
Ram anodes are 91% original

Paul