Galvanic Corrosion Fresh Water Aluminum

I have just had my Roamer hauled. The last time it was out of the water was 6 or 7 years ago. For the most part the bottom is in very good shape. There is some corrosion around the struts and at the point where the shaft exits the hull. The zinc on the keel and the zincs on the shafts were in like new condition. The zincs on the rudders were missing and my guess it was the a bolt issue. I am going to try and post some pics for general discussion.
One of the service guys at my marina suggested that I look to see if the cutlass bearings were made out of brass or Phyenloic. They should be isolated and not made of brass. The shafts should be isolated as well.
The cutlass bearings are in like new condition as well. Too bad if they have to be replaced.
The reason the boat was hauled was to replace the rubber in the coupling assembly, (stuffing box). These couplings have been cleaned and are brass with Chris Craft stamp.
The plan now is to isolate the shafts, check for proper grounding, install new annodes, and replace cutlass bearings with correct material. The bottom job will wait til the fall. I am also going to replace the shore cable. We are also going to look at the CAPAC and see if it is working. Any comments or suggestions are most welcome.

Absolutely did not mean zinc.

Some pictures of corrosion

Some additional corrosion pics

here are some additional running gear pics

Galvanic Corrosion an engineer's story

Dear Roamer Owners, The following report is what I received from my local service manager. I have just edited out any personal references.

Good news is, we’re one of the very few places in these parts that do understand metal hulls and what it takes to maintain them properly. As I mentioned, I’ve worked on them for a couple decades now, and specialized in metallurgy at GA Tech where I graduated in Mechanical Engineering. Much of my first ten years in the metalworking industry was devoted to metallurgy and corrosion…we manufactured bearings, for which corrosion (and how to survive it) is the primary concern in many cases. I serve as operations manager for our company, so our Service departments and managers report to me through our corporate service director. I don’t have to fix too many boats personally these days, thank goodness. But I do get involved with unusual situations, older boats, big jobs like re-powers or replacing an old obsolete powertrain with a new system, things where having an engineering background and 30 years experience working on boats is helpful.

I’ve owned an aluminum boat(s) for over 20 years myself and currently have a 28’ Marinette, which this Spring will be hauled out and painted correctly for the first time since it was born in 1973. I can’t wait! It’s hull makes yours look brand new, though, so I’m getting ready for a real challenge. Before I start, if you want to just maybe trade boats, let me know!

In terms of corrosion, there are three main ways to prevent it – first paint the bottom correctly, second isolate all dissimilar metals underwater, and third have the correct anodes.

My experience has been that’s the order of importance as well, although paint and dissimilar metals are close. If the first two are correct, the third (anodes) becomes much less a factor. By the same token, if the first two are incorrect, corrosion will proceed regardless of anodes, and the anodes themselves will be rapidly consumed. Their effectiveness starts to diminish the day they are installed, anyways, because they corrode, deplete, and become less active over time…so you want to do everything possible with paint and dissimilar metals, to give your anodes a very long effective lifespan.

Here is the right way to paint an aluminum hull – remove 100% of the underwater gear (water pickups, shafts, rudders, etc) and sandblast the hull to bare metal (we use a water/sand blaster that’s very effective and leaves the perfect rough surface for paint to adhere). Then one very thin coat of Interlux Vinyl Ester Primewash which is a 3-part zinc chromate. Then 5 or 6 coats of Interlux Interprotect Epoxy, same material used to repair and protect blistered fiberglass hulls. Very important to paint the insides of the shaft logs, that’s a prime area for corrosion as it’s close proximity to the stainless shafts. Then we “fair” any visible blemishes and pock-holes from the corrosion, mostly for asthetics, we use an Interlux epoxy called VC Watertite in most cases, although for large areas we’ll use a West System fairing mixture. Then we sand as necessary to get it all smooth, then finish with two or three more coats of Interlux Interprotect Epoxy. Soon as that coat is dry enough to touch without coming off on your finger, we immediately follow with one topcoat of Interlux Trilux antifouling paint, made just for aluminum. We let it dry, then give a second coat of Trilux the next day. You could add more coats for a longer protection against algae (there is no other sort of fouling on these lakes).

The painting itself costs $139 per foot. The other parts of the process – blasting, removing all the stuff, fairing, etc – is additional as required, but the worst I’ve seen has only cost again what the paint cost (so in other words double). Looking at your bottom, my guess is the estimates you’ve already gotten in the mid $8k’s are about right. The worst bottom I can remember was about $10,000 and it was a big cruiser.

Here is the right way to protect against dissimilar metals – isolate them electrically from the hull, whatever that takes. For thru-hull water pickups, it means plastic plates and rings, so the bronze never touches the aluminum. For rudders, it means plastic bushings at the shaft and cable connections. For propshafts, it’s a little more involved and requires the manufacture of plastic bushings to isolate the bolts, and plastic shim between coupler halves. It also usually requires a little machine work on the coupler halves. For trim tabs, it’s always different because there’s so many kinds of tabs. We also make sure both propshaft cutlass bearings, and also the bearings for your rudder shaft logs, are non-metallic and correctly installed. We also blast the rudders and epoxy them just like the hull, especially important if they’re stainless (that’s a big surface area).

Bronze is the worst underwater if electrically “common” to your hull, stainless is next. Plain steel isn’t too bad, of course it rusts all by itself. But your hull itself will act as an anode for the bronze prop and stainless shafts, less so for the plain steel, steadily corroding, if they are electrically “common”.

To make sure your hull isn’t protecting your neighbor’s bronze prop, you need a galvanic isolator properly installed on the ground coming into your boat’s AC electrical system, and also to make sure that 100% of both the AC and DC grounds in your boat all go to exactly the SAME point (usually it’s a threaded stud) in such a manner as to prevent your hull itself from ever being used as a conductor for any current, for any devices on your boat like marine air, radios, lights, etc. All that helps eliminate what they call “stray current corrosion” which you hear horror stories about.

What it costs to isolate the underwater metals depends on what has to be done, but normally a twin engine boat with generator for everything in that respect would be under $1,000.

For anodes, you have two basic types, the zinc type for saltwater and magnesium/aluminum alloy type for freshwater. You want to use the freshwater type. Many are available off the shelf, and I’ve also worked with a company in Canada to have custom made anodes on several boats. We put one on each shaft, one on each rudder, a longer one for the keel and a diver’s dream on the transom. If the trim tabs are stainless, we put one on each. If they’re steel, it’s your call, they probably won’t do a whole lot if properly epoxy coated. It is possible to over-anode a boat and that can be very destructive to your hull, there are formulas you can use to calculate correct anode amounts, but again, if it’s painted right and dissimilar metals isolated, this anode combination will work just fine for many, many years. Over the years I have gravitated to a process with better paint / less anodes where years ago we used less effective paint / more anodes…believe me, the better paint method is superior.

Properly painted, isolated, and anoded your Kapoc meter if working correctly will read about 0.8 in the winter and 1.2 in the summer. And yes, never paint the surface of your Kapoc meter thru-hull. You can always get a feel for its functionality by pressing the button, and grounding the shaft to the hull with a wrech or something…it should drop about .1 or .2. If not, either the meter is faulty, or the shafts are not isolated. But the meter itself as you know doesn’t protect the boat, it just sorta gives you an idea how the anodes are doing, and if some underwater metal has gotten “grounded” to the hull somehow.

Cost of anodes varies a lot but the whole boats should be less than $500 even if you go with custom made, for example on the keel.

Aside from the corrosion issues, you’re at this time replacing a coupling. Couplings are supplied undersize and should be “fit and faced” with your shaft. The process is first to remove the shaft, and set on V-blocks and checked with a dial indicator for straightness. A new shaft should be within about .001 everywhere for runout, I’ve never checked a new shaft that was over .002 even on a big long one. Can you use a shaft that’s .005? Sure, but it’ll vibrate more. If it’s .010 or .020 you probably won’t be able to stand the vibration, depending on where it’s bent and how.

When you’re comfortable with the shaft, then you chuck the new coupling in the lathe and use a boring tool, reamer, or if it’s close sometimes just a lapping mandrel with compound to create a nice “slip” fit…don’t ever want a press-fit coupling. It should have .0005 clearance ideally but no more than .001 or so. You make sure the set-screws for the coupling have a “dimple” machined on the shaft, so they don’t cause a burr when you tighten them (the burr will make it impossible to remove the coupling easily later).

Once you’ve got the perfect fit, you install the coupling onto the shaft, tighten the setscrews, and chuck that shaft in the lathe, then use a regular cutting tool to “face off” the coupling surface where it bolts to the transmission coupling half….that tolerance is .001 maximum runout. I’ve seen brand new couplings be only three or four thousandths off, and I’ve seen them .030 off. But one thing is sure, they are never, never perfect when they come from the manufacturer. If you don’t follow this process, you will have unacceptable runout, and more vibration.

Runout at the coupling face will cause your shaft to “whip” in the middle….each end is held steady, one by the coupling and the other by your cutlass bearing. But if there is runout on the coupling face, when you bolt that coupling tightly together, it tries to “bend” the shaft. That few thousandths runout at your coupling multiplies to a LOT two or three feet away at the middle of your shaft.

Some techs who don’t have access (or time) for a machine shop will try to use shims to compensate, but that’s almost impossible for a coupling that has electrical isolation (plastic) parts between the halves, and it’s tough to work on for the next guy. So we always fit and face the coupling properly. I think it’s essential, and it’s exactly the same process that happens hundreds of times each day in manufacturing plants where they align electric motor shafts to pump shafts, driveshafts, etc.

In your case, since neither of your shafts are isolated and you need to fit and face one anyways, I’d recommend you pull out both shafts, both couplings, have them both fit & faced and electrical isolation pieces made. That process is about $150-200 per side and WELL worth it. Your boat will vibrate far less, and hull will corrode far less. It’ll take another week or so for turnaround, but again I’d highly recommend it.

Then if you are going to get it painted in the next year, that’s probably all I’d do for now. If it’s going to be two or three years, I’d replace anodes and have our shop pressure wash your corrosion areas and clean as best they can, and apply several coats of barrier epoxy to them.

While you’ve got the propshafts out, if you find your cutlass bearings are metallic, I would definitely swap those to non-metallic right now. That will help save your struts from corrosion. The bearings themselves are less than $100 each, and replacing them is one hour if they slide out easily…but if they’re brass, they probably won’t slide easily (corrosion). So we have to take a hacksaw, or sawz-all, and cut a groove into the bearing front-to-back which relieves the pressure and lets them slide out easily. That process will take two hours per side, instead of one hour. Even if they are brass, but were installed recently, it’s POSSIBLE they might slide out without the need for cutting through them…but cutting them isn’t a horrible job, just a little time consuming that’s all.

That’s my recommendations for your boat. The impressed current thing does work and it does help, but I don’t think it’s 100%. Two reasons I say that. First is from what I see on Mercury outdrives, including the ones on my own houseboat. It’s a pretty expensive option to get MerCathode, and you’ve gotta believe they have engineered it pretty well! But the drives still corrode over time, even with MerCathode, and even in freshwater. My houseboat with MerCathode is a 1992 and I’ve replaced the bearing carrier (most likely piece to corrode, acts as the anode for the stainless propshaft) twice, so even with MerCathode it’s gone in less than 10 years and really looks awful in only a couple years. Second reason is metal hulled ships I’ve seen in yards along the coast, most saltwater metal hulls used impressed current systems, but you know what they still get hauled-out, blasted and painted and they still corrode. Of course, it’s saltwater.

On the other hand, I’ve hauled-out boats we painted and isolated correctly 6 or 7 years ago that still look brand-new, with anodes having very little if any activity. There is a nice 32’ Marinette on Dock E you could checkout, called Water Music, that was done at least 6 years ago and it’s still perfect. I don’t know for sure how long the system will last, but I think if you change anodes every 5 years or so and repair any collision damages etc, it would out-live us both.

Let us know how we can help you Steve, you’ve got a real nice boat and it’s clear that you’ve taken great care of it. Whatever we can do to help in any regard keep it in top shape, please let us know.

Galvanic corrosion running gear isolation and Shafts True?

Since the Roamer is out of the water and undergoing galvanic isolation I decided to check the trueness of the shafts. Here are the results for our Roamer knowledge base. As always opinions, experience, and resources are most welcome. For general knowledge I have never hit anything and the props are perfect. Perhaps the previous owner had some collision but the vibration has been very minimal at most RPM points. I usually cruise at 2100.


The Port shaft is not perfect, but pretty straight and I would say serviceable. I couldn’t measure anything over about .003” runout anywhere on the shaft.

The Starboard shaft is fairly bent. If you support the shaft on V-blocks at each end, next to where the coupler goes at one end, and where it rides in the cutlass bearing on the other, then the tapered portion where the prop goes has runout or “wobble” of about .024” which is about the same as a sparkplug gap. A new shaft would be .001 runout at most, usually less.

The taper is the worst place it’s bent (which is normal, that’s where it bends if you hit something with the prop) however in the very middle it’s bent as well, about .008” and bent a little less than that on the coupler end.

This shaft is 1.375” diameter so to give a perspective, with .024” runout at that diameter, it translates to (22*.024)/1.375 = .38 inches runout at the tips of a correctly tuned 22” diameter prop. That’s about 3/8” and in my experience would cause noticeable vibration, probably premature wear in a few places like the cutlass and packing if you run it fast and for many hours, but the main annoyance would be vibration (makes screws and bolts loosen-up around the boat, too)

It takes 1-2 weeks to get a new shaft and I don’t know the pricing on 1-3/8 shaft precisely, we recently got a couple 1-1/4” shafts about the same length that were around $500 each plus shipping, I would expect the 1-3/8” to be a little higher due to the larger diameter.