BajaBruno
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Aluminum boat zinc anode
My little 20' Bayrunner has a 2"x2"x3/8" zinc anode bolted to the transom. It was caulked onto the aluminum and through-bolted (uninsulated SS
bolts). A rather crude wiring system had it connected via the 12VDC neutral to the peripheral systems.
I'm no expert on this, but I wondered if anyone knows why an aluminum boat should need this, or what it is designed to protect? I originally thought
this was a ground plate (which I don't know why I would need, either), but closer inspection reveals it is a zinc.
If I do need this, should it be connected to the neutral circuit? Should it be isolated from the hull?
BTW, my little boat has no AC circuit and rarely spends more than a day or two in the water at any one time, usually anchored far from a marina.
I tried to post a photo, but to no avail.... 
Christopher Bruno, Elk Grove, CA.
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longlegsinlapaz
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No! No! That's the plug you're supposed to remove when you go out fishing to drain out all the water that comes aboard with all those fish you
catch! Hi Amigo!
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Barry A.
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I have had an alum. boat for 30 years (Gregor) and I use it similar to the way you describe-------no zinc plate on mine. I cannot immagine what that
is for.
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Don Alley
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A great board to find technical boat help is THe Hull Truth
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BajaBruno
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Thanks, Don.
Legs, is that why you won't go out in the water with me? Because I don't know where the drain plug is? 
Christopher Bruno, Elk Grove, CA.
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AcuDoc
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I have a 16.5 foot Klamath tin boat and just replaced the anode. It attached to the bottom of the motor mount. It was really on its last leg. I
suppose it is for was ever you get by having the boat in the water and also if you are in marinas a lot. Regardless it needed to be replaced and just
did so yesterday.
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Pescador
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that is a zinc sacrificial plate but it is really of questionable function since you have at least two sacrificials on the motor. They used to use
those when everybody had CB or Citizen Band radios on their boats for communication and someone explained to me that it was for ground plane but that
did not really make any sense either.
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Roberto
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I'm not an expert on aluminum boats, as I've never had one, but isn't there a difference in galvanic potential between zinc and aluminum? As I
understand it, galvanic corrosion CAN occur on aluminum in seawater.
As far as a grounding surface, I agree, it makes no sense.
[Edited on 4-28-2008 by Roberto]
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longlegsinlapaz
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| Quote: | Originally posted by BajaBruno
Legs, is that why you won't go out in the water with me? Because I don't know where the drain plug is? |
That and you probably woulda made me row!
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Taco de Baja
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Anodizing is apparently done to help form an oxide coating on the aluminum to help protect it. Its probably more for boats that are in the water a
lot
Also, the, zinc anode is supposed to have a good contact with the hull, the set up you have, being insulated with caulking, probably does little to
nothing.
http://www.kastenmarine.com/aluminum.htm
| Quote: | Anodizing, a process of electrically causing the formation of a tough oxide film on the surface of aluminum, slows pitting, but anodizing will not
prevent pitting or crevice corrosion.
Aluminum is very active galvanically and will sacrifice itself to any other metal it contacts either directly or indirectly. Aluminum is anodic to
everything except zinc and magnesium, and must be electrically isolated from other metals. In this case, paint, bedding, and a non-conductive plastic
or rubber isolator should all be used together. Unlike tankers, small metal boats are not designed with an appreciable corrosion allowance
Regardless of the bottom paint used, zincs must be used to control stray-current corrosion, to which we can become victim with a metal boat, even
without an electrical system! With a scratch at the bow, and another at the stern, the boat itself becomes the preferred path for any ambient currents
in the water.
Zinc anodes should always be used on an aluminum boat, and generally in the same quantities as with a steel boat, in order to prevent stray-current
corrosion.
After the first few months, inspect the zincs. If they appear active but plenty of material remains, the zincs are doing their job. If they are
seriously wasted, the area as well as the weight of zinc should be increased.
Of course, welding zincs on is best, but for an aluminum boat, the zincs will instead usually be bolted to studs welded onto the hull plate, or bolted
using stainless bolts into a heavy bolting plate welded to the hull. Good electrical connection between the zinc and the hull is
imperative. |
Truth generally lies in the coordination of antagonistic opinions
-Herbert Spencer
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BajaBruno
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Very good, Taco! I read the source article you quoted at http://www.kastenmarine.com/aluminum.htm . It was very informative.
On a related note, I knew that the stainless fasteners through the aluminum hull were causing problems, but I thought simply providing a plastic
spacer would be enough. The article says that waterproof paint, epoxy, or caulking (he recommends all three) is required along with the plastic
isolator.
As Pescador mentions, the Honda outboard motor is protected by several zincs, but it sounds like a zinc attached to the hull is also required.
Christopher Bruno, Elk Grove, CA.
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Barry A.
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I am getting more and more confused by the min.----
I have had stainless steel nuts and bolts thru my hull on my 30+ year old Gregor Alum boat all that time and there is NO evidence of any corrosion at
all. This boat has been outside and upside down on it's trailer all that time (when not being used), never under cover.
The only possible corrosion on the entire boat are microscopic "pits" over the entire outside hull-----which too me are not a problem.
How come I have not had problems??
barry
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