Sacrificial Anodes on Aluminum Hull

Discussion in 'Boat Design' started by STH, Aug 31, 2021.

  1. STH
    Joined: Aug 2021
    Posts: 2
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    Location: Montgomery, AL

    STH New Member

    My sons and I recently wired a new Tracker Grizzly 1648 and our work was heavily influenced by the discussion here ( Wiring aluminum boat https://www.boatdesign.net/threads/wiring-aluminum-boat.17557/page-7 ). The outboard motor is an electric start 2021 Mercury 40 Hp that includes sacrificial anodes on the both the foot and the mount. The motor is clearly grounded, but I have also grounded the hull by running a heavy gauge wire (4 awg) from the negative bus bar to a bare spot on the hull (see attached photo of battery box). We use the boat mostly for fishing on freshwater lakes, but are considering using it in salt or brackish water. Based on this description of our boat and the intended use, would the addition of sacrificial anodes on the aluminum hull itself be necessary or recommended?

    Thanks for any advice that is offered!

    Steve STH Battery Box.jpg
     
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    You only need anodes if you are leaving the boat in the water for weeks at a time.

    Disconnect the battery and hang a clip on "fish" anode.
     
  3. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Why Change from Zinc to Navalloy® Anodes? https://performancemetals.com/pages/why-change-from-zinc-to-aluminum-anodes

    An interesting article. We have always used Zinc anodes for our ocean boats, those that sat in the water say 6 months or more during the summer. The service manager recently recommended aluminum anodes and my response was that as the hull is aluminum why would I want to add another 10 square inches of aluminum and expect the deterioration of the anode as compared to the rest of the hull. (not all the hull is exposed to salt water)
    He threw the name Navalloy out there and this article gives some more information. They do not specifically say that it is strongly recommended for aluminum boats and that was a distinction that I am looking to get info on.

    We recently pulled our boat and I inspected the aluminum anodes and I will admit that the corrosion of the aluminum anodes were significant and appeared to look as corroded, perhaps a bit less, than a standard zinc anode.
    Of course the anodes were smaller than what we normally have had on so it makes sense to see more corrosion on a small anode
    The advantage from the table does show that the Navalloy does work in all waters.

    But I agree with Blueknarr, if the boat is only in water when in use, ie for short periods of time, anodes probably would not be required. BUT as they are cheap, you could always mount one and check its deterioration over time.
     
    Last edited: Sep 2, 2021
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  4. STH
    Joined: Aug 2021
    Posts: 2
    Likes: 0, Points: 1
    Location: Montgomery, AL

    STH New Member

    Thanks for taking the time to respond. It is encouraging that I may not need anodes after all.

    Blueknarr: I was not familiar with a 'fish' anode, so I looked around online and it looks like a potential solution. I might give it try. As a follow-up to your comments, I would be very interested to know why I need to disconnect the battery. Our battery switch is left in the off position when the boat is not in use. In your opinion, would this count as disconnecting the battery?

    Barry: Thanks for the Navalloy link. I too am curious about its use with an aluminum hull. I will see what I can find out in that regard. Also, the picture below is the aluminum anode on the foot of our outboard motor after one week in freshwater. It was shiny prior to that week, so I am not sure what I am seeing here. My initial thought was that the anode was just beginning to be consumed. After reading the Navalloy link, however, it appears that the anode could be developing a coating and is on its way to being insulated. Any thoughts on that?

    Aluminum Anode 1 Week Freshwater.jpg
     
  5. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    You connected the hull to the negative side of the battery.
    If there is any short in any device attached to the battery, then the hull could become electrically charged and greatly increasing the rate of galvanic corrosion to the hull.

    I would do all possible to guarantee no electricity to the hull when it is simply resting in the water.

    Different alloys of anodes protect better in fresh/brackish/salt water. Choose the best alloy for your needs. With a little enginuity any anode could be converted into a 'fish' type and for far less than the advertised cost of some of the metal fish.
     
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  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If the motor has anodes and is grounded to the boat, you already have protection. Inspect them on a regular basis and change them if they are heavily corroded.
     
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