Hull protection

Discussion in 'Powerboats' started by Mik the stick, Nov 22, 2014.

  1. rxcomposite
    Joined: Jan 2005
    Posts: 1,954
    Likes: 153, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Here is from Wiki. "For this to work there must be an electron pathway between the anode and the metal to be protected (e.g., a wire or direct contact) and an ion pathway between both the oxidizing agent (e.g., water or moist soil) and the anode, and the oxidizing agent and the metal to be protected, thus forming a closed circuit; therefore simply bolting a piece of active metal such as zinc to a less active metal, such as mild steel, in air (a poor conductor and therefore no closed circuit) will not furnish any protection."


    Therefore, withouth any electrical connection, the two dissimilar material in an electrolyte is nothing but a battery with no connection between the leads. It is there (battery) with its potential, but not doing any work and will last a long time.

    On a wooden boat, if the sacrificial anode is not connected electrically to the shaft/propeller/engine/ground, the propeller will get eaten by the electrolyte. If there is an electrical connection the anode will dissolve and coat the propeller. That is why it is called sacrificial anodes.
     
  2. TANSL
    Joined: Sep 2011
    Posts: 5,721
    Likes: 179, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Thanks, Rene, for your clear explanations.
    My mistake was to forget that, to get exchange of electrons, the circuit must be closed and seawater, although it is an electrolyte, does not close the circuit. The example of the battery is very enlightening.
     
  3. rxcomposite
    Joined: Jan 2005
    Posts: 1,954
    Likes: 153, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Youre welcome TANSL. Just so happen I have to explain these to two young NA who are arguing about zinc and aluminum anodes. I had to research.
     
  4. Mik the stick
    Joined: Dec 2012
    Posts: 189
    Likes: 0, Points: 0, Legacy Rep: 6
    Location: Devon

    Mik the stick Senior Member

    Having read the above replies I assume the zinks on the seaplane tender are so small I did not notice or identify them. For me this makes steel the third choice as a building material unless the boat is 100ft or longer. During WW2 Steam gunboats were not a success for many reasons one was the building effort was not far short of a destroyer, with a great deal less ability. I like wooden boats because of the smaller requirement for sacrificial zinks. So this would seem to make aluminium the best choice. I believe a boat in aluminium costs more be cause aluminium is about 3 times the price.
    Zinks are not necessary if you could Guarantee isolation of the metal from the water, BUT you CAN'T. Paint being a very good insulator is still not perfect, I have pictures of a corvette in the Atlantic. Its paintwork looks like a patchwork quilt.
     

  5. Mik the stick
    Joined: Dec 2012
    Posts: 189
    Likes: 0, Points: 0, Legacy Rep: 6
    Location: Devon

    Mik the stick Senior Member

    Researching how to protect steel (because I like steel boats) it seems that epoxy paint is a very good defense. However it requires the boat to be landed cleaned and repainted about once a year. I would cover the hull in epoxy and glass fiber laminate inside and out as if it were a strip planked hull. Then paint with epoxy paint. I think the result would be easier and cheaper to maintain with an almost perfect resistance to rust. Makes me wonder how they repaint large cargo vessels once launched.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.