stainless stringer on mild steel hull

Discussion in 'Metal Boat Building' started by John Dickie, Apr 3, 2019.

  1. John Dickie
    Joined: Apr 2019
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    John Dickie New Member

    I'm slowly building a Charles Wittholz Departure 35.The plan is a steel hull,wooden deck,wooden deck beams,of which he approved.He specified a 4" mild steel deck stringer along the sheer.Due to the potential for corrosion with planking fastened to it,is using a stainless steel stringer fully welded to the hull an option?We have a steel here,3cr12,welded with 308L to the hull.Any advice?
     
  2. M&M Ovenden
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    M&M Ovenden Senior Member

    I think you will end up with different corrosion issues with stainless. It needs oxygen to maintain it's self protection. I would just stick to steel and make sure it's blasted to white metal, has a good paint scheme and good bedding for the wood.

    Can you post a diagram of how the deck and beams are to be attached ?

    Cheers,
    Mark
     
  3. waikikin
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    waikikin Senior Member

    Go with the stainless, timber to mild steel is a curse after a few years. I'd be making sure you pre drill the staino "on the bench" before welding in.

    Jeff.
     
  4. John Dickie
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    John Dickie New Member

    the NA has specified these brackets at 356mm centres between the frames.The deck will be solid quarter sawn Muninga. IMG_20190404_113339_resized_20190404_113433358[5802].JPG
     
  5. M&M Ovenden
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    M&M Ovenden Senior Member

  6. rwatson
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    rwatson Senior Member

    I hate that deck arrangement.
    They should have extended the plywood deck over the steel edge, and capped it. It's going to hold water like crazy, even with sealant.

    If you can afford stainless, it will help.
     
  7. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    That stringer is a flange and is there to give enough sealing surface between a plywood deck and the hull. The lip on the outside is there to protect the ply edge and provide space for the sealant to form a gasket. It is a solution that was designed before epoxy was common and employed everywhere. A modern solution would be to bed in thickened epoxy and as rwatson said cap the ply with a hardwood strip or glass overlay.

    Going to a traditional laid deck changes things. There is no need for the raised lip anymore, the coverboard is solid not ply. If the flange rusts just remove the coverboard and repair. Use mild steel, paint with a good epoxy based system, bed with butyl rubber and replace as needed in 20-30 years. You will have more trouble with the laid deck anyhow then with the mild steel flange.

    3cr12 is not a material to be used. From an internet datasheet:

    "Grade 3CR12 stainless steels can be employed in applications for which aluminium, galvanized or carbon steels provide undesirable results, owing to its resistance to strong acids and alkalis, and cracking resulted from chloride stress corrosion. However, unlike grade 304, grade 3CR12 has minimal resistance to crevice and pitting corrosion in the presence of chloride.

    Under ambient conditions, grade 3CR12 has improved resistance to water and chloride substances, as the corrosivity of chloride contents will be mitigated by the nitrate and sulphate ions. One of the major drawbacks of grade 3CR12 is that the surface of the material is subjected to mild corrosion when exposed to any type of environment."
     

  8. John Dickie
    Joined: Apr 2019
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    Location: South Africa

    John Dickie New Member

    Thanks for the advice everyone-I'll stick to epoxy painted mild steel,bed the coverboard down and cut that lip off.Drawings off to the laser cutters on Monday.
    Thanks again
    John
     
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