Stainless Steel Boats

Discussion in 'Metal Boat Building' started by S Steel, Jun 27, 2015.

  1. S Steel
    Joined: Jun 2015
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    S Steel Junior Member

    I have looked at the issues of stainless steel boats and now have a viewpoint:

    First off, most boat design links warn about stainless steel in stagnant water that doesn't have enough oxygen in it. Then they warn about crevice corrosion.

    Well, welds are built up and ground smooth so I am not worried about crevice corrosion on the hull.

    As for the water it is much simpler. Stainless steel that has been tested in salt water tides loses about 0.001" per year. So a 1/8" thick hull could lose 1/4 of its thickness in 31 years.

    Basically, a stainless steel hull in salt water must be painted on the outside. Then I might as well go on and require that a stainless steel hull in fresh water be painted on the outside. Now to paint the stainless steel it must have a brushed or sand-blasted finish. So the only risk here is something like, will stainless steel with a #4 brushed finish hold paint ? (Well, it holds powder coating on my mailbox.)

    Now why is stainless steel specified if the outside of the hull must be painted ? Well, the inside of the hull will be resistant to rust without painting. And because the hull is stainless steel then the deck can be stainless steel without galvanic corrosion. And so the deck will be resistant to rust without painting.

    The next issue is what kind of stainless steel to use ? Well, since the outside of the hull will be painted then I would like to use 430 stainless steel for the hull. The reason to use 430 stainless steel is that it is economical but still has a high chromium content. Also, there would be very little galvanic corrosion between 430 stainless steel and 304 or 316. But the fact is that 430 stainless steel should be pre-heated before welding and that would be difficult with large parts. So I must specify 304L stainless steel for a welded boat hull. Then 304L stainless steel will be about 40% higher cost than 430 stainless steel but much easier to weld.

    Then the stainless steel for the deck can be either 304L or the more expensive 316L. In fact, 316L stainless steel would be the correct choice for the unpainted deck.

    But really, the material cost is not the largest cost of the boat. Just buy the stainless steel material wholesale and personally take the business risks
    .
     
  2. waikikin
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    waikikin Senior Member

    I have done some application of ss to deck of small craft, a 40' mooring lighter. Deck was 5mm to a 3 meter wide band to cl(1.5m each way) & "side decks" 4mm, a substantial belting to deck edge also. Basically the plating cost was 3-4 x steel, welding consumables was around x 10. I looked over her last week infact around 14 years after installation, none of the usual rust streaks of a steel workboat apparent, at the time of rebuilding she was 40, the owners wanted durability & much attention applied to limbers & not creating traps within the steel topsides & bilges- even a 25mm plate keel rather than the original box keel.

    Jeff.
     
  3. gonzo
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    gonzo Senior Member

    The welds changed the composition and crystal structure of the metal. How are you testing or calculating what you have now?
     
  4. S Steel
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    S Steel Junior Member

    Shop-built metal boats are hand welded and ground smooth. I would use TIG or MIG welding which limits the damage at the edges of the weld.

    Stainless steel in 304L is highly recommended for welding.

    Stainless steel in 430 bends just like mild steel but there are several warnings about welding 430.

    The 304L stainless steel costs about 40% more than 430
    .
     
  5. gonzo
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    gonzo Senior Member

    I didn't say there was damage at the welds, but a change in chemical and physical structure. What alloy and grade do you have after it is welded? If by "shop-built" you mean commercial builders, I don't know of anyone building in stainless.
     
  6. TANSL
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    TANSL Senior Member

    Whenever a metal is welded, base metal components are burning, disappear, and, if not protected with inert gas, can be incorporated foreign elements to the welding. Not only with stainless steel. It is clear, then, that in welding can cause changes in the chemical structure. I do not understand very well, and I would like, please, that you will explain, which and how are changes in the physical structure. Thank You.
     
  7. S Steel
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    S Steel Junior Member

    If the boat design is welded metal than the design has allowed for the characteristics of the weld.

    And welding is well known and widely used. We can't just pop-up and say that welding has been found to be unsuccessful.

    The weight of the boat is an issue. I like the metal boat designs with 3mm plate and not thicker.

    The cost of stainless steel is an issue. A 15,000 pound boat would have a stainless steel material cost of about $26,000 or make that $30,000. (Well, 304L stainless steel plate is sold on coils and by the pound. It should be less than $1.75 a pound. For structural shapes and for instance, a 2 x 2 x 0.125 hot-rolled angle in 304L should be less than $80 for twenty feet. But cold rolled structural shapes are near ornamental and not much more expensive. One more note, there is a 304 brushed tube beyond cold-rolled and it has a nice glow to it instead of a satin brush.)


    Here is a link to a painted stainless steel boat:

    https://www.youtube.com/watch?v=kj49E9m4wf0


    The question is, does a #4 brushed finish hold paint ? Well, the #4 finish is a standard and affordable mill finish available on stainless steel plate
    .
     
  8. waikikin
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    waikikin Senior Member

    Welding is successful, stainless to stainless, steel to steel & staino to steel. A few years back the BT challenge yachts had staino decks, cockpits are good in it(painted). Your costing looks cheap, maybe just the raw plate & geometrics for the build? For us mig wire goes from maybe 35/40 for steel to 300+ a roll, grinding consumables cost some more.
    Jeff.
     
  9. TANSL
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    TANSL Senior Member

    The bimetal strip, consisting of two different metals welded by explosion, allows to combine structures of different metals, for example, steel hull and aluminum superstructure. Also, I seem to remember that there are electrodes allowing welding carbon steel to stainless steel.
     
  10. Milehog
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    Milehog Clever Quip

    He doesn't want to address that. SS has it's place but it can be tricky.
    Steel and aluminum's quirks can be reliably dealt with with reasonable effort. I haven't seen evidence of SS boat construction reaching that standard.
     
  11. Jamie Kennedy
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    Jamie Kennedy Senior Member

    gonzo is referring to intergranular corrosion that occurs at the grain boundaries after welding stainless steel. I think 304L and 316L are both more resistant to this form of corrosion but I can't remember how. Here is Wikipedia...

    https://en.wikipedia.org/wiki/Intergranular_corrosion

    p.s. Based in this article I gather that 304L and 316L they are made to be resistant to intergranular corrosion welding by reducing the carbon content and by adding some element that form carbides to further reduce the carbon available to form chromium carbine at weld boundaries. It is the formation of chromium carbide precipitate after welding that can reduce the chromium content and make it more susceptible to corrosion. I think there is also some dissimilar galvanic type corrosion that can take place at these grain boundaries around welds also. I never really had an occasion to study this. I only had to teach it a couple of times, and you know what they say about those that know and those that don't... ;-)

    Here is where these grades fit in with the other austenitic stainless steel grades...
    300 Series—austenitic chromium-nickel alloys Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
    Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
    Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.[6]
    Type 304—the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506 (not to be confused with A2 tool steel).[6]
    Type 304L—same as the 304 grade but lower carbon content to increase weldability. Is slightly weaker than 304.
    Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.
    Type 308—used as the filler metal when welding 304.
    Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.
    Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.
    Type 316L—is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications, as well exclusively in the fabrication of reactor pressure vessels for boiling water reactors, due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.[6]
    Type 316Ti—variant of type 316 that includes titanium for heat resistance. It is used in flexible chimney liners.
    Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
     
  12. S Steel
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    S Steel Junior Member


  13. daiquiri
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    daiquiri Engineering and Design

    The level of weld finishing is only a part of the equation when it comes to crevice corrosion. Smooth welds help avoiding it in areas where welded joints are exposed to a well-aerated area and not covered by other material like gaskets, screws, bolts, washers etc.

    This type of corrosion happens wherever there is a differential aeration between two adjacent parts of the same metal, meaning that one part of the same piece of metal is close to well-aerated zone and the other part is hidden in a moist niche. Even a stagnant water on a flat surface triggers the differential-aeration corrosion, with no welds involved at all.

    When it comes to corrosion in general, the biggest part of the job is done at the design stage, by carefully thinking the shape and layout of metallic components which will be used in the construction. This is even more true for the crevice corrosion.

    Cheers
     
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