STEEL HULLS with Composite Superstructure / Topsides

Discussion in 'Boatbuilding' started by brian eiland, Jun 16, 2013.

  1. brian eiland
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    brian eiland Senior Member

    Regarding Hull Form

    ....interesting sub-title within Kasten's "Metal Boats for Bluewater"

    Efficiency and performance are high on the list amongst the myriad considerations that go into shaping a hull. With metal hulls, there is always a question of whether a vessel should be rounded or "chine" shaped.

    Assuming two vessels are of equally good design, whether the hull is rounded or single chine will not have much impact on their performance, i.e. they will be more or less equivalent. Here are a few considerations that may be of some benefit when considering the choice between rounded or single chine hull shapes...
    •If one were to take a single chine hull form and simply introduce a fairly large radius instead of the chine, the newly rounded vessel's wetted surface would be less; displacement would be less; and initial stability would be less, and the comparison somewhat skewed.
    •If instead one were to design the two vessels so that they had exactly the same length and displacement, exactly the same sail area and rig, no "reverse" to the garboard area, and with hull forms as similar to each other as one could make them, one would quickly observe the following:
    •In terms of interior hull space, a chine hull form will often be slightly less wide at sole level and slightly wider at the waterline level, so possibly a bit less room to walk around but larger seats and berths.
    •The single chine hull form will have slightly greater initial stability (greater shape stability), and will therefore have slightly greater sail carrying ability at typical heel angles under sail.
    •The single chine hull form will have greater roll dampening (faster roll decay).
    •The rounded hull form will have a slightly more gentle rolling motion.
    •The chine hull form will have slightly greater wetted surface.
    •This implies that the rounded hull form will have slightly less resistance at slow speeds where wetted surface dominates the total resistance.
    •The chine hull can be designed to equalize or reverse that resistance equation at higher speeds due to wake differences resulting from the chine hull being able to have a slightly flatter run.



    Aside from these generalities, relative performance would be difficult to pre-judge. We can however observe the following:
    •Given the same sail area, when sailing at slow speeds in light airs, one might see the rounded hull form show a slight advantage due to having slightly less wetted surface.
    •When sailing fast, a chine hull form will be more likely to exhibit greater dynamic lift, especially when surfing.
    •Especially in heavier air, one might even see a slight advantage to windward with the chine hull.


    Given that those observations do not reveal any special deficiency with regard to a single chine hull we can additionally observe the following:
    •When creating a new design, wetted surface is one of the determining factors of sail area.
    •Having slightly greater wetted surface, a single chine hull should therefore be given slightly more sail area, so its slightly greater wetted surface will become a non-issue.
    •If the chine hull is given slightly more sail area, it will therefore be subject to a slightly greater heeling force.
    •However the single chine hull form will have inherently greater "shape stability" in order to resist that heeling force.
    •One can therefore expect the sail carrying ability to be essentially equalized.
    •Therefore with good design, there is no performance hit at low speeds, and there is ordinarily a performance gain at high sailing speeds.


    Among the above considerations, the one factor that seems to favor the rounded hull form most definitively is that of having a slightly more gentle rolling motion. In other words, a slower "deceleration" at the end of each roll. On the other hand, rolling motions will decay more quickly with a single chine hull form. Even these factors can be more or less equivocated via correct hull design.
     
  2. Yobarnacle
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    Yobarnacle Senior Member

    I have worked some old tugs with steel riveted hulls and wooden houses and decks.
    They lasted a long time!
     
  3. michaeljc
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    michaeljc Senior Member

    In salt or fresh water? Some of the early steels were more corrosion resistant. Try that with modern mild steel.
     
  4. Titirangi

    Titirangi Previous Member

    If by early steels you meant wrought iron hulls, the very small carbon content in puddling iron (wrought iron) if high quality did hold out against rust longer than steels do. Mild steel in it's current composition is really no different from mill rolled steel production 80yrs ago just slightly higher alloy content from recycling.
     
  5. Yobarnacle
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    Yobarnacle Senior Member

  6. MikeJohns
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    MikeJohns Senior Member

    I've had a lot of involvement with riveted steel vessel repairs. From around 1900 on Iron is very rare and nearly all the hulls are actually steel when tested. But open hearth steel often with higher sulphur content but they aren't any more corrosion resistant.

    Cor-ten and other higher strength steels are only justified for weight saving in shipbuilding , or for plating smaller boats but never for their corrosion properties since they need coating just the same and corrode at the same rate in marine environments .

    There's plenty of ancient steel boats around and they can last indefinately if well designed and the paint is patched whenever it is breached.
     
  7. jonr
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    jonr Senior Member

    Why not galvanize the steel? A hull is too big to dip in a tank, but thermal arc spray can do it.
     
  8. Yobarnacle
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    Yobarnacle Senior Member

    Epoxy works well on steel.

    We have our hulls waterblasted. Next morning a thin film of rust has developed.
    The epoxy primer adheres best to this thin rust film. Better than to bright steel.

    With some touch up repairs for scrapes and dings, the epoxy is good for at least 5 years.
    Commercial vessels get hauled every 5 years by law. Might last a lot longer on a yacht.
     
  9. Yobarnacle
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    Yobarnacle Senior Member

    Ever watch work in a body shop?
    Old metal cut out. surrounding metal buffed bright.
    heated with a propane torch and lead rubbed on to tin it.
    Excess lead removed with steel wool while still hot.
    New steel patch likewise "tinned" and clamped or pop riveted in place.
    Heat applied to solder the two tinned mating surfaces.
    some filler around the edge and sanding, painting.

    Just heat a workable area of your hull, smear on the lead as it melts, polish off excess with steel wool, move on to adjacent area. You could tin the entire steel hull with lead this way.
    Better than galvanizing! :D

    But I'd STILL recommend epoxy overall!
     
  10. MikeJohns
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    MikeJohns Senior Member

    Protective coatings for corrosion should be less noble than the metals they coat not more noble. With lead coating you'd accelerate any local corrosion from coating damage. Not to mention the health hazards of lead.

    Aluminium can be metalsprayed onto steel as a protective coating just as Zinc can. It's not hard to keep steel from corroding in a marine environment.
     
  11. Yobarnacle
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    Yobarnacle Senior Member

    Hmmm. I never heard that.
    Not disputing you, but if the coating needs to be less noble, why did we use white lead and red lead on steel for so many decades?
     
  12. MikeJohns
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    MikeJohns Senior Member


    Red lead is lead oxide while White lead is lead oxide in a carbonate and hydroxide compound. Neither are in their metal form so contact nobility isn’t an issue.

    They were a good binder and pigment with drying oils which made them good for mastic anticorrosive paints. But it’s a long way from actually coating steel with lead. But if you could get a thick enough coating and never breach it it would be a good inert coating especially underwater as for example would metal spraying copper (which is close to lead on the galvanic scale). But any breach in the coating in a marine environment would result in a deep and rapid local pit corrosion.
     
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  13. Yobarnacle
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    Yobarnacle Senior Member

    Thank you MikeJohns

    I BELIEVE you. :)
     
  14. brian eiland
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    brian eiland Senior Member


  15. Yobarnacle
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    Yobarnacle Senior Member

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