DNV-GL Design Pressure And ISO-12215 Online Calc Mismatch

Discussion in 'Boat Design' started by zstine, Mar 2, 2021.

  1. rxcomposite
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    rxcomposite Senior Member

    Midship 2.png

    Agree. I would arrange it this way. With the exception of the girder, which sits at the deadrise. Girders poses a different computations as it is large and supported by bulkheads. It is also heavily reinforced and if I remember my DNV rules, keel plate (in fact the whole bottom) is supposed to be single skin, not cored. Can be up to 6m apart.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    I disagree.
    The stiffener should be close, as Tansl notes, some 50-60mm from the knuckle.
    Since what is supporting the knuckle...nothing...ergo it flexes under load which = failure.

    Every knuckle needs a support to prevent it flexing between frames ...
     
  3. rxcomposite
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    rxcomposite Senior Member

    AH- You mean this?
    Stiffener.jpg
    Or this?
    Stiffener 2.jpg

    It is allowed under LR rule. This from the hard copy of LR Design Details. No longer appearing in the new downloadable version.
    I followed the first rule. If the angles on the chine are shallow, the whole of the panel is treated as one. This naturally occurs in the forward end of the bow where the rise angle is stiff and the knuckles shallow. The plate is considered one and stiffener subdivided accordingly (This goes the same for calculations of frame strength). The formula for "Effective Width" of the plating completes the equation. This is what I have shown in the drawing.

    There is a formula in ISO about the contribution to strength of the small knuckles but is very small, hence I do not consider it as a factor.

    In cases where I have to add stiffeners in the knuckles, I use this. Not 50 mm away from. Not shown but in knuckles, I add 50% thickness to the plating thickness.
    Stiffener position.png
     
  4. zstine
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    zstine Junior Member

    I just calculated the angle as 151 degrees. While the spec may say at 149deg you don't need a stiffener but at 151 you do, in real life there's no discontinuity at 150 degrees... In other words, my 151 degree will provide some support an I can likely run a smaller stiffener on that chine. Even if it was 149 degrees, I'd likely still run a small stiffener.
     
  5. zstine
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    zstine Junior Member

    I am going to use one of the DNV GL specs because they are available free on-line where ISO is not, at least not that I've found. so I Can't use ISO... I just found another DNV GL spec for "Yachts", which after cursory review looks more straight forward than the High Speed Light Craft spec I was trying to apply. So, I will work with the 'Yacht' spec and see how that correlates to the 2003 special craft <24m spec that I had run last week.... Note that none of the current DNV GL specs appear to be specific to boats under 24 meters.
     
  6. zstine
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    zstine Junior Member

    Regarding the Chines.... My engineering intuition tells me that the chine is a stress concentration because it adds stiffness. There are several methods to reduce stress, adding an extra layer(s) of glass tape over a chine is one. To maximize stiffness in the panel, it seems to make sense that stiffeners should be placed in the middle of the panel as far from the chine to make maximum use of the chine's added stiffness as rxcomposite originally showed. But then the chine likely needs an extra layer of glass and while this maybe stiffer overall, it also may be heavier. So I thought that putting the stiffener over the chine would reduce the stress concentration in the chine joint by allowing the stiffener to take the loads in the joint. This, I believe, results in a lighter construction at the cost of some flex in the center of the panel. I have attached 1) 29ft Trimaran Laminate Schedule with core construction showing no stiffeners other than furniture and floor. no extra support of the soft/rounded chines. 2) A laminate schedule of a 44 ft catamaran that has hard chine. In this case they taper the core away and the chine is solid glass. I assume the solid glass is actually more flexible than the cored panel, which helps reduce the stress concentration of the chine. and 3) My proposed laminate schedule for the stiffeners. I want to locate them over the chine, bisecting the angle, using 2 or 3 layers of glass tape as shown.. TBD.
    Fig 1) upload_2021-3-10_17-59-26.png
    Fig 2) upload_2021-3-10_18-0-25.png
    Fig 3) upload_2021-3-10_18-23-2.png
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Not quite no.

    One needs to understand the purpose of the support. So looking at basic structural design of a beam that is experiencing a UDL, we have this:
    upload_2021-3-11_9-7-41.png

    Under an applied UDL, the beam bends and has a central defection, delta.

    If this same beam now has a joint at this centre, how different is the response?
    The max deflection is in the same location, but this joint has in-plane membrane stresses creating a reaction load at the joint for equilibrium.
    Coupled to this is the deflection, so it is flexing and has an applied load.

    So this reaction load will slowly work on the joint in addition to the UDL. Ergo, if design just for the UDL, it is weaker, as the stiffness will be insufficient.
    This is common on aluminium hull plating. Many arrangements do not support these joints with stiffeners, and so the 50mm "rule" is a minimum rule of thumb to allow a stiffener to be placed as close as possible, without the HAZ influencing the strength of the joint.

    So if we now look at this in the context of composite hulls...the same is true, but for differing reasons:
    upload_2021-3-11_9-16-48.png

    When the angle is greater than 150 degrees, adding the stiffener next to or on the joint seems "easy" enough.

    But as that angle changes and becomes much less than 150 degrees, as the tumble-home or flare or some other hull geometry changes this angle, placing the stiffener next to the joint is considerably easier to make, and it should follow the natural curvature of the hull and thus remain in-plane too. Since as this angle approaches 90 degrees, there is a lot of gap filling required to take up the 2 now very different angles of these surfaces meeting at the joint.

    In a similar manner as you note here:

     
  8. zstine
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    zstine Junior Member

    I'm confused about the "width of plate" that rxcomposite shows in the post #31 sketch. The sketch shows the 'width of plate' encompassing 3 stiffeners. Per DNVGL-RU-Yacht, pt3,ch3, "s=area in m2, supported by the element (plating, stiffener, floor, girder or frame). For plating, the supported area is the spacing between the stiffeners multiplied by their span". So shouldn't the width (at least for DVN GL) be just the spacing between the stiffeners? Am I correct to assume that the "width of plate" rxcomposites shows is something specific to ISO or other spec, but not neccessarily applicable to DVN GL...

    BTW, Yes you are correct that DVN GL doesn't allow for the bottom to be cored. I am considering removing the core along the keel, especially around the daggerboard trunk, but not the whole bottom.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    You are conflating two very different issues.

    1 - The panel size, as this is an input for the area and hence pressure to be applied.
    2 - effective breadth, the amount of contribution the plate for the stiffener provides.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Adding extra layer of glass in chines or knuckles is standard practice.

    Right. Cored is much stiffer. Locating the stiffeners is not a standard practice. Only a compromise. Will explain later.
     
  11. rxcomposite
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    rxcomposite Senior Member

    No I am not. The panel size is what is to be reinforced by an X number of stiffeners. Knuckles may provide additional stiffness but not that great.

    I am not against putting stiffeners into the knuckles. I just try to avoid it Spacing.png . It is the idea of offsetting that does not sit well with me as the rules do not offer that solution.

    What I am looking for is an optimum stiffener spacing that will give me a good aspect ratio. In 4:1 AR, the stiffeners are so close together. It may pass the short side deflection but not the long side deflection because of long moment of arm. To cure deflection, I will need bigger stiffeners which increases the "effective width of the panel" to a point where the end section looks like a corrugated panel.

    Here is the exercise. Greatly exaggerated of course.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    For clarity:

    The comment is referring to zstine's post.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    The 'effective' breadth is relating to how the panel of plating behaves, when supported by a stiffener.
    Different stiffness creates different responses. The effective breadth of plating can only be considered to be part of the stiffness, the second moment of area, between the points of zero deflection.

    Thus effective breadth and panel size are two very different terms for two very different meanings.
     
  14. rxcomposite
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    rxcomposite Senior Member

    If I go crazy on the rules, this is what happens. Going crazy.jpg
     

  15. rxcomposite
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    rxcomposite Senior Member

    Agree. The effective width of the panel equation forms part of the solution.
     
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