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

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

  1. TANSL
    Joined: Sep 2011
    Posts: 6,562
    Likes: 412, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is difficult to analyze the formulas used by the CS or the ISO, because they do not give any explanation, but some formulas used in the thin plate theory are based on the idea that the deflection is not greater than the thickness of the plate. From this point of view, studying the deflection of the plate is of less interest than calculating the maximum stress (traction or compression) that the most critical layer can withstand as well as the shear force in the core of the sandwich, checking that the stresses are not exceeded. of design. This check is not advisable but is mandatory. In this way, a "compliance factor" is obtained which must be, in absolute value, greater than unity.
     
  2. TANSL
    Joined: Sep 2011
    Posts: 6,562
    Likes: 412, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The input data of my previous post are those required by ISO 12215-5: 2019.
    • Overal procedure: there are 5 possible calculation methods from which you must choose the one used in each boat.
    • Type of craf: recreation, passage, work, ....
    • Type of hull: monohull or multihull
    • Design category: A, B or C
    • Chine beam: there is a procedure to determine it, even in ships with round bilge.
    • Operating mode: planing or displacement
     
  3. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Zstine
    I ran the bottom design pressure calc with several methods to find a common ground.

    Jasper and Heller- 68 kN/m2 (ratio of vertical acceleration to gravitational constant set to 4.2 typical)

    Lloyd’s – 65.3 kN/m2 (set to sheltered waters G1, max speed 10 knots, planning + light displacement), panel at 3.9m from transom

    J & H is known to approximate LR in calculating pressures.


    Using ISO with the data given, Offshore category, displacement mode derived, panel at 3.9m from transom

    ISO-10.6 kN/m2

    I did not run it with DNV as it has an entirely different definition of operating condition. Not similar to ISO.

    The difference is large because of the way the condition is set. I can’t say now if Vectorlam is not calculating bottom pressure correctly as I do not know the formula used.


    Sandwich Structure

    You are doing it all wrong. You have too high aspect ratio of 4. It will work for high modulus carbon, aluminum, or steel. Eglass has low modulus. Design around AR of 2 or 3.

    Panels should only be analyzed with fixed edges. Simply supported are for hatch covers only.

    1708 is a biax. Biax loses strength in the 0/90 direction and have only 40-45 % of its strength remaining. Worse if it is a stitched uni with mat. You get only 10-12% remaining strength. You should have a minimum of 1.1 mm for the inner skin and 1.5 mm for outer (ISO rule), Try plain WR first for the exercise (14500 tensile modulus, 14000 compressive modulus, 190 tensile strength, 147 compressive strength, in N/mm2). I do not know what strength Vectorlam assigned to Biax. Again, composite software should have accurate mechanical properties data or at least allow for fabric rotation angle in relation to load direction. This is where most online composite calculator fails.

    You H80 almost made it except for shear failure. Cores are derated to 50%, or 65% in case of Linear PVC (ISO Rule). Try 20 mm for 10.6 kN/m2 load.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,906
    Likes: 855, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    As noted, the only way you can establish this, is by doing this by hand and checking the variables within each set of rules.... notwithstanding that they are 16 years apart in the derivation too!!
    You simply cannot reply on a simple software to give you results that you can have confidence in if you cannot confirm the calculation your self, doing it by hand with pen and paper.

    If at the end you conclude that one of them is wrong,..based upon doing the calculation by hand.. then that is your answer.
     
  5. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Can you post a midship drawing or a sketch with dimensions (the linear distance between that supports the hull at waterline, not exactly breadth). My formula is asking for a girth distance and have only estimated it.

    Something like this?
     

    Attached Files:

    Last edited: Mar 5, 2021
  6. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    If the program is asking for the service area then it is establishing maximum wave height it will be operating on. From there, along with the operating speed, the vertical acceleration can be established. If the vertical acceleration is computed, the impact area per unit length can be solved. The girth and length needs to be specified so that pressure can be established (F/A).

    ISO 12215-5 does not ask for girth or computes vertical acceleration. This differentiates the two methods (LR, HnJ) and ISO uses to determine bottom impact pressure.

    Vectorlam probably uses one of the two basic format hence the discrepancy.
     
  7. zstine
    Joined: Sep 2013
    Posts: 74
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: New Jersey

    zstine Junior Member

    Wow you guys have been a big help and giving me a lot of information to wrap my head around!. sorry I was gone for a few days... Let me start with design pressure. We seem to have 2 groupings of results, one group of results around 10.5 kN/m2 and a second around 65 kN/m2. I understand the results are based on different conditions and assumptions, however, I currently only have a copy of the DNV-GL and cannot review the other calculations. I believe the full 'ship' pressure loading sections of DNV-GL get into ocean waves and vertical acceleration... I think similar to the J&H and Lloyd's calc above. But the <24 meter 'special craft' section calculates pressure based only on 1) Lwl 2) Tc (canoe draft), 3) Kar (area ratio of panel size to Lwl & BeamWL), (4) Kl (long'l distribution factor) and (5) Foc (operating Category 1=100%, 2=95%, 3=85%)... What this all boils down to is whether I design my scantlings for 10.6 kN/m2 with a FoS of 4 = 42.6 kN/m2 or design for the 65 kN/m2 without any FoS as I Assume there is not a FS applied to that loading per their respective specifications. Not a huge difference really, but I haven't personally reviewed the analysis that resulted in the higher figures and I hesitate to adopt a result I haven't personally reviewed and understood... It may be moot if the 1.1mm and 1.5mm skin thickness drives the design.

    The high aspect ratio on the panel is largely due to the choice to run longitudinal framing along each chine, which is fairly close together. That and I arbitrarily decided to have 5 transverse bulkheads/frames with lightening holes. I could of course increase transverse frames to reduce the panel AR if that results in lighter overall construction... i think I may be at 1.1mm and 1.5mm skin minimum with plenty of strength above the pressures calculated. So in that case, more framing may not be desired. I plan to build with 5 strips of core per side, planked over a female mold and am using freeship to develop the plates.

    1708 fiber orientation issues noted. I will review other weave options and investigate mods to framing and panel AR.

    Here is the mid-ship girth at the waterline you asked for... if I understand what you needed?
    upload_2021-3-7_14-28-19.png
     
  8. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    From my book DNV, the factor of safety for composite is 3 same as LR.

    ISO uses 2
     
  9. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    It is the minimum. You may go higher if the the designed stress is exceeded.

    Don't quote on that. LR inputs are much more numerous and calculations are much more complex.

    Bulkheads can also be non watertight. If you cut holes in it, better use just a ring frame. A single skin laminate hugging the skin or plate. It is much taller than a frame of a tophat design.

    Or you can just use transverse frame (tophats) in between bulkheads to reduce AR. That support the longitudinals. Bulkheads supports primary longitudinals like girders so it can be much further apart.

    Now that you have shown the midship, can you show us where you place the longitudinals before we go any further.
     
  10. zstine
    Joined: Sep 2013
    Posts: 74
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: New Jersey

    zstine Junior Member

    Attached shows the (Red) longitudinal frame locations running along each chine made by the 5 hull planks. I won't need a longitudinal along the nearly 90 degree chine directly below the deck-hull joint.. You can also see the developed curve of each plank. Since the hull tapers, the framing is higher aspect near the ends, particularly the bow. Framing will also be added to support dagger board loads and rigging as needed. I haven't gotten that far along. I'm imagining the longitudinal frames to be about 1 inch wide, by 2 inch tall and the Ring frames to be 5 or 6 inches tall, but I need to work that all out.
    upload_2021-3-9_3-12-24.png
    I'm using an older free download of DVN-GL. Looks like this chapter was last updated in 2003, and I believe it was active until 2016. I think in 2016 the spec got rid of the "special Craft <24m" part but I don't know the spec's well. If it worked then, it should work now... assuming I stick to it front to back, as has been pointed out by others. the attached excerpt shows the page with the FS..... EDIT, I just found a copy of DNVGL 2019 and it's quite different. I will look into it and update.
    upload_2021-3-9_3-30-30.png
     
  11. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Midship.png The proper way to define the arrangement is thru midship in an example shown below. You need also to specify what angle is the chine. Below 150 degree, that is a natural stiffener. Above 150 degrees, that is a secondary stiffener which contributes little to stiffness. That way the width of the panel and the stiffener spacing can be established.

    Let me see how you arranged the longitudinals. I see you have a center girder. That can be eliminated. I was wondering where you got that 12" witdh of panel.

    After defining what is correct, I will show you the results of the calcs. What works and doesn't work. View attachment 165935
     
  12. zstine
    Joined: Sep 2013
    Posts: 74
    Likes: 9, Points: 8, Legacy Rep: 10
    Location: New Jersey

    zstine Junior Member

    Thanks again. I modified the sketch of my plan midsection. As shown, I was thinking of running stiffeners along each plank joint. Perhaps a bad idea? I know that none of the planks are more than 12 inches wide at any point, hence the 12 inches. I had trouble with the example you attached. keep getting an error when trying to view it, So maybe I'm still missing something...

    upload_2021-3-10_3-56-31.png

    I have been reading and attempting the analysis in the DNVGL High Speed Light Craft, Part 3, Chapter 1 Design Loads, datd 2018 amended 2020 (https://rules.dnvgl.com/docs/pdf/DNVGL/RU-HSLC/2020-10/DNVGL-RU-HSLC-Pt3Ch1.pdf). It is MUCH more detailed and complicated than the 2003 spec I was using. I'm having some difficulty with it, but I will keep plugging away. Be aware that for a one-of-a-kind boat built in the USA, I'm not required to actually comply with any design specification. I'm just trying to learn to analyze the hull and scantlings design to make a good, light boat and to get smarter. Thanks for the help.
     
  13. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Looks like the 1st chine from keel is less than 150 degree?
     
  14. rxcomposite
    Joined: Jan 2005
    Posts: 2,279
    Likes: 316, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Have you decided that it is DNV GL you are going to use as design load and safety factor and not ISO? You can only choose one. You cant mix rules.
     

  15. TANSL
    Joined: Sep 2011
    Posts: 6,562
    Likes: 412, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    @zstine, putting a stiffener right at the junction of two plates is not a good idea. Move the reinforcement 50 or 60 mm.
    Also try to take advantage of "natural reinforcements", if there are any.
    Even if you are not obliged to comply with any rule, the designer is obliged to carry out the best possible project and, in scantling, for those of us who do not know much about this topic, the best way to ensure a good design is to follow the rules of a CS . But when you decide to follow a regulation, you can't skip what you don't like, you have to follow it to the end. I, for your boat, would choose the ISO.
    Good luck and feel free to keep asking. A clear question, and an appropriate answer, can save you a lot of time and improve your learning curve.
     
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.