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

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

  1. zstine
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    zstine Junior Member

    Hi,
    I have been running the bottom pressure design calculation per DNV-GL Part 3, Special Craft (http://rules.dnvgl.com/docs/pdf/gl/maritimerules2016July/glrp-e.pdf) sub-part C, Advanced Composites, para 6.2. I have also run the online calculator from on Vectorlam.com, which is to be ISO-12215 compliant, using the same boat specs. The DNV and ISO results for the bottom pressure should be similar… But they are not close. Either I made a mistake somewhere (been over the thing a thousand times) or the online calculator is putting in a factor of safety or something I don’t understand. And some comparative results in the Vectorlam calc don’t make sense. For example, if I change the operating category from offshore to inshore, per DNV the factor changes from 0.95 to 0.85, but the Vectorlam result changes from 9.22psi to 4.86psi. Way too much change! I’ve attached results of the online calc as well as some info on my boat geometry. Can anyone see an obvious error in my calc below? Does my 1.69psi calculation or the 9.22psi sound correct for a 25ft light displacement (1,100lb) dinghy, trimaran? Is there any documentation on what Vectorlam is doing behind the scene? Like did it apply a FoS of 4 such as required for laminates per para 7.5, DNV?
    Thanks!

    excerpt of calculation done in excel (Displacement mode Pb2, per para 6.2):

    Lwl is waterline lg [meters] .... 7.8
    Tc = 0.062 * Lwl - 0.26 .... 0.2236

    kar (sailboat) = 0.673-0.52*(u^.75-1.7)/(u^.75+1.7)) .... 0.422765446
    u =100*Ad/Ar .... 8.218277449
    Ad.Plate = 1.2m x 0.3m or 2.5 x .3^2 (lessor of) .... 0.225
    Ar =0.45*Lwl*Bwl [in m^2] .... 2.7378

    kl max of 1 was used… 1
    foc is given as 0.95 offshore .... .95

    Pb2 = 11.76 * (3 * Tc + 0.23 * Lwl) * kar * kl * foc [in kPa] .... 11.64158, Which is 1.69 PSI
     

    Attached Files:

  2. Ad Hoc
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    Ad Hoc Naval Architect

    Hi Zstine

    Why do you think they should be close?...
    All rules, are different, ergo all results will be different.

    It matters little what the results in terms of the Pressure are, the important part is, what scantlings do you end up with? Since each set of prescriptive rules, whether DNV, LR, or ISO etc, has their own 'fudge' factors based upon previous historical feedback - lesser so for ISO as this is just an Institution/Body not a Class society. Thus they will never be the same in their derivation, but, the end results... will be "in the same order of magnitude".

    So, what scantlings do you get using DNV and which do you get using ISO?

    If you think one of them is too heavy...then no one is forcing you to use the "heavier" one.

    The rules you select play in important role in the whole life and duty of the vessel you are designing. Since it also influences the plan approval stage - cost - the build/production phase - cost/experience - and the equipment you source - is it available and with a cert etc. Hence cherry picking one part of an exhaustive prescriptive set of rules and to compare with another - will never yeild the results that you expect, they will always be different. How much does it affect the final scantlings - is another question.
     
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  3. zstine
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    zstine Junior Member

    I think they should be similar for several reasons. First, DVN paragraph 6 states "the following design pressure formula are in the style of ISO-12215" which leads the reader to believe there is similarity between the 2 specs. Second,the scantlings can be designed independently by engineering analysis based on loading without the use of the prescribed scantling section of the specification since everyone need not comply with the spec in its entirety. Using it as a tool not a requirement. 3rd, common sense dictates that 2 separate engineering analysis of the same condition should have similar results that are somewhat close (within 20-30%, not 545% ) or one of the methods is invalid. For example, If you perform "maximum shear stress theory" and also "Distortion energy, Von Mises" to determine the point at which a metal frame fails, the results should corroborate somewhat.

    If the online calculation for ISO is correct at 9.22 psi and the 1.69 psi is also correct (over 5 times difference), then an engineer would obtain vastly different scantlings based on these loads. Note, that DVN doesn't specify the analysis methods for the scantlings in section 7, but rather provides FoS for required calculations (fiber strain, core sheer, deflection) which are left to the engineer to determine. Finally, even when applying the DVN FoS for design of the scantlings (eg. 4 for strain and 2 for shear) the result is panel and framing of less strength than if NO FoS was applied to the online ISO calculated load. I do not have access to ISO-12215-5 to see what if any method of FoS is required therein, and I'm not paying $200 for that single section!

    So, I'm pretty sure one of those analyses is in error, not that the specs provide 5+ times differences in estimated loading. Like I said, the online calc has some suspicious results when I play with inputs, but 1.69 psi seems pretty low. so I'm not sure on that either. I always like to have redundancy in design and validation of analysis results.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    You need to be more explicit in the - para 6 - reference. There are many para's with that designation.

    Having said that...if you have quoted correctly - in the style - does not = the same prescriptive set of parameters.
    It just means, in the same round-about -kind of way.

    Since if you read 7.3 for sailing craft in ISO it states Ncg - the dynamic load factor is not sued in pressure determination.
    This is one reaosn why getting a copy of the rules is important. So you can see where such differences are in their interpretations.

    So why does this output, show 2.43 g?

    upload_2021-3-3_10-16-28.png

    The rules you cite are in fact GL's which became DNV-GL but is now back to DNV.

    As with any Class rules you can.
    But it is all about the output, what the scantlings end up being, not the multitude of variables for the input.
     
  5. zstine
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    zstine Junior Member

    The quote is from DNV-GL Special Craft, part C, Advanced Composites, Paragraph 6. Design Pressures (below).

    The DNV spec calculates planning hull pressure (Pb1) and displacement (Pb2) and doesn't limit one to sailboats, but just says Pb2 is typical of sailboats. You are to use whichever is higher. Pb1 uses Ncg which I calc'd as .99 g's via DNV. It is not used in the calc for Pb2 displacement pressure. If you look at the 2 right columns in the ISO-12215 results spreadsheet you will see the Ncg accelerations are different, because I was playing with deadrise. Further, you will see that even though have different g's, the design pressure is equal for both 10 and 30 degrees deadrise. This, I presume, is because the online calculator ignores Ncg for determining design pressure for sailboats as you stated.
    upload_2021-3-2_20-39-52.png
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I have no idea where a Part C is??
    The references always begin with a series of numbers as part of the table of content, not letters.?! It starts with a series of numbers...then subsections use a letter...thus still not clear to me where you are citing the above.

    Having said that... it states ISO draft of June 2000.....those GL Rules are 2016-ish.... a major major difference!!! 16 years is a lifetime in terms of rule development and changes.

    As noted, you need to get yourself a copy of those June 2000 rules to ascertain where are those differences.
    Notwithstanding the dynamic load factor noted above, is not in ISO...yet it seems to be in that Vectorlam...
     
  7. rxcomposite
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    rxcomposite Senior Member

    zstine

    Something seems to be wrong in displacement. L x B x d x Cb = 29.2 ft3. You have 17.7 ft3

    If inputs are correct, ISO bottom pressure is 9.8 kn/m2 (1.41 lbs/ft2). Close enough, you are getting 1.69 lbs/ft2

    Offshore for a 25 footer boat? Check ISO for length limitation in this category.
     
  8. zstine
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    zstine Junior Member

    Thanks for the reply rxcomposite.

    The hydrostatic data in my attachment was generated by FreeShip software, so I took it for granted. I see 25.8'L x 2.5'B x 0.83'd x 0.33Cb = 17.66 ft3

    Thanks for running the ISO bottom pressure. TANSL did as well and we have all gotten results bout 10 kn/m2 give or take. So I'm happy with my analysis. What that tells me is there's something seriously wrong with the VectorLam online tool or maybe the way I'm using it. I think I'll send them and email...

    Offshore for DVN-GL just means you use 95% of the full "Ocean, Cat I" bottom pressure when design the scantlings. For inland, Cat III, you would use 90%... yeah, not really planning to sail this boat offshore, but its not going to hurt to be a little conservative with the loading.
     
  9. rxcomposite
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    rxcomposite Senior Member

    ISO and LR comes very close when determining bottom pressure. ISO tends to be on the greater side as it lacks the finer correcting coefficient LR has. LR has more inputs.

    I am not sure about DNV asking for a safety factor of 4. I know that in DNV composites, the modulus of elasticity is projected (linearized) and 1/3 of that is yield. LR uses 1/3 of ultimate strength, ISO divides ultimate by 2. Thus ISO has SF of 2 or 1/2 of the ultimate, depending on how it is expressed.

    I am not in my design reference right now but I know that "Ocean" or "inland" determines wave height, vertical acceleration, and ultimately bottom pressure. Different rules have different definition of this distance from shore so you get different values. I once compared DNV to ISO and values are largely different because of the way they define it. So unless you are evaluating rules, stick to one guide.

    Whats specifically wrong? Pressure? Lam scheds?
     
    Last edited: Mar 3, 2021
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  10. zstine
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    zstine Junior Member

    The pressure is wrong for some reason. The design pressure for my boat was determined to be 9.22 PSI or 63.6 kN/m2, as can be seen in the results attached to the 1st post. That calculation is supposed to be ISO-12215 compliant, but we now have three independent corroborating analyses giving approximately 10.5 kN/m2 (~1.5 psi) pressure. So either the tool is wrong or the way I'm using it is wrong.

    VectorLam has a separate section to calculate sandwich composite panels, which is quite nice to use (assuming it is correct). I intend to enter the design pressure we found into that tool to help determine my laminate schedule, while applying the appropriate FoS per DNV-GL. Yes, I agree that sticking to one guide is the way to go and that's my intent. The DNV-GL spec I'm using doesn't identify the method to determine fiber strain, core shear and deflection (i will use vectorlam). It states the required FoS and 'control' attributes that must be identified in design doc's, like fiber orientation, ply thickness, modulus of shear and elasticity... I may be using an older version (dated 2016 i think)
     
  11. rxcomposite
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    rxcomposite Senior Member

    I will run again the calcs to establish consistency of bottom pressure using LR, ISO, DNV, and Heller and Jasper.

    What is the size (length and width) of your firmly supported sandwich bottom panel? Any particular core brand/type you plan to use and the type/weave of fibers for skins. The answers to the calculations will prove if Vectorlam is wrong.
     
  12. zstine
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    zstine Junior Member

    Thanks Rx.. The distance between transverse frames is 1.2m (48in) and between longitudinals is 0.3m (12in). I put all the needed variables below. I haven't settled on a laminate, but I'm considering (outside skin) 2 layers of 1708, with a 3rd below the W.L, then (13mm) 1/2 inch Diab H80 core, 2 layers of 1708 inside. I am also considering using 1/2 inch Nida Core H8PP honeycome, vice H80, b/c it is way, way cheaper.

    Description Variable Metric English

    Displacement… D 500 kg 1,100 lbs

    Length over all… Loa 7.9 m 25.9 ft

    Length water line… Lwl 7.8 m 25.6 ft

    Beam at waterline… Bwl 0.78 m 2.6 ft

    Velocity operation… Vo 12 knt

    Dist between Trans Frame.. 1.2 m 48in

    Dist between long’l frame.. 0.3 m 12in

    Area of plate (4’x1’)… Ad 0.36 m2 576sq.in
     
  13. TANSL
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    TANSL Senior Member

  14. zstine
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    zstine Junior Member

    I ran the laminate (2x 1708 - diab H80 .5in - 2x 1708) through the Vectorlam panel calculator with a distributed load of 1.5 psi and a 48in x 12in panel (it's all in English units). I ran it with both simple support and fixed edges (results below). The simple support gave worse case with a FoS of 4.4 on 1% deflection (1 required by DVN-GL), an FoS of 12.36 on ply failure (4 required), and FoS of 7.7 on core shear (2 required). This tells me that 2 layers of 1708 over 0.5 in H80 is significantly overbuilt. So I quickly ran it again with just 1 layer of 1708 over H80 and the FoS still doubles the requirements... Deflection FS=2.06 (1 req'd), Ply failure FS=9.98 (4 req'd), core shear FS=7.71 (2 req'd).
    upload_2021-3-4_13-48-51.png
     

    Attached Files:


  15. zstine
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    zstine Junior Member

    I will do my best to answer, see table below... I don't have access to the ISO spec to see the definitions of some terms. Sorry for my ignorance, I'm not sure what mLDC stands for. Also, this is a mushroom cross-section hull like the Corsair/Dragonfly Trimarans, so I'm not sure where to take "Chine Beam". And I don't know what "operating mode" is.
    upload_2021-3-4_14-29-47.png
     
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