Designing a 9 meter sailing catamaran

Discussion in 'Boat Design' started by Pammie, Jul 25, 2018.

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PammieSenior Member

Another ISO related question: I calculated hull loads with part5 as well as part7. Ofcourse part7 is not official yet but in its final stages.

An short overview (all in kN/m2) :
Part 5:
bottom: 49,85 - 24,48
deck: 8,69 - 5
Part 7:
bottom: 14,89 - 9,17
deck: 8,85 - 7,08

Quite a difference. Any opinions?

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TANSLSenior Member

The final ISO papers for small boats can be very different from the preliminary ones. Do not trust the provisionals. At the moment the only solution is to do a direct calculation or apply the rules of a CS.
The bottom should not change much from one option to the other. On the other hand, in the inner parts of the hulls or in the wet deck, you should apply higher pressures than those foreseen in 12215-5, taking into account slamming. You could, but it is only an opinion, to apply the formulas of the 12215-5 but taking the design pressures for those special panels from a CS.

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PammieSenior Member

What do you mean with direct calculation? Well, it might be able to handle the maximum load so lets see for that. In the mean time I'll try to contact the ISO people and ask for their opinion. Maybe they are active on BD.N?
In part 7 ncg for sailing multi's is taken 1,2, In SSC I believe 3,2.
Load on wet deck is higher: 33 kN/m2

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TANSLSenior Member

Direct calculatios : go to the principles, that is, apply the theory of plates and the theory of continuous beams to calculate the various elements of the structure. For this, normally, one agrees with the entity that has to certify the construction, the design pressures applicable in each case and the mathematical model that can be used. It is not available to almost any designer because you have to know about structures, elasticity and something else, not to mention the expensive calculation programs needed. That is why those of us who know little about structures take refuge in the regulations of the Classification Societies or in the ISO standards. With them a formula is applied without worrying, almost never, of the theory that justifies it.

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HeimfriedSenior Member

Hi Pammie
since you posted a xls-File of the data regarding your short beam shear test, I looked into this and in addition at a website with sketches and formulas describing the test according to three slightly different standards as I understand.
Interlaminare Scherfestigkeit – Lexikon der Kunststoffprüfung http://wiki.polymerservice-merseburg.de/index.php/Interlaminare_Scherfestigkeit
All three geometries of the testing sets make it impossible that the specimen could pressed through the gap between the supports (e.g. sketch "c": diameter of the upper roll applying force is 10 mm plus two times 2mm thickness of specimen gives 14 mm, the gap is just 6 mm. As it seems, there is no mesurement of the bending, just applying force with the upper roll which comes down with an (exactly set) slow velocity (e. g. 1 mm/min). The test stops as soon as first cracks occure and the then applied force is decisive for the calculation of (interlaminar) shear strenght.
(Your test geometry is, according to your sketch, 40 mm roll + 2 * 16 mm = 72 mm, the gap is 80 mm. So the pressure roll could deform the specimen to an U-shape and press it through the 80 mm gap. Next point is, the supports should not have sharp edges.)

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AlikSenior Member

Part -7 is full of mistakes and is being re-worked. I know it because I was in the group...

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AlikSenior Member

No materials will work as pure core and pure skin. There should be some design assumptions for calcs of laminate. You can count plywood as core, but then You need to add fiberglass on inside of plywood, I think this will be extra weight.

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fallguySenior Member

As the casual observer; this has always been my take on the downside of ply plus core. Even just walking on light wovens is hard on them, so a minimal fabric for even foot traffic seems prudent. I'll return to my quiet observation post. Good luck Pammie.

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Excellent, always good to learn about the design side, rather than just rule application; which is the end result of such. But can get a bit heavy....just forewarning you!

Ok. So this should be addressed into two parts, as they are different.

This assumes that you have calculated the structure and thus you know what the required core strength needs to be, for the design and panel aspect ratios you have assumed for your design. Is this correct?

Choose a rule you wish to comply with and stick to it. As I noted before, don't mix and match. Even if the it is a partial update. Only use fully updated/approved rules.

Last edited: Jul 29, 2018
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rxcompositeSenior Member

Pammie- Ok, I see why you are doing the tests. ISO requires it. You don't need to. Predictive values can be derived in Table D1. To quote ISO, "the core is capable of transmitting only shear force". See fig D.1.

In order for a laminate to be called a sandwich, you must have a top and bottom facing sandwiching a core. Plywood can be considered a core if you laminate both sides with fiberglass. Plywood can also be the facings if there is a soft core in between.

Placing a core on one side of the plywood will not work. It only acts as insulation. At this point, you are too far ahead in your build to consider cored laminate. If you find that your panel is inadequate to carry the calculated load, add stiffeners. It is common in the forward section, the bow area, the frames/stiffeners are closely spaced. It is the simplest remedy to enable the panel to sustain the load.

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rxcompositeSenior Member

If that is the latest official rule, follow it. As far as I know, part 5 is for monohull and part 7 is for multihull. With cats, there is less pressure on the bottom as as it is generally long and narrow and the demihull carries only half the total displacement. It is the longitudinal stiffness that needs to be looked at.

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AlikSenior Member

Part -7 is not yet official, it is still under re-work and I didn't get recent version for review yet. There were a lot of issues to sort out, so do not use part -7 for calcs.
We use part -5 this is fully legal, but for vertical accelerations we use doubled chine beam. This well works for catamarans, we designed dozens of them

Longitudinal stength of small catamaran is usually no issue. If one complies with local strength, longitudinal strength will comply with factor of safety ~20.
Transverse strength could be checked, however such check might only reliable with FEA.

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rxcompositeSenior Member

Agree. Small craft has adequate strength globally but I do double checking on the girders that support the crossbeams.

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PammieSenior Member

I started calculating the minimum thickness of sandwich laminate (10.5.4) which is obliged besides Annex H. With loads according Part5 the most loaded bottom plate requires 12,1 mm. ISo is not very clear about how to calculate plates around natural stiffeners (I've seen LR orDNVGL? using overlaps. See picture.
The other assumption is the foam maximum shear strength which is 1,2 N/mm2 according to the specs. In this spec the measurement method (ISO 1922) is mentioning. (Spec is added in post #2) Would this spec count as a test? I calculated 85% of this specified value? So: 10,3 mm sandwich means 12,1 - 1,7/2 = 11,3 mm foam.
Though I suppose minimum shear strength will follow from Annex H calculations but Table 13 specifies minimum shear strength at 0,25 N/mm2 which compares to 50 kg/m3 density. My density is 80.

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15. Joined: Dec 2015
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PammieSenior Member

@Heimfried: great reference! Getting (free) information about test methods is difficult. But test should have been C393... which has a longer span. My original question isn't relevant anymore when I am able to calculate the total effect.

@Alik: I'll forget Part 7. Pure core, pure skin: I understand. There is glass at the inside of the plywood: tri 830 - plywood - foam - carbon UD 300 - biax glass 600. It seemed a good idea at that moment, will have to see now what it does.

@Ad Hoc: higher density core: as prescribed by Gerr and used in a study plan from a well known designer. I dimished skin thickness, not core dimensions. I'm working on the calculations.

@RX: Well. My part 5 calculations uses total weight for one hull, but changing Mldc by half gives only a very small change in load. Annex H points to CLT software for unbalanced sandwiches. Do you have any suggestions what to use? eLaminate doesn't support SI units and open office, CompositeStar, NAFEMS Downloads engineering analysis and simulation - FEA, Finite Element Analysis, CFD, Computational Fluid Dynamics, and Simulation https://www.nafems.org/about/technical-working-groups/composites/downloads/

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