Building a “C-Class” Catamaran

Discussion in 'Multihulls' started by Nordschleife, May 23, 2021.

  1. upchurchmr
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    upchurchmr Senior Member

    It would be more believable if there were comparative properties quoted between TPT and other lamination schedules.
    Speaking from an aerospace perspective, if there was a real improvement Aero would have jumped on it a long time ago, and they haven't.
    Another question is - do you want isotropic properties in the final laminate?

    If you don't do an aerospace style analysis of the requirements of the laminate, it is a doubtful that you will match needed properties to actuals for any given laminate.

    Out of Autoclave is certainly cheaper. Aerospace has consistently rejected it due to the reduced properties and therefore increased weight.
    Stiffness is a function of the stiffness of the fiber and the % of layup in a given direction - not the epoxy.

    I doubt the above statements.
     
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  2. Erwan
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    Erwan Senior Member

    You might be right I don't know, I am not a structural engineer, or FEA proficient, but I d love to have some support on these topics
    I am not advertizing anything, just providing some "seeds" for other half -geeks to make their own research and therefore build their own independent judgment.
     
  3. oldmulti
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    oldmulti Senior Member

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  4. upchurchmr
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    upchurchmr Senior Member

    Making comments you don't understand or can't support is not helpful.
    Sort of like a bunch of politicians around here.

    Misleading people leads to firmly held opinions which cause boats to be built poorly.

    Why do people say - I don't know about this, BUT, this is a fact.
     
  5. upchurchmr
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    upchurchmr Senior Member

    TPT and spread ply provide the possibility to make a laminate closer to the required dimension.
    If a layer of old laminate is .060 thick, but you only need .010 to make the Laminate "good", then you have saved weight. If they cost the same per # then you saved cost.
    But if you put down 6 plys of .010 vs 1 ply of .o60, then you have increased labor cost. Assuming the cost per # is the same - which I don't believe it is (I haven't checked lately).
     
  6. Skyak
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    Skyak Senior Member

    TPT has been around a while. I first noticed it in North 3DL sails. I don't know if it even has the production capacity to do commercial aircraft yet but I would be amazed if it is not used in military applications. The tapes are narrow so they are placed by machine and can follow loads -not straight lines like prepreg.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Stearing narrow plys around a contour is indeed used in aerospace. However the ply is the same thickness as typical prepreg, because it is prepreg, made from full width UD tapes. Sometimes called "slit-tape".
    Not thin ply in any manner.
    However, the supposed cost savings and weight savings is typically reserved for fairly unique parts and not found in "regular" parts. The laydown equipment and process are expensive, and needed production rate savings are not always found.
    It is not used for typical skins or covers, generally because analysis methods cannot account for the changing geometry. If you cannot predict the strength, then test to confirm it, it won't be used anywhere I worked.
     
  8. Skyak
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    Skyak Senior Member

    There is the answer for why TPT is not in aircraft. If you use a new advanced technology to build toys for rich guys you get paid when you hand them the part, maybe sooner. When you build for aerospace you can't even get an order until you have a validated part off production intent process. In this case it needs to go all the way back into the design software. That's the kind of development that only gets done on DoD contract...or America's Cup...

    One more thing about thin plys, they make smaller stress risers at flaws and crossings and it's OK that ply orientation is only 22 deg because that still gets you to 90 in less thickness than prepreg. My first thought was amateurs are kidding themselves using TPT because they don't have the precision placement, but thinking it through a bit more I see TPT as more forgiving and with simple rules. Sort of a poor man's load path.
     
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  9. Erwan
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    Erwan Senior Member

    Thanks for these interesting comments, and if I do not abuse, could you develop a bit more these interesting points about "simples rules" and "more forgiving" ?

    Thks in advance
     
  10. upchurchmr
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    upchurchmr Senior Member

    With some simple template tools, amateurs can indeed make layups that at first glance require precision (expensive) equipment.
    Amateurs just substitute care and time for equipment.
    And it is not always necessary to have high priced analysis tools for "different" material and geometry parts. But if you really are trying to get the most out of the material (cost and weight) then you need to do good testing of the finished product. And iterate the design and testing until you reach your goal.
     
  11. rxcomposite
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    rxcomposite Senior Member

    Thin Ply Technology has been around for awhile, early 80’s I remember. It started when the advanced composite technology was declassified and the private sector took up the challenge and developed further the technology.

    Prepregs and “out of autoclave” was the norm using especially built walk in curing oven.

    It is not limited to unidirectional fibers but used also in fabrics. Typical is style 7781 in satin weave fabric available as fabric and prepreg. When cured, it is only 0.009” in prepreg and about 0.010 to 0.012” hand laminated. As a requirement example, a typical cored panel subjected to 38 psi pressure (FAR rule, typical) would require only 0.007” thick of laminate. UD Carbon fibers, available only as prepreg at the time is about 0.006 to 0.007” thick. Sometimes up to 80 layers are used.

    The mantra “thinner is better” is widely used and there is a rational explanation for that. Unidirectional fibers are sensitive to out of plane loads as shown in the engineering constant attached below. Modulus starts going downhill quickly at about 7 degrees off axis load. Note how sensitive is the carbon uni fibers compared to Eglass. Fabrics which are woven typically uses the twill or satin weave, skipping two or 3 strands of weave. This reduces the upward/downward undulations typical in a standard basket weave. Uni’s are not woven but still about 3% of the fibers go astray. Not a perfect world.

    Filament winding method paved the way for automatic fiber placement machine. This machine lays several strands of filaments collimating into a certain bandwidth accurately laid up side by side. As always, a continuous strand of fibers is better than the cut and overlap variety common in fabric layup. Little increments for greater strength.

    Engineering principles has been around since the 80’s. It is still being used today as a basic calculation albeit more complex as filaments are wound at different angles/different directions. The only difference between Advanced Composites and regular Fiberglass Technology (which uses tables and charts) is the amount of engineering that goes with it and the quality control (such as coupon testing) to ensure product consistency. Thus, when you want Advanced Composites, you pay more money.
     

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    Last edited: Jun 21, 2021
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  12. rxcomposite
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    rxcomposite Senior Member

    Since you have mentioned it, I will explain further.

    What is 22.5°? It is ½ of 45°. UD laminates are strongest only in the direction the fibers are oriented. In order to achieve ALMOST equal strength in all directions, the thin UD plies are stacked together by incrementing the angle of the fibers. The simplest quasi isotropic laminate is -60/0+60 degrees. The more complex one is 90/+45/0/-45/0 but is not symmetrical. It works with thin ply laminates as the plies are so close together that in a cored laminate, the distance of the innermost ply to the neutral axis is almost neglible.

    To further increase the isotropic qualities, the stacking is 90/67.5/45/22.5/0/-22.5/-45/-67.5/90. Every 22.5 degree. (the last ply is optional if it is to be stacked with the same set). We now have a laminate consisting of very thin plies stacked together in the sequence shown and its reduced modulus can be calculated by the matrix theory or averaging the resulting modulus using the engineering constant. The properties of the whole is taken as ONE resulting mechanical properties.

    22.5° is the least known shear inducing stress. The shear brought about by the fibers closing (like a scissor) in shearing the resin or interlaminar bond (glue line). In depth engineering tells us if the resin used will not fail within its published/claimed shear properties. Resin failure is delamination as you have mentioned.

    The analysis just goes deeper and deeper.
     
  13. rxcomposite
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    rxcomposite Senior Member

    That is right. Boatbuilders don't need the in depth analysis.
    The most needed is analyzing a plate for longitudinal strength in hogging/sagging and the transverse strength brought about by the beam of the boat.

    In most cases, a 0/90 weave style will suffice and throw in at least 30% of biax for torsional strength. Boat uses thick plies, in the order of 1 to 2 mm and the distance (of the individual ply) to the neutral axis is what is important. Just keep the woven biax closer to the NA, not in the outermost ply. It has only about 45% of the strength of the 0/90.

    It can be solved by the tabulated method ISO and LR uses. In most cases, fiber stacking is the most efficient method.
     
  14. Erwan
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    Erwan Senior Member

    And that what is amazing with the Canadian C-Cat main tube, it is very slim, so the distance to the neutral axis is minimum.

    Too bad they put their C-Cat on the back-burn

    Cheers
     

  15. rxcomposite
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    rxcomposite Senior Member

    I meant the neutral axis in the thickness of the laminate, cored or single skin.

    This long narrow hull have very small wave making resistance which makes it very fast.
     
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