Designing a 9 meter sailing catamaran

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

  1. Pammie
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    Pammie Junior Member

    I got confused myself a little bit :)
    The most forward frame of the aft section is called S6. The most aft from the forward section is S7a. Bottom plate A6 is in between and has a length of 950 mm.
     
  2. TANSL
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    TANSL Senior Member

    I do not know how you are focusing your study but I would not analyze each and every one of the panels. I would study a section in the center of the ship, of a separation between reinforcements of 910 mm and a zone in front, to 8.85 * L of the Ap, with the separation of reinforcements that there is in that zone. Both can be fictitious sections, that is, they do not need to be exactly the same as the existing ones. You study the bottom, side and deck in each section. You take the most unfavorable values that you get and apply them in the whole length of the boat. When you begin to detect problems of overweight or lack of resistance, try, first, to add a new stringer (may be a local one)and then try to replace the initial materials, which should be very simple, by biaxial, cores or any other of the solutions that can be applied in composite materials.
    In addition, the first thing you should consider, because it can lead to very different results, is if you want to get the minimum weight structure of all possible, or that combination of layers that gives rise to the cheapest structure in materials, cheaper in hand of work or a compromise between all this (in this last case I would resort to an expert designer). A structure will be good or bad depending on the degree of compliance with the requirements. As I say, different initial requirements lead to totally different results. It is a no-brainer but it is the first thing that must be taken into account.
     
  3. Pammie
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    Pammie Junior Member

    A few questions:
    Is it a problem I mix longitudinal and transversal sections? As I understand it Annex H and propably also CLT SW calculates over the short side so I have to differentiate between x and y plate properties. Correct?

    Is it a problem my "deck" (that is according to load) jumps from side to top at A8?
     
  4. TANSL
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    TANSL Senior Member

    The ISO considers the longest side and the shortest side of a panel without prejudging whether the long side is on the "X" or "Y" axis. There is a pressure (pressure that does not depend on the orientation because what is considered is the pressure in the center of the panel) that is exerted on a panel of dimensions "a x b" and whatever the orientation of the panel, the results will be the same.
    Whenever there is a discontinuity in a structure, the means must be provided to resolve the concentration of tensions that may occur there. The solution is usually a console that joins the two pieces of transversal beam in each "step".
     
  5. Pammie
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    Pammie Junior Member

    @TANSL: I calculated the load of every panel but will work this out for the most loaded ones. I studied Annex H to understand how this works and am now studying CLT and software for that. I've checked eLamX2 and Composite Star but both are not able to work with non symmetric laminates (they put the neutral line in the middle). Besides that I am not sure what to do with curved plates in CLT. How to decide what loads I should take into account (local, global, moments for side plates?) How to convert a pressure load to a strip load? Etc.
    Minimum weight is a nice thing but I will not be searching for the end of it. I'd be happy to keep this hull below 220 kg (155 kg now).
     
  6. TANSL
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    TANSL Senior Member

    I do not know what CTL is.
    The position of the neutral axis is fundamental to make the study layer by layer in composite materials. If there is a system that does not calculate it correctly, forget it immediately, it does not work.
    You can do the calculation for as many panels as you like, but as the boat you will not do it with different patches, in each zone (center, bow and stern) you will have to make all the panels equal to the most loaded.
    The ISO defines a design pressure for each zone, bottom, side, etc., and, depending on it, determines the minimum conditions of each panel. (CSs do something similar). I do not know anything else. If you want to send me the drawings of your boat, longitudinal profile, frames and profile of the deck, I would tell you how I would focus it, I can not do more.
    I recommend that you try to understand how the ISO works and that you forget about the rest. Initially, you also forget the triaxials, mixed cores, and other sophistications. Learn how to scant up a "normal" boat and, little by little, you will add improvements.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    That's because you're rushing into 'details' before you have established the principle behind the structural arrangement.

    See the image Angelique posted here.

    A simple 3D sketch of what you have and where will quickly show where you supports are or at least, intend to be. Then you can begin to define your structure. Can't do simple 2D frame analysis without understanding the bigger picture i.e. the 3D arrangement.
     
  8. 07MAM
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    07MAM Junior Member

    Good day sir!

    I am really interested in your posting about Catamaran can I asked for help if there is a Block Coefficient that is appropriate for our Philippine water in designing a Catamaran. If not, can I asked any recommendation what is the better Cb to used for designing a Catamaran Ferry with a heavy top load.

    Thank you so much in advance. Hope you can read this message of mine.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Pammie,

    Here is a diagramatic way of establishing the panel proportion.

    1. Start at the midship section where the hull is widest and most probably have turn of the bilge producing a natural stiffener. The ISO diagram is self explanatory for chine hull. For hulls with bilge radius and not so defined knuckles, here is how.

    2. Draw a line 50 degree up from the horizontal. Slide it over the baseline until it touches or form a tangent to the bilge radius. This becomes the "A" span, At the tangent draw another line to the intersection of the hull and deck. This becomes the "B" span.

    2a. The "A" span defines the bottom panel. As such, the bottom pressure derived must be used for calculating the panel strength/stiffness.

    2b. If the "A" span width becomes too impractical, it should be treated as >150 angle.

    2c. There are exception to the rule (not defined in ISO) if the bottom has a girder and a floor, or if the bottom curvature has become tight that placing a center girder is redundant, example b must the used. The bottom forms a "D" section satisfying the keel strength and modulus requirement.

    3. If the "B" span shows a very noticeable knuckle or bend of less than 130 degree a second line must be drawn, forming a "C" span.

    3a. If the "B" span shows a very large depth of curvature of panel, it must be divided into two.

    3b. If the "B" span is longer than the frame spacing (discussed later), then it must be divided into two again provided that the end goal is for a longitudinally framed hull. For transversely framed hull, the spacings of the frames are brought closer together.

    4. The spacing of the frame shown in the third sheet is the preliminary length and the distance between the natural stiffeners (span without stiffeners) is the preliminary aspect ratio.

    4a. Divide the distance between the frames by the distance between the span. This is the rough aspect ratio of a panel. You should be looking at 2:1 or greater.

    4b. Since a panel is described as a plate bounded by the edges of the frames and stiffeners, you musr subtract the base width of the frame to the spacing and subtract the base width of the stiffener (if there is any) to the span of the panel. This is defined as "l" and "s" by the ISO rule. You now have the final Panel Aspect Ratio. Panel Aspect Ratio of 2:1 or greater is recommended but if less than 2, panel aspect ratio correction factor must be used in Table 5. Use the correct coefficient in the bending moment rule.

    4c. If the panel is to be evaluated in the short side and long side position, you might want to investigate the eAR rule Table H3.
     

    Attached Files:

    Last edited: Aug 7, 2018
  10. rxcomposite
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    rxcomposite Senior Member

    There is no perfect Cb. Just a range appropriate for the SOR.

    With heavy top loads, it implies that it is a workboat. Since cats have naturally slim hulls compared to monohulls, the Taylors number or speed to length ratio will play into how much blockiness (greater Cb) the hull will have.

    A blocky hull implies also a flat or near flat bottom craft, thus lowering the center of gravity, or allowing the engines to seat deeper. Necessary for a heavy topload. Start another thread lest we hijack Pammie's thread. Include this as first response.
     
    07MAM likes this.
  11. 07MAM
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    07MAM Junior Member

    Estimated Block Coefficient for Catamaran Vessel https://www.boatdesign.net/threads/estimated-block-coefficient-for-catamaran-vessel.60819/#post-836877
     
  12. Pammie
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    Pammie Junior Member

    @TANSL:
    CLT stands for classical lamination theory.
    It is a good idea to calculate the hull as if it were a 'normal' laminate and compare this to Annex H calc and CLT.
     
  13. TANSL
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    TANSL Senior Member

    Thanks Pammie for the explanation. I was confused and I did not see what was obvious.
    About sandwhich, in my opinion, but it is only my opinion, just in case you need a laminate of a very large thickness, or with many layers, it would be interesting to check if the sandwhich type construction improves something: it reduces the weight or reduces the labor . There are constructive reasons that may advise you to use the sanwhich but use it without a very specific reason, it does not make much sense, in my opinion.
     
  14. Pammie
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    Pammie Junior Member

    @Ad Hoc: Hmm... I think I do have a good idea about the structure of the hull. Why do you think not? Maybe you have this idea because the longitudinal structure is not that visible in my sketches? Added pictures of the current situation that shows the structure. My biggest point of attention is to get all details of ISO right and do calculations that make sense.
     

    Attached Files:


  15. TANSL
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    TANSL Senior Member

    Well, you already have the 3D structure that is what Ad Hoc advised you should do. Although we already knew that from post # 1.
     
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