Catamaran scantlings

Discussion in 'Boat Design' started by Mikthestik, May 19, 2016.

  1. TANSL
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    TANSL Senior Member

    This information is quite complete. A GA drawing would be necessary to see transverse bulkheads, frames and longitudinal stiffeners, which has the ship as well as the structure of the wet deck and its transverse beams.
    Max speed?
     
  2. Mikthestik
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    Mikthestik Junior Member

    Hull speed about 9Kts with 30hp.mik
     
  3. Mikthestik
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    Mikthestik Junior Member

    I looked again at Richard Wood's "Romany" and found that my guessed scantling number of 0.96 using Lindsey Lord's formula from Boat Strength gives 8.5mm planks and inner/outer E glass of 245g/369g which is almost exactly what is specified for "Romany". Note Building to LL scantlings would produce a stiffer boat as Vectra and dynell seem to be difficult or impossible to get.
     
  4. TANSL
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    TANSL Senior Member

    I attach data from a GRP catamaran, calculated in accordance with SSC Rules by Lloyd's Register. Although, as I have noted ppreviously, without knowing how the structure of your boat, there should be no comparison.

    Total Breadth of Hulls at LCG 3.760 m
    Maximum Air Gap: 1.040 m
    Draught : 1.000 m
    Waterline Length : 14.926 m
    Long Centre Gravity : 6.407 m
    Significant Wave Height : 2.500 m
    Displacement Mass : 36.910 m
    Allowable Speed : 14.000 knots
    Deadrise Angle : 16.600 deg
    Frame spacing : 0.500 m

    thicknesses
    Keel : 17 mm
    Bottom Panels : 6.3 mm
    Side Panels : 5.7 mm
    Main Deck : 4.6 mm​
     
  5. Mikthestik
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    Mikthestik Junior Member

    Thank you Tansl Now I am going to do a weight estimate to compare with the spreadsheet resultd then try to make a drawing.mik:D
     
  6. Mikthestik
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    Mikthestik Junior Member

    My weight estimate using lyndsey lord scantling rules worked ok. I found the attached document very interesting, as an alternative/comparison with Dave Gerr rules. I have never heard of MARINA as an organisation. However Par has told me in a previous post if 9mm planks are required about 12mm ply is equal in strength. I am confident Par is correct. The required planking (page 22) for a 6M boat is 19mm OR 9.5MM MARINE PLYWOOD.:confused: mik
     

    Attached Files:

  7. gonzo
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    gonzo Senior Member

    The strength of planks are plywood are very different. To get full advantage of the characteristics of each material, the structure should be designed accordingly. Usually, plywood hulls are thinner than planked.
     
  8. Mikthestik
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    Mikthestik Junior Member

    Thank you Gonzo. I therefore must accept that the Marina organisation must have undertaken some sort of structural analysis in conjunction with their frame sizing and spacings. It also makes sense of the hull of a Tiki 21 in 6mm ply. At the same ratio a Tiki 21 would have 12.7mm planks, much nearer to what Gerr's rules calculate.:D Mik
     
  9. TANSL
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    TANSL Senior Member

    You should explain this better because it could be totally wrong.
    Current methods for calculating scantlings of small boats (between 2.5 and 24 m in length) use exactly the same formula to determine the minimum thickness of a panel, either strip planking or plywood. What happens, as with any other material, is that the minimum thickness does depend on the minimum bending stress parallel to the smallest dimension of the panel, for each material. According to that, as expected, everything depends on the quality of material used in each building system but apparently not dependent on the building system.


    In another vein, if you want to design a modern ship, I would not use the calculations that were used 100 years ago.
     
  10. Mikthestik
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    Mikthestik Junior Member

    Tansl I think I understand what you are saying. The frame spacing on Marina Hull rules I not large and panels would be small. What would you recommend for the deeper boats I think adding 5mm per size would make them very strong. Mik 2016-06-03_210055.png
     
  11. TANSL
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    TANSL Senior Member

    Sorry Mik but I can not comment without knowing how the boats structure (frames and longitudinal), without knowing the type of construction you want to use without knowing the type of wood or plywood and, therefore, its mechanical properties .
    Neither my professional deformation allows me to look at a boat similar to yours and say how it should be yours.
    Sorry, with the data I have, I can not quite help and I can not give you an opinion knowing that if I'm wrong, the problem will be yours.
    BR
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Mik

    No, it may seem like it is, but to anyone who does this as their daily job is very simple and straight forward. It only seems daunting to those that have never tried before, owing to all the rules that must be adhered to for compliance.

    Well, if this is the case, then i would suggest either contact Richard for such, or any naval architect to advise you. As you have already noted:

    Then what is the purpose of posting this data if by your own admission it is meaningless?

    All you are doing is now confusing him, by posting data that is unrelated and now you have shifted away from commenting whilst still posting!

    Mik,

    If you're serious about this project, you should employ a NA/designer to do the calculations you seek. Otherwise you shall get endless non-committal replies that pretend to offer assistance, but merely confuse you further.

    If you're willing to spend a small amount of money, I can put you in contact with a colleague of mine who knows more about small GRP cats then I can shake a stick at...and he's only about 3hours drive from you too. But, you will have to part with money.
     
  13. gonzo
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    gonzo Senior Member

    In post #16 TANSL said that the "information is quite complete", then tells the OP that he can't do it with that data. I think the OP needs the help of a naval architect, engineer or shipwright.
     
  14. TANSL
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    TANSL Senior Member

    Gonzo, if you read in a little more detail the post # 16, you can see that, although I say, indeed, that the information is complete, also I say that it would need to know any more information. Without knowing the GA, the distribution of reinforcements, ie, how the structure, without knowing the maximum speed, without knowing the navigation area, without knowing the mechanical properties of the material used, etc ... is not possible to make a serious work.
    Reread the post # 16, please. Surely you, as a designer, will agree.
    Do not compare, please, professions of shipwright and na or engineer. All three are very respectable but the knowledge required each are totally different and what one can calculate the others do not have to know how to do. I would not ask scantlings to a shipwright, nor building the boat to a engineer or na.

    Ad Hoc, I can, I guess, think and show certain data, but when someone asks me a specific answer to a particular ship, should not give an opinion but give the result of my calculations for that boat. Apparently you would act differently ?. If I confused Mik, only he can say. You can not. I just tried to tell him to seek advice from someone qualified and do not be guided by opinions, unprofessional, which can lead him to very serious problems. Do you think my attitude is wrong ?. What your professional ethics would propose about it?. Thanks, as always, for your constructive comments.
    Let me ask an innocent question, thanks in advance, trying to clarify a question certainly produced by my enormous ignorance : why you offer the services of a GRP technician when Mik is talking about types of wood?. This, I don't know to Mik but to me, yes, it is confusing me.
    Speaking at this time, in this thread, in your first intervention, about money does not seem very smart. Is it?
     

  15. Mikthestik
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    Mikthestik Junior Member

    My profession before I became disabled was Electrical/electronic in nature. The requirements of that profession forced me to learn the things I know. So what am I to do, being disabled I can get about with my stik hence the name, but there are few mental challenges left. I have chosen to try and learn something about naval architecture. I have spent money on books and tried to design or partially design a boat to a point where if I was fit and up to sailing I could bring a drawing of sorts to someone like Richard Wood and say this is what I want. Between my meager bank balance and my lack of physical fitness it is a source of enjoyment for me but it is never going to happen. I have an enquiring mind to much time on my hands and sitting in front of a TV doesn't appeal. I have learned much from your replies. I'm sure I will continue to do so Mik:)
     
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