Catamaran scantlings

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

  1. Mikthestik
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    Mikthestik Junior Member

    I have decided Dave Gerr scantling rules are fine for mono hulls but not cats. The best rule of thumb I have is from Raggi_Thor. 9mm for up to 4m boats plus 1mm per extra meter. Some on this forum delight in rubbishing Dave Gerrs work, which is why he no longer contributes. I read somewhere his formula produce a boat which is heavier than required. The Raggi_Thor method gives planks 4mm less in thickness.
    Could someone supply me with some boat details of say 20 -50ft wooden boats with scantlings, i.e hull/keel/deck thickness so I can judge for myself how good these formula are.
    I thought your insurer sent a man to check the boat if scantlings and anything else was amiss then no deal. mik
     
  2. TANSL
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    TANSL Senior Member

    A formula that gives the thicknesses based solely on vessel length, in my olpinión, is unreliable. In any case, as you know very well, the thickness of the hull will vary greatly depending on the quality of the wood you use, the construction system and the distribution of reinforcements. So, with the data you have provided us, I do not think anyone is able to give you information that will serve you for something useful.
     
  3. gonzo
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    gonzo Senior Member

    Dave Gerr's scantlings correspond to Herreshof's rules for plank of frame and strip planking. He is very specific on the limitations and constraints of his rules. They are conservative and not appropriate for high speed or race boats. A range of scantlings for 20 - 50 foot wooden boats of a variety of construction methods will fill a library. There is no way to post that in a thread. You can look at many of the published books that give information about particular boats. Also, many museums, like the Smithsonian and the Maine Maritime museum have a huge collection of wooden boat and ship plans.
     
  4. Mikthestik
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    Mikthestik Junior Member

    building boats to any class rules seems to me to be a black art. If your insurer does not like it there is no point in building the project. In my previous post 16 or 20mm is the choice. From John Teal's book Youngs modulus for ply is 1.8, for GRP is 1.3 to 1.6 Therefore ply is stiffer/ better than GRP. Now the formula for solid skin GRP boats is 1.8 + WL(in meters)/1.8 = 8.1mm. So if the boat is built in 9mm ply with 6oz/sqYD internal and 10 oz/sqYD external sheathing. The framing would have to be correct as you pointed out.
    1. I am confident the boat will be light strong and abrasion resistant.
    2. I am not confident a company would insure it, or more likely refuse a claim because I don't know whose rule it adheres to. If Loyds specify 7mm ply fine If not there is the problem. I think few people will know fore sure till tragedy strikes. Thats why I asked for details wich are easy to get and actual scantlings which are not. mik
     
  5. Mikthestik
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    Mikthestik Junior Member

    Thank you gonzo I will search.:)
     
  6. gonzo
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    gonzo Senior Member

    Scantlings by Gerr or Herreshoff rules are not related to classification societies. They are engineering formulas derived from empirical knowledge.
     
  7. Joakem
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    Joakem Junior Member

    Why don't you use ISO 12215?
     
  8. TANSL
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    TANSL Senior Member

    ISO 12215-8, which deals with catamarans, has not been published yet, is not in force. ISO 12215-5 and 6, for monohulls, can be applied but with certain limitations.
     
  9. Mikthestik
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    Mikthestik Junior Member

    Trying to get it is the problem I won't do financial stuff on the net. I downloaded a document which I think is 1927 racing rules by herreshof and it has scantling rules which apply to the yachts.The one which applies to planking is Cube root of displacement in cu.ft times 0.14 gives plank thickness for wood of hard pine or heavier. The displacement of my project is 177.43 cu.ft =88.72 per hull = 0.624"/16mm. The Selway Fisher cat19 uses 6 and 9mm ply The formula gives 8.74mm. Most of their other boats use thinner ply than the formula predicts so they do something differant.mik
     
  10. TANSL
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    TANSL Senior Member

    In any scantlings of a current project you must take into account various parameters that can make the thickness of the hulls, for one or another project, they are totally different. When the thickness of the hull is calculated, what is done it is to study the design pressures to which is subjected a panel in a particular area of ​​the hull. Each area of ​​the hull is subjected to a different pressure so a panel as calculated for the center of the boat, for example, may not be sufficient for a panel of the bow. Another important variable is the size of the panel, and the length/width ratio. The panel dimensions depend on the separation of the reinforcements delimiting the panel. Therefore, we can not compare panels in two different vessels if, for example, the separation of the reinforcements is not the same in both vessels.
    These and many other considerations were not taken into account 80 years ago so calculate scantlings for an actual boat in accordance with those prescriptions may not lead to an optimal structure in any case, although it could be, probably, enough (or too strong).
     
  11. gonzo
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    gonzo Senior Member

    Are you building a plank of frame catamaran?
     
  12. TANSL
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    TANSL Senior Member

    By reading what he writes in the post # 9, one might infer that it is planking building method. How do you read it, Gonzo?
    In any case, what does it matter to you?
     
  13. Mikthestik
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    Mikthestik Junior Member

    Because you gentlemen have answered my posts before, I'm sure you realize I am an armchair sailor/designer:eek:. My project was to come up with an oceangoing catamaran design similar to a Richard Wood design. Construction to be double chine in ply, or If I could not get what I wanted then strip plank round bilge hulls. With 15mm ply skins it is difficult to get the empty weight low enough. Now I know (because it is on his website) he uses 300gr glass cloth and 9mm ply on the "Romany". I can't get that low as my posts indicate. To ask him how he gets 9mm (even if I could) feels rude.mik
     
  14. TANSL
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    TANSL Senior Member

    If you wish, I can do some calculations with my SCT software, which calculates scantlings according to ISO 12215. For this I'd need a general arrangement drawing and all possible details about how do you want to build your boat.
     

  15. Mikthestik
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    Mikthestik Junior Member

    I really struggle with Tech drawing,and was going to try and draw it once i was sure every thing was right and proper. I have enclosed the spreadsheet with the details from which i will eventually try to draw. Note the midsection coefficient of 0.76which is supposed to allow for double chine.
     

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