Laminate model boat panel calculation

Discussion in 'Fiberglass and Composite Boat Building' started by BTG YACHT DSGN, Aug 17, 2010.

  1. BTG YACHT DSGN
    Joined: Oct 2008
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    Location: Poland, Warsaw

    BTG YACHT DSGN -sailing is believing-

    Currently I want to determine in more scientific way than just say ,,Well, let's put there two 100 g/m^2 WR cloth and that should do the thing go". That's why I went back to Larrsson's ,,Principles..." and started to follow the equatations.

    !To make it clear I'm building a model boat, radio controlled of 80 cm LOA!

    I have two questions about panels: firstly, Larrsson showed on fig. 14.5 , page 287 proposed panel arrangement (ie. panel 1A, 3E, 2B etc.). In my opinon this panels are large and in particular there's large difference to determine the value ,,c" of each panel. In my considerations, this value varies on each end quite a lot. Should one use a median value? Or maybe it's a common practice to calculate the panels of very small ,,I" value (let's say ,,short one's". Then this difference in depth ,,c" is getting smaller and smaller... It comes to mind an integration calculation. So what's the common practise in naval architecture to arrange panels... Is Larssonn's proposition just an example, or is it a sufficient engeneering model?

    And secondly: in the same figure, in equatation for Bottom pressure 3a) Pbsbase= (2*Mldc^0,33+18)*kls I don't know if this ,,+18" is practical to apply in my situation. I guess (please do correct me) that this ,,+18" is kind of constant, used to cover some dynamic conditions or wave slamming... Anyway the values I calculate using the original equatation are too big in my feeling... Is it possible that a modelboat has a Pbsbase of over 20 kN/m^2 ? While YD-40 has the same value for bottom at 65% Lwl aft of FP...

    I know that ISO scantlings were not designed for calculating model yachts, but can they be used and can I rely on the values I get when using their equatations step by step?

    Regards,

    Kuba
     
  2. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Hi Kuba,

    When you're looking at the ISO standards, upon which the Larsson/Eliasson calculation is based, you're looking at a set of empirical guidelines that are believed to represent common practice for boats in a certain size range (2.5 to 24 m LOA). It's not a complete and theoretically accurate model, just a framework of parameters and relationships that produce structural specifications typical of known successful designs, and stronger than known failed designs.

    Scantling formulae don't scale very well outside of their intended domain. In the case of your models, they don't even consider the correct loads- an 80 cm model's sailing loads are very light. A model this size will more likely be limited by its resilience on land- for example, whether the skin will bend excessively or crack if the boat is jostled in the car, or slips from its cradle at the dock.

    If you're concerned only with sailing loads- and not with what happens on shore- I'd think you can roughly follow the ISO method, but you'll have to do some re-deriving and re-calculating according to your engineering judgment. That 18 kPa base pressure, for example, might be quite a lot lower in a model (which sees vastly lower speeds and sea states than a full-size boat). You may have more success just going with a first-principles approach.
     
  3. BTG YACHT DSGN
    Joined: Oct 2008
    Posts: 104
    Likes: 3, Points: 0, Legacy Rep: 38
    Location: Poland, Warsaw

    BTG YACHT DSGN -sailing is believing-



    So it seems that an old, good ,,rule of thumb" still applies and that there are small chances to make a ,,light- weight revolution" by doing just the ISO calculations... :(

    Anyway, as I made some calculation, step by step according to ISO, I received some reasonable values. For example the

    hull panel at bottom, from stem to 20% of Lwl should be 0,424 mm thick (Wf=0,35, fibre weight 220 g/m^2),

    hull panel further aft, so bottom from 20% to 40% of Lwl should be 0,723 mm thick (Wf=0,35, fibre weight 375 g/m^2),

    hull panel at bottom from 40% to 70% of Lwl should be 0,325 mm thick (Wf=0,35, fibre weight 168 g/m^2)

    hull panel at bottom from 70 to 100 percent of Lwl should be 0,342mm thick (Wf=0,35, fibre weight 176 g/m^2)

    So, the results are reasonable and can be further reduced, becaused I choosed Kdc factor of 1, which is for ,,A" category... Just as if it was an ocean yacht in full scale ;) . On the other hand, if I make the calculations with Kdc set to .25 the laminate might be so thin, that it will crash in land, the situation you mentioned, marshmat...

    Can you please also tell me how is it with this panel arrangement, as asked in my #1 post?
     
  4. Alik
    Joined: Jul 2003
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    Alik Senior Member

    Definitely ISO is not for model yachts!
     
  5. BTG YACHT DSGN
    Joined: Oct 2008
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    Location: Poland, Warsaw

    BTG YACHT DSGN -sailing is believing-


    I know that it's not intended for boats smaller than 2,5 m but can it be used as a guide?
     

  6. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    I don't think so - they have criteria based on minimal number of layers...
     
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