McNaughtons Scantlings - fibreglass weight sheathed strip construction

Discussion in 'Boat Design' started by rwatson, Feb 21, 2009.

  1. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Hi all

    Just working my way through the Mc's scantling rules I bought recently (they are cheaper to buy as PDF instead of books - great idea !)

    I would love to get verification that I inderstand the formulae.

    Their scantlings for 'glass weight over strips planking are expressed like this

    "If ordinary balanced weave glass cloth is used, the total glass cloth weight of each skin in ounces per square yard, equals 3.67 multiplied times the cube root."

    So, if the boat is evaluated to a cube root of 3.8 (3500 lbs displacement),

    or
    3.67 x 3.8 = 13.9

    or 13.9 ounces of glass per square yard on each side

    This sounds reasonable, but have I worked it out correctly do you think ?

    Any and all thoughts welcomed.
     
  2. apex1

    apex1 Guest

    Hello rwatson,
    have no idea if that works well, but I fear it does calculate the glass only?
    Real figures are provided here:

    http://www.r-g.de/en/
    change language, goto onlineshop, select your product and find laminate weight per m² incl. resin, for every single sort of fiber, weave etc.
    a program to calculate laminates: (must register)
    http://www.r-g.de/laminatberechnung_registrierung.html
    and this is a REAL test:
    http://download.r-g.de/rg_dokumentation_whz.pdf
    unfortunately (for you) in German only.

    I hope I could help a bit further!

    Regards
    Richard
     
  3. Alik
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    Alik Senior Member

    I wonder why use these amateur 'scantling rules' if official standards (ISO12215-5, ISO12215-5 simplified, GL, etc.) are available?
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Maybe a fear of VOR's and Vendee boats having ISO scantlings :p
    Reckon however they aren't :confused:
     
  5. diwebb
    Joined: Jun 2008
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    diwebb Senior Member

    have a look at Dave Gerr's book The Elements of Boat Strength for builders, designers, and owners. This book synthesizes the formulas and scantling rules in to simple tables and graphs for those who do not wish to spend a long time and a lot of effort to get a viable construction layup for their boat. I find it very useful and relatively easy to understand. Also difficult for a non professional to make an eror in selecting a suitable weight and thickness for the hull and other parts of a boat.
     
  6. Alik
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    Alik Senior Member

    Provided that there is no error in the book :)
    Actually, the method in that book does not work for planning boats.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Dave Geer's "scantling rules" work rather well, including planning hulls. What they don't do is produce the lightest boat you can get away with, which is an intentional side effect of his rules. He clearly states the rules are designed for well founded hulls, not light weight machines. This covers the vast majority of desires when looking for this type of information. He also points out that the rules can be shaved to a degree without harm, but understandably rejects any ill will, that may come about by someone looking for this type of short cut. If you need this type of hull, then the reasonable designer works out the loads and scales everything accordingly.
     
  8. Alik
    Joined: Jul 2003
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    Alik Senior Member

    They don't really have planning hulls in Gerr's book as they do not define planning speed. Gerr uses increment of thickness for over 10kts(?), and this sounds rather strange because there is no dependence from Froude number. And actually there is no definition of design loads...

    I mean Gerr's books are always great reading - I study his "Mechanical systems" right now, that is excellent. But for structural calculations there are better professional sources supported by software. I still believe for amateur designers simplified method presented in ISO12215-5 is the best.

    This is my calculator for planning boats, based on ISO simplified method, works for craft under 12m.
     

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  9. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    So many answers .... but no solution to the question. I dont know why people assume I have no other source of calculations than McNaughtons.

    Also, funny how close McNaughtons comes to the calcs on the 'easy ISO calculator' since they are 'amateurs' !!!

    Still, if anyone wants to throw in a comparative figures, feel free

    Length 8.5 metres

    Beam 2.24 metres

    Lightest displacement 1200 kilos

    Heaviest Displacement - 1800 kilos

    Draft around .3 metres

    Freeboard - around 1 mtr

    Power Source - 50 -up to 70 hp outboard

    Top speed around 20 knots with lighter load

    Building materials - Ply ,or epoxy over strip planking
     

  10. ricardoribeiro
    Joined: Mar 2004
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    Location: Brasil

    ricardoribeiro Junior member

    Você tem o easy structural calculator xls planilha para sailboats?
     
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