Wood-Metal Composite

Discussion in 'Boat Design' started by DCockey, May 7, 2017.

  1. PAR
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    PAR Yacht Designer/Builder

    With modern adhesive/sealants, this issue can be mitigated to a degree.
     
  2. goodwilltoall
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    goodwilltoall Senior Member

    Thinking more about it, if steel stringer could be have welded partial brackets or channels for the wood to free float inside but with tight tolerance.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Isn't that a contradiction..free to float..but tight tolerance?...and what would be its purpose?
     
  4. goodwilltoall
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    goodwilltoall Senior Member

    Purpose - take up expansion differentials between wood n steel.

    Picture metal stringer with 4" tubuler square welded outside say 3' o.c. 1×3 stringer goes through allowed to just barely slide. Slotted at screw location.

    Or perhaps even polyureatheane
    adhesive at inside to accomodate anticipated movement but remain secure to framing
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    The same contradiction.

    Adhesive oh..to be secure, aahh...but then...to allow movement.

    So, i'll spell it out in simple terms. You can't have both....either it is being secured or it is allowed to move freely.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I have now received my MT...i read the article differently. The author is mixing up values and I suspect as he has an agenda, a motive for doing so.

    He says "....the weight of shell plating per unit area is the density times the thickness, and the strength is yield strength times thickness squared. This favours materials of low density: a figure of merit is the square root of yield strength divided by density, with the highest figure indicating the "best" material....."

    A load that is applied to a panel/beam can easily be calculated as the stress = My/I or = M/z. Where z = modulus. The modulus of a panel of plating is simply bt2/6.

    So he is mixing this up by rearranging so that M, the bending moment = s.Z proptnl to s.t^2. (hmm..bummer, i thought this new website format would allow me to use Word and its symbols and cut n paste..but not..same as old one....crap!)

    So the bending moment is proportional to the yield strength x thickness squared. If the M remains the same, either the yield strength of the material or its thickness will influence how much M it can carry. It says nothing about the allowable deflection, and says even less about favouring a material of low density!

    Looking at proper charts of materials and placed in their correct order of properties, for minimum weight that gives a strength limited type of design parameter wood is very low down:

    [​IMG]

    And then looking at the amount of stored energy in a material, again wood low down:

    [​IMG]


    I’m not knocking wood (excuse the pun) at all…every material has its pros and cons and every design must be evaluated on its merits and SOR. Thus, no one material is better than any other, it is what’s best for THAT design.
     

    Attached Files:

  7. DCockey
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    DCockey Senior Member

    "bending moment is proportional to the yield strength x thickness squared" (1) This is maximum bending moment without the yield stress being exceeded. Displacement is not considered.

    Rearrange (1) and take square root: thickness is proportional to square root (bending moment / yield stress) (2) This is the thickness needed for a given bending moment without the yield stress being exceeded. Also corrected "strength" to "stress".

    Panel weight per unit area = density x thickness (3) Based on definition of density.

    Rearrange (3) to: thickness = weight per unit area / density (4)

    Use (4) in (3): weight per unit area / density is proportional to square root (bending moment / yield stress) (5)

    Rearrange (5): weight per unit area is proportional to square root (bending moment) * [density / square root (yield stress)] (6) This is the weight per unit area in terms of density and yield strength for a given bending moment.

    Rearrange (6): weight per unit area is proportional to square root (bending moment) / [square root (yield stress) / density] (7) Thus the weight per unit area is proportional to the the bending moment divided by Chris' figure of merit: square root (yield stress) / density. A higher figure of merit means a lower weight panel.

    Let's consider the relative thickness of panels made of two materials, w and a, subject to the same bending moment (also same size, etc) using (2):

    thickness of a is proportional to square root (bending moment) / square root (yield stress of a) (11a)

    thickness of w is proportional to square root (bending moment) / square root (yield stress of w) (11w)

    thickness of w / thickness of a = square root ( yield stress of a / yield stress of w) (12) The ratio of thicknesses is proportional to the square root of the inverse of the yield stresses.


    The density of wood is also very low. That is why for a much thicker wood panel can be lighter than a much thin aluminum or steel panel for a given bending moment.

    Now let's consider displacement of a panel.


    maximum deflection of a panel is proportional to bending moment / (E * thickness^3) (13) From thin panel equations.

    max deflection of w / max deflection of a is proportional to [(E of a * thickness of a^3) / (E of w * thickness of w^3)] (14)

    Use (11) in (14): max deflection of w / max deflection of a is proportional to [E of a / (yield stress of a)^1.5] / [E of w / (yield stress of w)^1.5] (15)

    Rearrange (15): max deflection of w / max deflection of a is proportion to [(yield stress of w)^1.5 / E of w] / [(yield stress of a)^1.5 / E of a] (16)

    From (16): For panels with thickness selected such that the maximum stress equals to yield stress for the given bending moment, maximum deflection is proportional to the (yield stress)^1.5 / E (17) This only applies if the panel thickness is based on maximum stress.



     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Which it is not, or shouldn't be for low modulus materials.....see one of my previous posts on the checks required.

    Structural design cannot be dumbed down into a one liner.
     
  9. Rakesh Bhansali
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    Rakesh Bhansali New Member

    Thank you Guys..for sharing this important information...
     
  10. Rakesh Sanghvi
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    Rakesh Sanghvi New Member

    Very Nice Posts...and Images..Thanks
     
  11. Angélique
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    Angélique aka Angel (only by name)


    In section 3 some 1919 - 1948 Lloyd's Rules regarding the thread subject, don't know about its validity today...

    ‘‘ Lloyd's Rules
    Here are the scantlings for building International Rule yachts, 6, 8 and 12 metres, covering the years 1919 - 1948. These rules also cover the building of these yachts beyond these years in the methods described. These files comprise all of the sections and tables.


    -- PDF links --

    1) - Title, Preface, Index - 4 mb

    2) - Surveys for Wooden and Composite Yachts, Rules for the Building and Classification of Yachts - 7 mb

    3) - Composite yachts having steel frames and wooden planks, Tables 1 - 8 for Wood Yachts - 6 mb

    4) - Composite Yachts - Tables 9 - 15, Anchors - Table 16 - 1 mb

    5) - Wood Yachts - Metric System - Tables 17 - 24, Composite Yachts - Metric System - Tables 25 - 32, Proving Establishments - 2 mb ’’
     

  12. CDBarry
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    CDBarry Senior Member

    Young's modulus of wood is between 2,000,000 psi and 1,000,000 depending on species.

    Steel is 30,000,000 psi.

    Take a plate of 4.08 lbs / ft2.
    It would be an inch thick or more in wood, EI for 1 inch breadth is 10^6 * 1 * 1^3 / 12 = 83,333.

    In steel it would be 0.1 thick, EI 30 x 10^6 * 1 * 0.1^3 /12 = 2,500.

    The effect of material density on stiffness per weight ratio is even more important for deflection.

    As to practical issues, wood over metal was very common for larger boats in Europe before WW II because of the difficulty of getting large timber. (Not a problem in North America.) More recently there have been a few minesweepers
    built in wood over aluminum to minimize their magnetic signature.

    The royal yacht Britannia was wood over metal, for example and there are currently a number of builders of larger composite yachts in the U.S.

    Also the practical issues of welding, etc. thin metal make metal plating impractical, but metal framing at some level (say, wood longs with aluminum web frames, girders and keel) might work.

    The ability to CNC the metal parts also makes the combination more feasible.

    The point of the article was that metal / wood composite construction might be worth considering for some one off or low rate production boats from both practical and structural reasons.
     
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