From Army idea, 4 mt. Diamond monocoque with Sika adhesived 0.7 mm stainless steel

Discussion in 'Boat Design' started by mustafaumu sarac, Oct 15, 2021.

  1. fallguy
    Joined: Dec 2016
    Posts: 5,861
    Likes: 1,159, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The comment was made regarding the statement that 'small obects have an inherent structural rigidity' which is just not true. You are simply doubling down which might work in politics, but not here.
     
    gonzo likes this.
  2. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    In engineering assumptions and frames of references must be stated. The claim that small objects have an inherent structural rigidity has no evidence of being true.
     
  3. sharpii2
    Joined: May 2004
    Posts: 2,135
    Likes: 260, Points: 83, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    I think it would be fair to say that it is easier to make a small object rigid than a large one.
     
    TANSL likes this.
  4. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    No, it wouldn't. Rigidity is a function of the material properties and the design. This is one of the situations where size doesn't matter.
     
  5. TANSL
    Joined: Sep 2011
    Posts: 6,946
    Likes: 542, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The flexural rigidity of a beam is usually defined by the young's modulus of the beam material, the moment of inertia of its cross section, and the length of the beam. Accordingly, saying that size does not matter does not seem very accurate.
     
  6. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    From your point of view, size matters. However, the larger the size the more rigidity, which shows the OP is wrong.
     
  7. TANSL
    Joined: Sep 2011
    Posts: 6,946
    Likes: 542, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    For flexural rigidity, the larger the size the less rigidigy.
    Rig = E*I/l
     
    Last edited: Oct 25, 2021
  8. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    For a rectangular beam I=bh^3. E increases to the cube, while l increases linearly. Ergo, rigidity increases with size.
     
  9. TANSL
    Joined: Sep 2011
    Posts: 6,946
    Likes: 542, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    No Gonzo, the "I" (moment of inertia of the cross section) does not change when the length of the beam is increased. Talk to your advisors, I know you have good friends at the University, tell them what we are talking about and that they explain it to you in simple terms.
     
  10. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If all changes maintaining the same ratio (getting bigger as related to the post) the depth and width increases as the length does.
     
  11. TANSL
    Joined: Sep 2011
    Posts: 6,946
    Likes: 542, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  12. upchurchmr
    Joined: Feb 2011
    Posts: 3,269
    Likes: 249, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    This is another example of needing to state assumptions.
    You two have different assumptions for a not very accurate statement = because it was not stated well.

    Just answering No was one of the more juvenile statements I've seen here in a while.

    Like two 10 year olds repeating - is too, is not, is too, is not, etc.
     
    bajansailor and TANSL like this.
  13. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Since I quoted cracked ribs on my answer, it isn't confusing that I was answering his statement. Nobody answered "no" however.
     
  14. Ad Hoc
    Joined: Oct 2008
    Posts: 7,274
    Likes: 1,164, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    That's correct.
    However, it does need a little further explaining, as this is a blanket statement.

    That does it nicely.

    It is misleading however, to say this:

    Clearly "size" does matter.
    Since size is not related to just its length - the out-of-plane length of the sectional shape. Size infers all 3 dimensions: length, breadth and depth.

    Because the frame of reference - the dimensions - have now been changed, from what Gonzo referred to, as the sectional shape (the depth and breadth of the section for the "I") to this:

    Gonzo did not state the length - just one dimension out of 3 possible - remains the same. Since it is clear he is referring to the product the - EI - which is the flexural rigidity and the shape of the section.

    The moment of inertia of a section does not have length as an input. It has height (the L, or which ever letter you prefer) and width (the B), as per clearly defined in each figure that outlines the 'shape', the nomenclature is for each figure and must not be inferred to be anything else. Mischaracterisating the "L" as length of section to be the out-of-plane dimension - L - belies a lack of understanding and comprehension.

    The 'Length' that Tansl is referring to....is the 'length' which is the out-of-plane perpendicular axis... but has nothing to do with the sectional shape Gonzo clearly noted.
    Hence the mischaracterisation for perhaps some cheap shot?? This is an engineering/boat forum, not the school playground gents.

    For those that do not understand here are typical second moments of inertia of sectional shapes, clearly showing "length of section" is not an input. Despite nomenclature sometimes using "L".
    Such as a full range of shapes HERE, (where they use 'h' as the depth)
    or
    a page out of any text book, shown below (where they use d as depth):

    upload_2021-10-26_8-47-52.png

    Definitions mean everything in engineering.
     
    Last edited: Oct 25, 2021
    gonzo and bajansailor like this.

  15. upchurchmr
    Joined: Feb 2011
    Posts: 3,269
    Likes: 249, Points: 63, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Should be enough detail for anyone.
    Perhaps assuming Gonzo had one definition for the length element was an assumption not warranted.

    Enough from me.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.