Limber hole size rules and frame strength?

Discussion in 'Metal Boat Building' started by Steelboat, Apr 5, 2022.

  1. rxcomposite
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    rxcomposite Senior Member

    Nothing much in the Rules about limber hole opening size. Nothing much on the drawing you provided either. If the two members are of the same height, then it must be two primary members as in fig. 3.14 . Lacking the gusset, then it must be one primary and one secondary attached together in which one would obviously be smaller than the other.

    Your drawing does not also fit the design detail of the chine in which limber holes are allowed. See attached.

    There is an explanation on the web opening in 1.25.1 Part 7 Chapter 3 of Lloyd's. Given the rule, it implies that something is passing thru it, a longitudinal perhaps to which it is connected (not shown and yes, there is a hole).

    Web frames have a safety factor of 4 so increasing the size of hole by a little bit would not drastically reduce your safety factor.

    In case of doubt and in the absence of relevant data, add a doubler as most everybody has said.
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly...do you need that doubler?...if you have an R25 mm i say no. As you still have sufficient shear area, as it is less than 50% depth.

    But, if you are set on having the doubler, you need to make sure you don't introduce a stress concentration in doing so.
    For simple shear transfer, welding to the web of the Long.t and the frame is sufficient...and it would pay you to allow enough of a 'gap' to weld the lowered edge to the long.t web too. But make sure it does not cross/over lap the the shell - long.t weld that is or will be there.

    Finally , ensure you to a proper return around the mouse hole.
     
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  3. Steelboat
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    Steelboat Junior Member

  4. Steelboat
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    Steelboat Junior Member

    Actually I am convinced that the boat would likely be fine without doubler, my tendency is to overbuild things, need to work on that.

    attached drawing, I am thinking about the overlap you mentioned.
     

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  5. AlanX
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    AlanX Senior Member

    Hi rxcomposite,

    At this point of the discussion it seems as if frames are generally designed with sufficient "safety factors" to accept "reasonable" notches.
    I think the various safety factor definitions should be defined before comparison.

    In my case for a clean member for his boat, it was five times the working stress (which is in my case is 65% of the yield stress).
    I call it a "reserve" or "excess" safety factor (is there a proper name for this?).

    In your case, the member has a safely factor of four, is that over ultimate strength or yield strength or working stress?
    If it is over ultimate stress (which I believe is preferred by our American friends) then SteelBoat's 25% maximum notch size statement is consistent.

    Something else to consider, is that SteelBoat's frame members are not continuously welded to the hull plating.
    If it was then we would not be having this discussion, as the limber hole would not be at the "extreme fiber".
    It would not be considered a notch but a hole in the web. But I am bending the rules here for Lloyd's rules for "web openings" presented above.

    Regards AlanX
     
  6. AlanX
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    AlanX Senior Member

    Update to my calculations. I miss-read the frame spacing as 19" rather than 26". Don't you hate that!
    The reserve safety factors (over working strength) have fallen to:
    • Clean 3.8
    • 17.5 mm R Notch 2.1
    • 25 mm R Notch 1.6
    Still okay.

    Regards Alan
     

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  7. AlanX
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    AlanX Senior Member

    Hi @Steelboat,

    Those longitudinal stringer may be secondary members?

    I will do some more checks:
    • What are the dimensions of the longitudinal stringer and hull plate thickness (shown above)?
    • What is the bulkhead spacing amidships?
    and
    • What is the beam and keel to sheer distance (if round bottom) or keel to chine (if hard chined)?
    Regards AlanX
     
  8. rxcomposite
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    rxcomposite Senior Member

    Different rules have different safety factors applied and is usually defined in the (marine) Rules but in general, they follow the civil engineering standards. That is, if it is a primary member(fully fixed ends), it uses 4, if it is secondary (partial fixing), it uses 3. There is even a tertiary (ends not fixed). Primary members are defined by the end fixicity. It is reinforced by gusset or increased in SM in the area of fixity.

    There many definition used. Safety factors, Stress ratio, Margin of Safety, Safe Working Load. In lifting arrangements where it is critical, it is sometimes 6 or 8 FoS for safety. Most marine formula uses stress ratio as design ratio (0.3, 0.5, etc.)

    Safety Factor is not yield strength as material yield strength may vary. It is a means of estimating a design stress to ensure that the factor does not approach the yield strength of the material. For example, if the safety factor is 2, then it is 1/2 (0.5 or 50%) of the ultimate strength of the material. If the material yields at 70% of ultimate, you have an extra (reserve) 20% before yield expressed as Margin of Safety (MoS). It is usually expressed as a whole number and a decimal such as 1.2. It has to be always greater than 1. So designers always aim for less than the yield strength of the material. If the confidence is high on the material because of manufacturers proof or simply making coupon test in accordance with the standards, designers aim for as low as 1.7 or 1.8 FoS.
     
  9. rxcomposite
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    rxcomposite Senior Member

    It is not exactly a design rule but design detail (improvement on shop practices) as AH has expounded. It is a means of preventing stress riser or simply a stress relief.

    See the images I posted above.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Seems to follow the the images of the last two I posted about limber holes.
     
  11. Steelboat
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    Steelboat Junior Member

    A picture is worth a thousand words. This may help a lot.:)
     

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  12. Steelboat
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    Steelboat Junior Member

    The longitudinal stringer is 1-1/4 high by 3/8" wide (32*10mm)
    Plating in this area is 5mm A36
    Bulkhead spacing. hmm. Plywood is bolted to the frames to form the interior but from what I understand they are not designed to carry loads like a GRP boat.
     

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  13. rxcomposite
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    rxcomposite Senior Member

    Not exactly as how a web frame is supposed to be drawn but I chose this one because it has a hole in it. Web frames are primary members and as such its ends (connection) is to be bracketed or filleted thus increasing greatly its SM.

    Attached also is how the longis (stiffeners) are attached to the web frames which I did not post before as it is out of topic. Longis are secondaries and its ends are partially fixed. Note the "holes" or stress relief points.

    Your boat might not follow the Class rules but no worry. It might be overbuilt and not built to Class. Class rules are minimum standards and good shop practices.
     

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  14. Steelboat
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    Steelboat Junior Member


    Thanks!
    So primary members = frames and keel bar.
    Secondary members = longitudinal stringers, chine bars.

    I have never seen the referenced lugs or "tripping brackets" between frames and other members. Perhaps this is done on specialty / larger vessels only.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Yup. Tertiaries are just stiffeners not connected in ends such as stiffeners in hatch covers. It just stiffen the plates to give it more rigidity.
     
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