watertight bulkhead

Discussion in 'Boat Design' started by Pammie, May 28, 2018.

  1. Pammie
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    Pammie Junior Member

    Dave Gerr writes about watertight bulkheads:

    upload_2018-5-28_10-35-15.png
    But what is the reason the stiffeners should end before the bulkhead? My watertight bulkhead separates the hull from the sacrificial nose (1,5 m, total length doesn't fit in my garage). So I can imagine letting the stiffeners run to the bulkhead is better to make sure the hull doesn't break after the watertight bulkhead?
     

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  2. TANSL
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    TANSL Senior Member

    I think it would be interesting to see what the current regulations, ISO for samll boats or Classification Societies, or current good practices say on that subject.
     
  3. fallguy
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    fallguy Senior Member

    Interesting.

    My guess (and that is all I offer) is the stiffener, if not all the way across like a locker base will potentially mechanically fatigue the BH joins or in collision, be a weak spot with the potential to damage (pierce) the BH, rendering it no longer watertight.

    So the stiffener is preferred to fail before the BH to the system failing at a more critical location?

    But then the thing seems like there is a weak spot in the margin between the stiffener and the BH tabbing. This is the preferred point of failure.

    But for someone building boats; you don't want failure points. It doesn't work well in the brain.

    I would imagine, as a wise man (Tansl) points out; there is more discussion elsewhere.

    Like, what about instead of a stiffener you have a full base going from side to side to make a watertight compartment above the WL on a watertight BH? Where or how do you design a point of failure then? Poorer or no tabbing at the BH? Or not allowed with watertight BH? That would be wrong.

    I'll be quiet and get the popcorn.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    None at all.

    It is personal choice and dependent upon how you wish the loads to be carried and shirked to the surrounding structure, and, the ease, or difficulty, of ensuring watertightness of the stiffener passing through the WTB.

    That's all. No magic...
     
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  5. Angélique
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    Angélique aka Angel (only by name)

    If you're really interested, and not just wanted to post the above truism on the subject, or next maybe even less, then you can look this up, especially ISO for small boats or Classification Societies, and if you want to share it, then relevant links + quotes + files can be posted here, so it can be discussed and maybe learned from.
     
    Last edited: May 28, 2018
  6. Pammie
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    Pammie Junior Member

    Thanks for your clear answer Ad Hoc: that's what I expected.

    sacrificial nose: I just mentioned that to make clear what's in front of the WTB. I choose to use Gerr's scantling method and compare that with other boats. Maybe not the best method, and I'm going to check (later) with ISO.
     
  7. TANSL
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    TANSL Senior Member

    The book "Boat Strength" by Dave Gerr has been a very important help for many years. However, I do not think that, at present, no regulatory body will accept scantlings calculated with their methods. I want to make it very clear that this is only a personal opinion because I have not made any management in this regard, nor with the creators of the ISO norms for small boats, nor with any Classification Society. Especially in the matter of composite materials, the focus of calculations has been greatly changed.
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    How will you make the fwd part sacrificial...will it just be "weaker" at a certain location, engineered to fail first?...or...will you make it a bolt on type of arrangement?
     
  9. Angélique
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    Angélique aka Angel (only by name)

    I would like to share my gut feeling about this, which is the best I can offer, and also would appreciate to have it reviewed here to learn from . . :)

    But first want to check if I've got my terminology right . . ?

    naming of boat hull parts.jpg

    1 = Stem
    2 = Keelson
    3 = Bulkheads
    4 = Longitudinal stiffeners - aka - Stringers
    5 = Transverse stiffeners - aka - Frames​

    As I understand it the stiffeners in the Dave Gerr quote are about item #4, the stringers, is this correct . . ?

    [​IMG]

    P.S. - Got my pic from post #1 in the thread Transverse/Primary Stiffeners, to which I've added the 1 to 5 numbers.
     
    Last edited: May 28, 2018
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Angelique

    Your understanding of the structural members (names and locations), via the excellent image, and DG's statement, is correct :)
     
  11. Angélique
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    Angélique aka Angel (only by name)

    Thanks John :), now I have to formulate my thoughts on this, after a night of sleep and a day of work, so I'll post later.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Ready and waiting for any question/queries you may have :D
     
  13. Pammie
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    Pammie Junior Member

    @TANSL: I understand that now.... The curvy learning path... :)

    Maybe I used the wrong terminology? Made some pictures from the situation at this moment. I will use bolts to align the nose, but the structural connection is by laminate. The bolts/nuts will be covered from the inside. I think the nose will break because : the width is smaller, it has thinner laminate, no extra supporting structure, but not sure where it will break. In my first calculations it showed that strength (especially sidewards loading) would be enough for normal use (acc. GL rules). But rxcomposite mentioned that the method I used was not very precise, so I decided to do it all over with ISO. (Will do this when I finished coupontests).

    IMG_20180529_093915[1].jpg IMG_20180529_094218[1].jpg IMG_20180529_095252[1].jpg IMG_20180529_095329[1].jpg
    Stiffeners: the strips (not complete, not glued in yet) in the 3th picture. These run through the full length of the boat. Last picture is the section I was not sure about. 3th picture: 2nd frame: I know the width is to small; have to fix this, but that is another discussion.
     
  14. TANSL
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    TANSL Senior Member

    In my opinion, there should be an "extra suporting structure" that will ensure the longitudinal strength of the joint. This can be achieved with longitudinal reinforcements, continuous, covering 2 or 3 frames on each side of the watertight bulkhead. Some holes are made in the bulkhead that allow the passage of longitudinal reinforcements, holes that are subsequently closed with watertight ties (I do not know if these pieces are called "ties" in English). Perhaps the attached drawing explains better what I mean.
     

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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Pammie

    There should be no....I think.....there should be, IT WILL...etc etc.
    You design the joint to do what you want it to do. It is not that difficult. All you do is workout what load will make the structure fail at the joint as if no join, just a continuous structure. Thus you then then the make the joint stronger/stiffer at THAT location...and then the attachment the same as if no joint. So the mechanism of connection, the bolts, fails first. You create the weakest link..but it must not be weaker than the hull if there were no joint. Because then it is deficient. You just make the surrounding hull stronger/stiffer, so the failure occurs where you 'design' it to fail.

    A classical sacrificial member..
     
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