Longitudinal frame twisting and different kind of holes in transverse frames

Discussion in 'Boat Design' started by Joonatan, Jan 31, 2025 at 8:12 AM.

  1. Joonatan
    Joined: Nov 2024
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    Joonatan New Member

    Hey guys,

    I have a few questions to you about designing boats (steel or aluminium BUT especially in aluminium)

    In the first picture you can see a simplified example of bottom plate and longitudinal frame. I have couple questions regarding that:
    1. Should the aim be that the longitudinal frame is always perpendicular to the bottom plate (yes I know it helps with scantling calculations but what is too much angle in general when boats are designed)?
    2. Is it bad thing, if the longitudinal frame is twisting a lot (as you can see in the picture) towards the front section of the boat? If you need to force the flat plate to it's place then it is bad but otherwise it is okey? is this correct?
    3. If twisting is bad, should the longitudinal frame be cut in few shorter pieces and then assembled so that it would not always be perpendicular to the bottom plate, but at least it would not be twisting that much.

    My second questions is regarding the holes in transverse frames.

    as you can see in the second picture, there are 3 different kind of holes. I have seen all of these used in boat building (when searching the internet). But is there a place for all of these different cuts?
    4. Why I should not use the middle one always if building a boat?
    5. If the left cut in the picture is helping to relief stresses, should it be used always?
    6. Which one of these holes should be used for the bottom plate of the boat?
    7. Could you give some practical examples when to use each of these cuts, or if some of them should be avoided always.

    I would really appreciate if some of you have wisdom for these questions. Especially if you could give some good practises/rules ("rule of thumbs") to follow in general.
     

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  2. montero
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    montero Senior Member

    I've always wanted to use a computer to cut oblique bulkheads to avoid twisting.
    Any construction will somehow work twisted stringer too.
    Second question , everything depends if the stress is minimal you can use even second cut. If it is a very heavy loaded frame probably always better use first and third.
     
  3. montero
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    montero Senior Member

    Check from 12 min :
     
  4. gonzo
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    gonzo Senior Member

    Longitudinal frames (stringers) are usually straight. You can do really complicated calculations for a twisted stringer, but it will probably be less stiff than a straight one. The mouse holes on the left is the best design because of the stress relief.
     
  5. TANSL
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    TANSL Senior Member

    Whenever necessary, and this happens very frequently, the longitudinals must be turned and twisted so that they are perpendicular to their attached plate. In both cases, the same calculation must be made: the first momento of stiffener area with its plate, which is not a "really complicated calculation".
    The holes have little to do with the stresses (which ones and where?) and a lot to do with making it easier for the reinforcement and the bulkhead or the floor to be welded correctly. I would propose another solution (but there are more, depending on the cross section of the reinforcement or other circumstances).


    upload_2025-1-31_19-40-30.png
     
    Last edited: Feb 1, 2025 at 4:33 AM
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Hi Joonatan

    Welcome to the forum.

    Yes, in general the stringers/girders etc are always normal to the plate.

    Yes, twisting over a short distance, i.e like one frame bay is a no-no. You can twist stringers but only over a long distance.

    The objective, as noted in no.1, is to always be normal to the plate.
    You achieve this by understanding how to do stringer runs. The runs follow the natural curvature of the hull, so as the stringer goes forward, it is very slowly twisted, but you hardly notice it, because the stringer remains almost perfectly normal to the plate. And example is shown below:

    upload_2025-2-1_17-33-10.png

    You only change this when the degree of curvature exceeds this objective, as noted in no.2. Then you need to rethink what you are doing and why, and how to stiffen the plate…usually introducing traverse members, like ½ frames become more prominent, or breasthooks.

    But in some structural members, like Engine Girder, they remain vertical, and then the web is not at 90 degrees to the plate. But the strength of the girder far out-weighs the perceived loss of strength by being at ‘some’ angle to the plate. Since engine girders are always oversized anyway.

    No!...not unless you wish to prevent cracking occurring.

    Because it will crack in the locations’ noted, owing to a biaxial/triaxial stress raiser.
    upload_2025-2-1_17-28-17.png

    same for the one on the right:
    upload_2025-2-1_17-32-4.png

    Yes, this helps to relief the stresses, because by having a mouse hole, as shown, you prevent the weds from crossing over each other – thereby no biaxial/triaxial stress raiser.

    Only the image on the left, as noted in no.5


    As noted never use the middle or the right hand figure.
     
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  7. montero
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    montero Senior Member

    This is not entirely consistent with what TANSL wrote .Cracking .
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Then he doesn't know what he is talking about by making such a statement.
    It is simple structural design basics...
     
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  9. TANSL
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    TANSL Senior Member

    Thank you Ad Hoc. You are as kind as ever.
    @montero I can be wrong, as can anyone else, but I would like to know, in order to learn something myself, where the inconsistency lies in what I have said. Thank you, I would like non-petty explanations that allow me to learn from my mistakes.
     
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  10. Joonatan
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    Joonatan New Member

    Hey everyone,

    Thank you very much for all of your responses! I really appreciated them. Really grateful there is such a forum like this!
    Now I am much more confident in some of the building methods I thought they should be done.

    Btw. Is there some classification rules for the sizing of the mouse holes? or some general rool to have in mind when making them? @gonzo @Ad Hoc @TANSL
     
  11. Waterwitch
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    Waterwitch Senior Member

    Mouse holes have to be large enough for the welder to be able to run a continious bead along the length the stringers. Also big enough for the welder to wrap the weld around mousehole on the frame to hull fillet joint. Weld bead size is designed according to material thickness.
     
  12. ropf
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    ropf Junior Member

    While the reasons for the mouse holes are completly clear, other questions arise.

    - What do yo if the bulkead has to be watertight? Filling them with some softer materials?
    - In the few boats I know from the inside and in the plans I have seen, bulkheads and ring frames are attached without mouse holes (with only one exception). Does wood or composite behave differently in this respect, or does it just not matter for smaller boats?
     
  13. montero
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    montero Senior Member

    Metal alloys are not perfect but they have great strength. Each hull construction technology has its own laws.
     
  14. gonzo
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    gonzo Senior Member

    I am not sure I follow the explanation. Are you saying the welder makes a bead around the edge of the mouse hole? That would not be correct. Also, depending on the design, the stringers may be stitch welded and not continuous.
     

  15. Waterwitch
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    Waterwitch Senior Member

    In the case where continuous welds are specified for all primary structure, welds are wrapped around the the frame, even the short fillet inside the mouse hole running the thickness of the frame; in which case the mouse hole must be big enough to do so.
    Iknow there are weld schedules that allow for stitch welds on stringers, and some that are continuous too.
     
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