Egg-Crate Stringers and Frames

Discussion in 'Boat Design' started by Mat-C, Feb 4, 2010.

  1. Mat-C
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    Mat-C Senior Member

    Can someone explain to me how you go about determining the scantlings in a planing powerboat when the stringers and transverse frames are made from relatively thin panels, rather than the top-hat sections that are referred to by most methods?
    I know that Tom Lathrop employed a variation on the theme in his BJ24, pic below, but I have also seen others made of composite, where the frames create and egg-crate like effect.
    I assume that the bottom plate is determined in the normal way - by finding the max unsupported span, loadings etc. But how to determine the scantlings for the frames / stringers?
    And how do you get around the higher point loading that is created by thinner contact area that the frame / stringer has on the panel that it is supporting?
     
  2. Mat-C
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    Mat-C Senior Member

    Oops, sorry, forgot to attach the pic.
    (Hope that's ok Tom....?)
     

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  3. tom28571
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    tom28571 Senior Member

    An interesting question Mat and my take on it may not be what other designers prefer. First, I like a stiff boat and also like a lot of ventilation in the hidden chambers of a wooden boat. Along with the ventilation, I like easy flow of any water that finds its way into the bilge to the pump or drain at the transom with as few traps and restrictions as possible. In the particular case of this boat, I want all the stiffness needed by the design to be provided by the bottom. The reason for this is to allow maximum ability to move interior structure around or, even to eliminate it in the case of a convertible design.

    First I looked at stiffening the bottom with stringers and quickly found that, when combined with the cabin and cockpit soles to form a torsion box, the vertical bulkheads could give vastly superior performance to any stringer system. Arranging these bulkheads along the long axis of the boat gives a combination of ventilation and water flow access much better than an "egg crate" style where ventilation is limited to limber holes which can clog.

    In the end, stiffness of the hull bottom and soles is easily met by the layout shown. I admit to overkill in the spacing of the bulkheads but felt that it is not worthwhile to loose a few pounds by eliminating any of them. The open area shown is for tankage and gets an additional layer of plywood plus a transverse bulkhead between the fuel and water tanks. The bottom edge of each bulkhead is fileted and taped which greatly spreads point loadings into and out of the bottom panels.

    Some of this design was arrived at by fear as it was my first large one and I did not want any failures. Bottom thickness on this 24 footer is 1/2" and bulkhead spacing is 7 1/2". Soles are 3/8" and over the tanks gets some girders on the underside. Other designers might do it differently but I am satisfied that this is a completely workable system that meets all my original goals.
     
  4. Mat-C
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    Mat-C Senior Member

    Thanks Tom - as always your replies are evidence of the logic that you use in your designs.
    I understand that much of this stuff is based on previous experience. I that how you determined the thickness of the bottom and the 'stringers'? And more to the point, how would you then go about doing the same if were entering unknown territory - like building in composites, or indeed just with a different sized boat?
     
  5. tom28571
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    tom28571 Senior Member

    Like any new designer wannabe should, I started with good material from those acknowledged as experts in the field. For scantlings, I went to Gerr as a starting point and then established for myself that these were reasonable. Also, like most beginners, I tried to be a bit conservative. All the bulkhead/stringers have to do is transfer bending stress for hull bottom to sole on the far side of the torsion box. The stiffness of any beam or structure is mostly a factor of the thickness spread between the outside surfaces. In this case, the "thickness" dimensions of the box are so great that the resulting structure is extremely stiff and there is just no bending going on.

    Computing the modulus of a panel is straightforward and covered in many books. I keep mentioning stiffness because if you satisfy that in most cases, strength is well taken care of. I don't remember all this stuff and always refer to the basic sources when needed. You are absolutely correct that relying on precedent in boat structure will turn out an very acceptable boat. It will not be optimum in stiffness or strength to weight ratio though.

    As for design for composites of synthetic materials, I don't get into that and so can't answer. There are many sources of information on synthetic composites though. Subscribing to "Professional Boatbuilder" (free) is a great place to start. Gerr also gets into it but I only browsed those sections.
     
  6. Mat-C
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    Mat-C Senior Member

    Gerr's rules seem to be considered to be a little conservative but otherwise fine... if you're happy to stick to the 'recipe' - ie X number of frames / bulkheads with a max spacing of Y between them. He's very clear about not straying from the script, which I guess for most designs would pose no problem at all. The 'egg-crate' approach, however, would seem to be a simple construction technique that has the potential to reduce weight... the unsupported span is smaller and the stringer / frames are much simpler and lighter, though more numerous....
    Clearly the BJ is quite different from this - my apologies for singling it out... your was the only good pic I could find that demonstrated what I was talking about....
    In the longitudinal sense, I can see where you are coming from - you effectively have a series of box girders, stacked side by side. But by eliminating all the bulkheads / frames, aren't you introducing the possibility of 'twist' into the equation?
     
  7. TollyWally
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    TollyWally Senior Member

    Racing is one of the foremost ways of observing the effects of straying from the script. As racers push the envelope things improve, break, or both. The script sometimes follows. At least that's how it seems to me.

    On the egg crate theme, it seems like in Tom's boats you have a solid and stiff box beam below you. Structure above this box beam in the form of seating, partial bulkheads, etc would appear to stiffen the topsides and further lock in the shape of the box beam below. That's my theory, I await the words of the designer to know if I'm close.
     
  8. tom28571
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    tom28571 Senior Member

    Well, I hesitate to make this thread a referendum on my design. It was indeed a mixture of using established scantling criteria and my own ideas as to what a good boat hull should be in order to do what I wanted from it. As to the possibility of twisting, it should be observed that it is not a parallel torsion box. It is rather the combination of this and the triangle formed by the V of the bottom and the sole above. It would be pretty hard to make this twist and I would think near impossible under the loads that such a boat might encounter. I did make a serious effort to keep weight low and the finished boat, including all tanks, steering system, instruments, and everything other than the engine weighed 1850# just before placing onto the trailer. This low weight, which is less than half that of any similar sized FG boat is partly responsible for the performance of the boat.

    The interior structure does add stiffness but I think the boat is really stiff even with out any consideration of the above sole structure. Local stiffening of the topsides does benefit from the transverse bulkheads, upper decks and furniture though.
     

  9. TollyWally
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    TollyWally Senior Member

    Tom,
    I sure like the premise of those boats of yours. If I could wave a magic wand I would transform my existing boat into something like your 28' model. I'd probably want it out of aluminum instead of plywood with a small inboard rather than an outboard. LOL, I realize I just changed much of the engineering that makes your boats so desirable with my magic wand but as a carpenter I don't want wood and I really like straight shaft boats.

    I've studied your site a bunch and I am fascinated with the ideas of low loading on the hull bottom and shaping the hull to trim the boat "flat" as it gets on plane. I like the way your boats are optimised for the speeds I actually WANT to cruise at.

    The diagrams about the water particles etc. have helped me understand (sort of) what is actually happening. Keep on doing what you're doing. I wish I was a customer but it's unlikely I'll get rid of my existing boat.
     
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