stringer at chine???

Discussion in 'Boat Design' started by metin_mehel, Sep 10, 2011.

  1. metin_mehel
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    metin_mehel mech.eng.

    Hi,
    In some designs there is a stringer that connects two plywoods. However it is hard to shape the stringer for that kind of connection. Because angle of the two adjacent plywood have different angles through the chine. What is the best manner to attach two plywoods to eachother by stringer? I think one of the plywood will be glued to a stringer and the other will be glued by fillet to same stringer...
    Thanks A Lot! :)
     
  2. PAR
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    PAR Yacht Designer/Builder

    You are mixing two different methodologies together, which isn't wise. Traditional plank on frame building methods (plywood over frames) require a chine log, which looks like a big stringer. The topside planking and bottom planking land on a rolling bevel, the full length of the chine log. This isn't a difficult task to accomplish, you just need a straight edge, a hand saw and a plane.

    The other method you mentioned is taped seam construction, which uses epoxy fillets and 'glass fabrics on the seams, to replace the chine log. The two methods generally aren't interchangeable. In other words, if you have a set of plans for plank on frame construction, then you should build to that method. Conversely, if it's a taped seam build then you should stick with taped seam techniques. You can convert a set of plans from one to the other, but judging by your questions, you are ill prepared to preform this task. It's a relatively inexpensive job for a designer (design specific) and worth considering. I do one or two conversions a year.

    There isn't any "best method" for attaching the sides of a boat to the bottom. With each building method, there are good and bad points to consider. For example, a taped seam build doesn't have chine logs and in many cases no stringers or frame either, but the reverse side of the coin is, this method is very heavy on working with epoxy and 'glass, which many find objectionable.

    Simply put, which ever method is employed, the topside to bottom planking joint, will be engineered for the loads expected and that's all that counts. If you're attempting to design a boat yourself and are asking these questions, you have much to learn in several areas of expertise, before you should consider the effort worthy of producing a boat, that will not drown you. If you already have an engineering background, absorbing the new disciplines will be relatively easy, as you'll know much of the structural aspects or at the very least, should be able to comprehend the concepts and muddle your way through with additional research.
     
  3. metin_mehel
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    metin_mehel mech.eng.

    http://www.boatdesign.net/nyd/K800/K800-frames1.png
    Dear Par, When you look at the chine of this boat you will see not a rectangle profiled stringer. I am asking how it is machined. Because this crossectional shape is changing through the chine. You know I have an experience about filleting and it is too costly and I think it is little bit week. However if two plywood can be connected to each other by a solid timber I think it will be stronger?
     
  4. metin_mehel
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    metin_mehel mech.eng.

    Yes I think it is chine log. How can I learn this technique?
    Thanks A lot
     
  5. michael pierzga
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    michael pierzga Senior Member

    WHOA !!!!! Many chine logs in that boat ! You will be an expert at rolling bevels with a hand plane by the time you are finished. Difficult to explain in words the tricks to cutting the rolling bevel...

    GOOGLE " rolling bevel boatbuilding"... "rolling bevel sheer clamp"... "rolling bevel plywood"... " rolling bevel Greg Rossel " and you will find plenty of good descriptions of the technique with pictures.

    http://rosslillistonewoodenboat.blogspot.com/2011/02/lapstrake-planking-cutting-rolling.html

    http://moy.org/Exhibits/6MetreNewBu...yId/48/A-little-lesson-in-rolling-bevels.aspx

    http://mojonunseyirdefteri.blogspot.com/2011_01_15_archive.html

    The book "Gougeon Brothers on Boat Construction" also includes a good description of cutting rolling bevels.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Again you are making assumptions about strength. Testing has clearly proven that taped seam methods, that eliminate the chine log(s) are more then sufficient for the role they preform in the structure.

    The design you show is a plank on frame build and very common. It can be converted to a taped seam build, which would eliminate the chine logs, but I don't think you currently have enough understanding of the principles involved to do so yourself. It would be a simple request for a designer to preform the plans conversion.

    Making a rolling bevel isn't hard to do and (again) requires only hand tools and a straight edge of sufficient length to bridge the distance between keel and chine log or chine log to chine log. Typically, it's not "machined" though it can be if you have a very well equipped shop (most don't). CNC cut parts preform these rolling bevels, but the average builder just uses their eye, a straight edge and a sharp tool.
     
  7. metin_mehel
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    metin_mehel mech.eng.

    Dear Par, I want to mix these two methods because of efficiency. First for the boats in length about 7m. we need stringers for strength, and also plywoods must be connected to eachother at chines. So if we connect two plywoods at chine by a stringer so we will eliminate extra taped seam job so the design will be efficient for both material saving and machining costs. Actualy I have never have a plan, I design myself. I am a mechanical engineer. So I cant request someone to smth.. :) What do you think about my comments about efficiency?
    Now I am searching rolling bevel method... What do you think instead of this extrajobs, we can use circular profiled stringer? It would be tangent at any location between two plywoods?
     
  8. michael pierzga
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    michael pierzga Senior Member

    Circular chine log ? Hmmm. Never seen one. It would need lots of gap filling epoxy. so much epoxy that it would be logical to change the construction to conventional epoxy biax taped chine then eliminate the rolling bevel chine logs.

    Batten seam construction is a challenge. The Rolling bevels are not that difficult to cut. The difficulty with so many chines is laying the floppy ply panels onto the chines and keeping the whole hull " Fair" . Carefully check your work with a long fairing batten

    What thickness plywood will you use for the planking ?
     
  9. metin_mehel
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    metin_mehel mech.eng.

    :) I liked it 'Batten seam construction'
     
  10. PAR
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    PAR Yacht Designer/Builder

    Under what engineering pretense have you surmised, mixing these two method would make the build more efficient?

    Again, (obviously repeating myself doesn't seem to help), the two methods don't mix well, as previously stated. If you tape the chine you don't need a stringer or chine log. If you install a chine log or a stringer you don't need the tape.

    I'm not sure what type of mechanical engineer you are, but frankly the hallmark of an engineer is the research he applies to his projects.

    Let me try an analogy that you might have a better understanding of. If you build a house, a typical wooden one will be platform framed. It has several different types of studs, footers, headers, rafters, floor joists and sheathing materials. It's "stick built", literally an assembly of wooden sticks. This is precisely what a plank on frame built boat is, stick built.

    On the other hand you could build the home from concrete blocks, which will have block walls, cast footers and lintels, covered in a protective sheathing or coating. This is the same thing as a taped seam built boat, which is a monocoque shell with a wooden core. This is a composite structure, not stick built.

    In this same vain would you combine the two home building methods? You can, but it raises new, unnecessary issues. You wouldn't use wooden footers under a concrete block wall, nor would use use cast concrete lintels bridging a wood framed door way. The same is true of plank on frame boats and taped seam. It is advantageous to tape the exterior seam of a planked frame boat, but it's not necessary and has the potential of causing more issues then it solves.

    Simply put, preform the research necessary to understand the different methods and maybe take some courses in both yacht design and structure. Other wise, the best advise you can receive, is to buy a set of plans and build to those set of plans, as engineered by the person that has already done the necessary research and gotten the prerequisite education.
     
  11. FMS
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    FMS Senior Member

    I don't want to belittle your point with this aside, but your analogy made me think of a counter-example, a show I watched on HGTV this month on new factory built homes for hurricane zones. They started with standard stick built wall sections. However on the line they sprayed an expanding slow curing adhesive. The OSB sheathing on the outside and even the drywall on the inside became part of the stick built frame with this adhesive expanding 4 to 6 inches on both sides of each stud and filling all gaps. The focus was on the superior strength for hurricane and tornado areas to traditional stick built homes.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Yes, FMS, I've seen the experiment, though it's still stick built, even if modified with polyurethane foam, not a composite or monocoque structure.

    More interesting applications of this same chemistry are being used as armor in war zones or likely target locations. The polyurethane still does what polyurethanes do, but it a different activation process and the result is less foam, but a much tougher skin.

    Lastly the round stock for a chine log replacement will introduce a huge point load situation, along the contact axis of the round stock. Any first year engineering student would catch this and avoid it like the plaque, but this poster seems to want to do an end run on conventions.
     
  13. metin_mehel
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    metin_mehel mech.eng.

    When we compare two cross-section under longitudinal bending condition,
    The stress= M*y/I
    M=moment
    y=distance to neutral axis
    I=second moment of area about neutral axis.

    As we obviously see in case-1 "I" is considerably bigger. So the stress is lover than case-2.

    If we consider the loads acting on the plywoods for forcing to seperate them, In this case the glued are is bigger than tapedone.
    So we should use stringers for strength of hull, if we use it at chine we also strengthen the joint. And shaping of stringers in outside is easier than chine log shaping.
     

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  14. DCockey
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    DCockey Senior Member

    Is there a reason you are rejecting the time proven method of a beveled chine log? Is it because beveling the chine log requires a little bit of skill with a plane, which can be obtained with a small amount of practice?
     

  15. DCockey
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    DCockey Senior Member


    First, your drawing of the taped seam is not representative of what I've seen used. Generally the width of the tape is a number of multiples of the plywood thickness, and the thickness/number of layes of tape used is selected based on the strength needed.

    Second, the gap between the edges of the panels is generally filled with thickened epoxy which effectively glues the edges together.

    So your conclusion is based on faulty premises.
     
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