Possible contract for Composites Engineer

Discussion in 'Services & Employment' started by rwatson, Jun 18, 2010.

  1. rxcomposite
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    rxcomposite Senior Member

    DNV do not accept cored bottoms on high speed boats (patrol boats), only single skin. LR do not differenciate. Cored bottom is acceptable, but there is a minimum thickness of the laminate inside and outside.
     
  2. rwatson
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    rwatson Senior Member

    Thanks for the input RX - just a couple of points of clarification

    ok, so what is on the sheets ? is that just info on sizes and weights for the different sections ?

    Oh yes, I understand about rolls of fibreglass only too well. In this case I will be the shop floor, and I expect a lot of cutting will be needed for a small boat and lots of differently loaded areas.

    Of course I expect hull lines, not sure what you are getting at here. And bugger offsets, I am going to get DXF cutting files to create mold shapes - who wants to loft stuff? For the developable surfaces, I also will get full scale patterns cut from DXF files to lay the cloth onto.

    That makes sense to me, though I would have thought that density and thickness ratios ar'nt rigidly coupled. For example, a cabin top could be say H80, and 25mm thick, but under the winches and around the hatchway you could have H200, still 25mm thick, to make layup easy ? They cut the sheets out of big blocks, so they can vary the thickness to suit as I understand it.

    All fascinating stuff. It will be interesting to see what is provided in the first set of specs.
     
  3. rwatson
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    rwatson Senior Member

    Thanks for that info RX. Makes sense to me - I cant think of a reason to do a cored bottom in a ballasted sail boat especially.

    And, as this boat will be powered up to 25 knots, the comparison to the patrol boats is very apt.
     
  4. HakimKlunker
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    HakimKlunker Andreas der Juengere

    My decent contribution: To start with Shakespeare / Hamlet: "There are more things between heaven and Earth than you have dreamed of."
    A proper calculation will help you keeping your property and your life. Alik is right: The 'designer' should provide the technical data. Even then you cannot be sure: I have worked with plans from so-called professionals, who later only could state that until now nothing was sunk, or that they estimated by what they call experience. You better ask for an appropriate certification.
    Building guidelines and scantling rules can be purchased, but they are not cheap for a one-off and you need to study (and understand) yourself. Same goes to professional service. Quality and safety have a price.
    I have not seen your targeted purpose: Is the boat to be made to US, or European, ISO or which standard? (Note for insiders... Pattaya local manufacturing standard is NOT advisable :D)
    But if from N/Z or Oz is rather traditional feud, hm? ;)
    My impression: You wonder about the laminate quite LATE - usually you need this value at least as a closer estimate for weight- trim- and displacement calculations. If you want to build soon and with a safe result, better employ a specialist.
     
  5. HakimKlunker
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    HakimKlunker Andreas der Juengere

    Yes and no: The hull shape also plays a role. The core has a limit of how much slamming it can stand. We talk about shear strength among others. The approach that makes most sense to me, is to determine first the expected pressures (bottom, sides, fore, aft), calculate a full laminate and then modify for core thickness and density. If the result by calculation does not satisfy, then better consider again. After all: In many cases a sandwich can reduce weight and so reduce resin costs and improve boat performance. Don't throw away your options before they are provenly inappropriate.
     
  6. apex1

    apex1 Guest

    What a bold statement Tunnels.

    So, in your opinion the majority of the industry is too dumb for their business?

    And you would make a sailboat hull lighter below waterline (and add the risk of water intrusion and failure), just to add more ballast then?

    Well, you are a real expert...................:confused:

    Regards
    Richard
     
  7. rwatson
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    rwatson Senior Member

    True Apex.

    Isnt it a strange how when you ask for expert help, many people provide sarcasm to big note themselves.

    All sensible hints and knowledge welcome (thanks for the usefull stuff so far) , but I have all the silly innuendo and charactererization I need in the general discussion threads.
     
  8. rwatson
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    rwatson Senior Member

    Makes no sense to me at all.

    You have to calculate the hull weight (including motors, gear, fuel etc) before you know what loads will be achieved at the desired speeds. The core/laminate schedule can be calculated very late on, as there is always a good margin on a boat this size. You can easily start with an initial hull worst case estimate based on similar boats or scantling rules, and then optimaize the layup for weight V strength.

    If you overestimate the hulls weight initially, then thats a bonus you can either increase the payload by, or increase the ballast.

    Trying to start with the laminate schedule is a silly idea.
     
  9. apex1

    apex1 Guest

    Well, not sooo senseless rwatson.

    In fact these calculations go hand in hand when starting a design. The NA almost always has some reference boat / s to start with a good estimation what would be the result going this or that way.
    If he would start with a (known) too heavy structure he would have to recalculate the entire game.

    Starting with a layup shedule of course is not the way to go.

    Regards
    Richard
     
  10. Alik
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    Alik Senior Member

  11. rxcomposite
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    rxcomposite Senior Member

    Similar to threads started before, there is a bias towards single skin laminate and cored laminate for bottoms.

    There are pros and cons and I am sure Class societies have taken them into account. A single skin laminate will be more forgiving as there is more flex and will absorb a shock better. I believe impact strength was covered in Eric Green's Marine Composite book.

    On the other hand, LR specifies a minimum outer laminate thickness, the reason behind this is a minimum "shore" hardness requirement. I am not into design mode at this moment so don't ask me how much. Shore hardness is to assure a minimum impact strength.

    LR also requires a break in the core whenever the core comes into connection with a major stiffener/frame/bulkhead. This is called a "shear tie" wherein the core is scarfed, laminated over, before the adjoining foam continue. So it is not entirely one continous core.

    There are many construction details that is required.

    Lastly, laminates are designed around the pressures derived from the design of the boat (shape, breadth, depth, displacement, speed, ect). We follow the design spiral. After all the weights are known, we design the laminate and predict the shell weight. If it gets too heavy, (displacement will be greater, pressures greater, speed slower) we go back to recalculate again untill we get it right.
     
  12. Alik
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    Alik Senior Member

    When we start new design, we either use reference design made before as basis for weight estimates, or use statistical weight factor related to LxBxH of boat.

    At concept stage, we develop preliminary structural calculations and preliminary lamination schedule (usually includes panels, stiffeners added as percentage), this also goes to table of weights and BOM of concept design. At this stage we play with materials in terms of weight, cost and desired performance.

    At engineering stage, more details are added for structural calculations, lamination schedule and BOM are clarified, structural drawings are developed.

    Conclusion: there is no sence to build heavy displacement boat in high-tech sandwiches and add ballast later. Do it properly from the beginning.
     
  13. apex1

    apex1 Guest

    What I say.............
     
  14. rxcomposite
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    rxcomposite Senior Member

    I quite agree Alik, ballast is unnecessary evil but as our seasoned Naval Architect philosophically says " What can I do? If the owner wants 3 decks on top (raising the center of gravity) and the solution is to add ballast, who am I to say no. As long as he is informed of the solution".

    Design is always the collaboration between the owner and the designer.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Sometimes the sheet is more than 1. Say page 1 of 2. Say Hull lamination will show in the second sheet the reinforcement placement.
     
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