Cantilevered Davits - Engineering Problem

Discussion in 'Boat Design' started by oceannavigator2, Mar 22, 2014.

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  1. groper
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    groper Senior Member

    It's a catch 22 scenario. If you let a professional design it, it will never fail but it will cost twice as much as weigh twice as much because they will not put their *** on the chopping block, or even on the same table for that matter.

    If you design it less conservatively, it may or may not fail, and it could be half the weight and half the cost :)

    A good way to go is to simply use the ropes and pulleys as a fail safe limit. So undersize these so that these fail before anything else in an unforeseen extreme loading situation...
     
  2. CDK
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    CDK retired engineer

    I fully agree!
    The bending forces are at their maximum value at the support point only. Nowhere else is it necessary to use such a heavy cross section. I would cut the arm diagonally, so the tip is approx 2 inches high instead of 6. The section between the support and the wall also doesn't need a bottom, a U profile with a few bridges welded in can do the job perfectly.

    And I would not use 316L but cheaper stainless steel like 304 or 304L and invest some time in polishing.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Oceannavigator2, I have redone the calculations with new loads, dimensions and operational requirements - as indicated in your last post. The objective is to minimize weight without going to extremes. It is still your money we are talking about, and I guess you don't want to see it plunging in the water.

    The boat load is 150 kg. The dynamic safety factor is f=2.0 (on the higher end, considering that it is a catamaran), giving a scantling load (and SWL) of 300 kg (approx. 3000 N). Each davit arm will carry half of that load.

    Either S235 galvanized steel (approximately equivalent to A36) or AISI 316L stainless have been considered, for low cost and wide availability. I respectfully disagree with the idea of AISI 304. It has inferior corrosion resistance, good for housing applications but not for boats.

    The maximum allowable stress is calculated as UTS/4 for mild steel (which equals Yield/2.5) and Yield Stress/2.5 for stainless. The results are summarized in the attached pdf, which also shows the rope tensions for various revving diagrams.

    Regarding the safety measures, both the CDK's idea of shear pin and Groper's idea of undersized ropes are valid IMO. However, the latter one is difficult to implement, because the Minimum Breaking Load (MBL, sometimes MBS or MTS) of commercial ropes is too high, and you should either go down to less than 5-6 mm (1/4") manila rope or up to bigger and heavier steel sections, in order to match the required values.

    Cheers
     

    Attached Files:

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  4. oceannavigator2

    oceannavigator2 Previous Member

    Daiquiri, you didn't have to do this and to such a quick pace. Thank you for taking rthe time to present the solution. It is perfect! I will use the S235 and paint it to black, as appropriate for the styling of the boat (all accessories are black).

    This solution is nearly finished. While we are still looking at this thread, does anyone have suggestions on a good material for the posts or columns at the support point? These columns are subject to both the compression load of the newly designed davits as well as the tension load of the main sheet (19.8 meter mast). The mainsheet tension is of course, composed of a directly vertical pull and a horizontal pull. The horizontal part would make me think I need some wide flanges at the base and top of these supports.

    I am off plans here, but see many examples of the same type of set up on other catamarans. Sheet controls are doubled back along the boom to the helm for maximum safety (releasing the sheet in an unexpected gust)
     
  5. oceannavigator2

    oceannavigator2 Previous Member

    I learned so much about cantilevers and steel here that I'm wondering if I could ask something else, just to learn.

    I would like to use these same stress numbers and look at cored fiberglsss cantilevers, just as an exercise. Maybe at some other time, it will come up. The data on epoxy or glass tensile strength is easily found. What is not easily found is the change in tensile strength as you increase the number of layers of laminate. Is it simply added? 300g glass + epoxy = X tensile, then 600g glass + epoxy = 2X??

    Also, the glass is one tensile strength. The resin is a different tensile strength. If you have 2 layers of glass, in some epoxy, how is the tensile strength calculated? Good basic skills to have, but that I am lacking.
     
  6. daiquiri
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    daiquiri Engineering and Design

    No problem, it took me more time to write the accompanying text than to do the calcs. :p You're welcome.

    By the way, found this pic in the internet: http://4.bp.blogspot.com/_ZsjySuynQ...6qtFIsVk/s1600/23 6.18.06 Goodby Puurfect.JPG
    and I think that guy has found a very nice and elegant solution for the tender. No need for support posts and columns. It also avoids complications with the main sheet traveler, which regards your question in the post #34.

    Perhaps you too could take it into consideration?

    Cheers
     
  7. oceannavigator2

    oceannavigator2 Previous Member

    Yes, that is the normal, traditional way to do it. The roof and support posts are already in place (scrap timber posts until I get the real ones. boat is not in the water).

    Since it already has posts and a roof, I would like to use this existing structure to raise the tender.

    The advantages over the traditional way in your picture are:

    1) The tender can be raised a full 2 meters higher which eliminates any chance of it being caught by a wave.

    2) The "view" from this sheltered area under the roof is not cluttered.

    3) Better diving from the roof! :p
     
  8. CDK
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    CDK retired engineer

    Basically, you are right, the tensile strength doubles. BUT.... craftsmanship (or the lack of it) is also part of the equation. Only the glass provides the strength, the resin just distributes forces over the glass fibers. The amount of resin vs glass and the direction of the fibers are critical.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Not wanting to lift my own chair but it is easy. See this thread, post no. 25.http://www.boatdesign.net/forums/fi...bon-fiber-chainplates-49724-2.html#post680929

    Strength is a unit of measure. Pressure= Force/Area. It does not matter if you have 1 square inch of section area or 1/8" section square. It comes out the same.

    It is the resin that shapes the usable strength of the laminate.
     
  10. daiquiri
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    daiquiri Engineering and Design

    If you chose to go for GRP beam, your design criteria would have to change. In steel beams, as you could notice from my analysis, the limiting criterion is given by the maximum allowable stress. That's because steel has a relatively high modulus E (a measure of how rigidly the structure behaves under a given load) and a relatively low ultimate stress (UTS). On the other side, a grp laminate will have a much higher UTS, but also a much lower E (i.e. is more elastic).

    Typically:
    1) mild steel: E_steel=200000 MPa ; UTS=350-450 MPa
    2) unidirectional grp laminate (f=0.6): E_grp=35000-45000 MPa ; UTS=1200-1500 MPa​
    Hence, a laminate beam (even when the best case is considered) will be much more flexible, meaning that its deflection will be an order of magnitude higher than that of a steel beam of the same size and under the same load (E_steel = 5 times E_grp approximately)

    Example:
    If you look at the pdf file in my post #33, you will see that a davit arm made of 70x70x4 beam will deflect by approx. 10 mm under a load of 1.5 kN (values highlighted by a shadowed box).

    Performing the same analysis for a grp davit arm of a nearly same size (75x75x5 mm ; 3.1 kg/m), one can find out that the maximum stress is inside the allowable boundary, but the deflection of the beam tip is around 160 mm. That's a very huge deflection for such a short beam, and in many applications would be considered as unacceptable. However, if you do accept a davit arm which behaves like a fishing rod, then you would end up with arms weighing only 10 kg each - a nice save.

    In order to obtain a deflection of around 10 mm, one has to go for a 160x160x8 mm square section of a grp beam, which weighs around 9.1 kg/m. In this case, the weight of each davit arm would be around 30 kg - no saving at all. The source of info for this analysis is Fiberline Composites.

    Summing it up, you can conclude that:
    - By adopting the allowable stress criterion, a significant weight advantage can be obtained by adopting a laminate (grp) davit arms. One has to live with high deflection under load, though.
    - By adopting the allowable deflection criterion, there is no advantage in using a composite beam.​
    In both cases the cost and the engineering complexity will be much higher.

    Cheers
     
    Last edited: Mar 26, 2014
  11. rxcomposite
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    rxcomposite Senior Member

    Right Daiquiri. If you plug in the material properties of a grp laminate in your spreadsheet it will fail. The only way it will work is using special resins, autoclave process, and increasing the section modulus.

    The resulting laminate will have the right E and Uts, perhaps lighter but will lack the plastic property or yield of metal which is safety in itself.
     
  12. rwatson
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    rwatson Senior Member

    At the risk of being boring, I refer you back to the solution I proposed earlier, of steel cored foam and glass,

    By the time you Fritz around with glass calculations, special layers for the loaded sections, and the sheer engineering futz involved, you could get an experienced boat fitter to create a cheaper galvanized solution that you can protect with foam, and carve yourself an attractive shape and glass for protection.

    Whole boats have been done this way, and you have the best of both worlds.
     
  13. oceannavigator2

    oceannavigator2 Previous Member

    I agree with the suggestions and will use the galvanized steel for the davits.

    I just wanted to learn more about this subject to further my understanding of composites.

    Reading rxcomposite's link. Entire boat is silvertip. Glad I didn't cut that corner.
     
  14. daiquiri
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    daiquiri Engineering and Design

    And I hope the goal has been reached. :)

    Cheers
     

  15. oceannavigator2

    oceannavigator2 Previous Member

    A little shakey still, but getting there. Thank you.
     
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