Cantilevered Davits - Engineering Problem

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

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  1. Barry
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    Barry Senior Member

    One should not underestimate dynamic loading or at least design dynamic loading.

    A very interesting UTUBE video is called
    Tacoma Narrows Bridge Collapse, 1940 Sound Version

    A different set of circumstances to be sure, but an interesting video to view when a person is considering lowering factors of safety or wondering " Is there something that I did not consider?"
     
  2. Rastapop
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    Rastapop Naval Architect

    Any engineer should be able to do that for you without trouble.

    Where? I haven't seen any (the OP possibly?).
     
  3. oceannavigator2

    oceannavigator2 Previous Member

    Nope. Couldn't be me. I titled the thread "an engineering problem."

    Engineering can go wrong however, when it contradicts reality. Despite engineers in this thread saying it would be impossible or deadly, this is reality. Standard equipment on one of the most famous Morelli and Melvin designed catamarans in the world.

    [​IMG]

    My point is only that one should keep an open mind when designing, not simply rule things out just because it is not something one works with in day to day life.

    People in all areas of life are quick to say things are impossible. After reviewing all the data, aluminum and composite seem to be the best choices.
     
  4. rxcomposite
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    rxcomposite Senior Member

    If your box beam is the crossbeam, it is also meant to handle torsional load from the two hulls twisting when hit by a transverse wave. The biax alone is not enough as the 45 degree implies loading in a symmetrical panel and loses effectiveness if the torsion is low or axial. The triax handle the in between biax and unis.

    So the question is if you hang your davit on the cross beam, will it handle the additional twisting local load?

    That goes the same for the davit. It is not a straight down pull. Sometimes, due to swinging, the pull angle will misalign. If the dinghy is smaller than the spread of the davit, you have inclined lifting angle. Worse if you use a lifting sling with shallow lifting angle.

    The gusset at the right angle decrease the shear of the hard point where it meets. If small, it acts like a bracket increasing fixicity. If large, it has the effect of shortening the length of the lifting arm, increasing capacity. A tapered doubler is the simplest approach.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly..
     
  6. daiquiri
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    daiquiri Engineering and Design

    Sorry ON2, but this is an unfair comment. It uses oranges to demonstrate that apples are wrong.

    The davit in your opening post, which is the one discussed and analysed in my subsequent posts, has nothing to do with the davit shown in this last post. They are two completely different animals. Different length, type of constraints, deflection modes and reactions on the support. Hence allows for completely different engineering solutions.

    Let's play fair, please.

    Cheers
     
  7. oceannavigator2

    oceannavigator2 Previous Member


    Sorry, the post was not intended to be negative. I certainly did not mean to offend any of the very helpful contributors. It was meant to encourage. As a reminder that sometimes, we can also look at what others have done and do a little reverse engineering. That before we simply dismiss something like the idea of composite davits, that we look around in the world to see if it has been done, how it was done and if we can improve on it.

    My apologies if the post was offensive. That was not my intent. I am very grateful for your contribution to this thread. :)
     
  8. oceannavigator2

    oceannavigator2 Previous Member


    Whew... took a few reads to get that. ;)

    I think I could reduce the torsion load on the aft, box crossbeam a lot by taking the long part of the upside down "L" and burying it into the bridgedeck panel, while making contact with the aft crossbeam for its entire face. Same idea, either aluminum or composite.

    Apologies in advance for the quality of my rendering software. :p

    [​IMG]
     

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  9. rxcomposite
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    rxcomposite Senior Member

    I did some quick calcs from the base dimension given to you earlier. Not too far from Daiquiri's dimensions and thicknesses and not too far from what i have posted earlier.

    This is a 156 kg load with a load factor of 2, assumed laminate values at yield, Uni as caps, Biax as shear web. Uni @ 0.54 glass content, biax @ 0.50 or better.

    This is a tapered beam, so the uni on top/bottom can taper, keeping width constant. I made it more square than rectangular to compensate for side loads.

    Remember this is only a preliminary sizing technique. I usually do a layer by layer analysis and use fiber directionaly factor to compensate for losses. Make a test piece.
     

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  10. oceannavigator2

    oceannavigator2 Previous Member

    Ok, I'm on this. Thanks!

    Today, one of the very wise 85 year old men came by the shop. I discussed the davits with him in terms of sor, service life, etc.

    He had done aluminum davits in the past and said "do it right" "it's not worth it" and you know, he's right. I should do this right and never have to worry about it again. I will make up the laminate rxcomposite suggests for the short part of the upside down L. Only question is what to use for the long part.
     
  11. groper
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    groper Senior Member

    The reaction forces are equal , thus for the davit loading the laminate is the same either side of the junction. However you are moving the goal posts again because earlier you said that the vertical posts take the sheet loads from the main sail etc. All of these must be taken into account also and again without defining these loads , the professionals won't be able to design anything for you...

    BTW, the laminate RX detailed is within 0.5mm of what I gave you way back on page 4. The difference is also part of the increased section modulus I used...
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed. However, with respect you're going about this the wrong way.

    You're seeking opinions from fabricators whom are ostensibly biased towards one material/method or another. Fine, ask a steel fabricator which is best of course they'll say steel..or a composite fabricator and of course, composite...etc etc.

    The davits have a function to perform. Looking at just the statics is only part of the story and is just a set of numbers. The fabricators, of which ever material, are not concerned about numbers nor what they mean, only what they consider "best". Best for what??

    If you're serious about doing this right, you need to consult with a naval architect whom can not only "just run the numbers" but provide with you all 3 options (if you wanted), based upon not just the numbers for the strength but also the integration into the existing boats structure without creating any undue problems and longevity of the davits. In other words, hard facts of the whole life cycle from conception to finish installation to 10 or 20 years down the road, with everything taken into account.

    Otherwise, all you're doing is replicating another person's bias of a material and no real appreciation why it is other than the experience of that single fabricator. Which of course generally has very little to do with the hard facts of design and risk analysis, as I noted in my post #74 and ref'd this, as an example of such.

    But it's your choice whose "right way" you wish to follow...
     
  13. oceannavigator2

    oceannavigator2 Previous Member


    Very well put! I think that is definitely what is happening.

    I am sure all of these materials are valid, if engineered properly and I would imagine the thoughtful and detailed posts made by each contributor are accurate. I mean, stepping out of thought experiments for a moment, in real life we see davits and posts made of all three suggested materials, plus carbon fiber laminate as well.

    We also see davits you order from a catalog and bolt on with backing plates, stress calcs be damned.

    Surely all will work. The old man was saying I need to use composite so I don't have to deal with the inherent problems of metal. The old man loves to spend my money! :p But he has always been right.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Yes Groper. We are all very close except that we have used different sections. If we all stick to the same size sections, I am sure it will come out more or less the same as this is based on the first principle of design. the tried and true engineering principles. In fact it can be analyzed the long way, through the transformation of material properties. That is like analizing steel caps on wood beams.

    I have the same format D uses but its limitation will analyze only a constant wall thickness, constant section, and a homogenous material propery. I used a formula that allows me to input three different material property and thicknesses. Still has problem as the Vshear would not compute properly for a tapered beam. It is too simplified.

    I now look at the problem on a detailed view. This 30-40 foot cats don't have much load bearing on deck, much more on the superstructure which is just meant for walking. You had one posted before, right? So will the superstructure main (vertical) beam handle the load?
     

  15. oceannavigator2

    oceannavigator2 Previous Member

    I can help in afew respects with this question.

    The main deck (bridgedeck) on this 50 ft cat is 1 and 1/2" thick 8lb foam with 64oz of glass on either face.

    The aft box beam is 4ft tall, a little over a foot wide and runs the full beam of the boat.

    The laminate, inside and out of the beam is comprised of alternating layers of uni and triaxial, all infused to ideal glass to resin ratios of 50/50. Post cured. All over 3/4" foam. The layers are: about 9 alternating layers of 34oz triaxial and uni. There are 4 bulkheads inside this beam along its length. It is what holds the boat together, other than the main beam that takes the rig loading.
     
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