Cantilevered Davits - Engineering Problem

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

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  1. rxcomposite
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    rxcomposite Senior Member

    You can always cover the steel with nicely painted and curved GRP panels. :cool: Weld some ears into the steel davit and bolt the GRP panel. Seal joints with silicone.
     
  2. groper
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    groper Senior Member

    Is the Fibreline RHS section made from UD Fibre? I doubt it... they quote their modulus at half the normal value for UD fiber. It's nowhere near as good as a custom laminated beam ...

    Buckling is controlled by the shear Web inside the beam located above the support post. The rest of the beam is controlled via the core material in the walls. There is not more than a days work to make this...
     
  3. Rastapop
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    Rastapop Naval Architect

    Can you not just buy aluminium rectangular tubing with thicker plating? (Really wondering)

    It looks like a minimal number of large joints....
    If you can build an entire vessel from aluminium, and if you can buy aluminium davits ready made - this doesn't seem to me to be an issue that can't be overcome.


    Percentage-wise the weight increase will be quite significant.
    And I think aluminium will take exposure much better than steel would.

    This is the killer.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Structural beams cannot be made of just UD fibers because they are subject to a combination of in-plane bending, torsional moments and transverse forces. And they are all cyclic.
    http://www.fiberline.com/armering
    http://www.fiberline.com/structures.../mechanical-prope/mechanical-properties-struc

    Regarding the published mechanical properties, they appear to be identical to the minimum values required by the EN 13706-3 standard, so I guess that's a way to play safe against possible liability problems. See Ad Hoc's contributions to this thread, regarding the AISI 316L issue. The actual mechanical data might be higher than those used in calculations, but even the latter ones appear to be pretty much in line with results of several coupon tests of laminates I've read about - which more often than not reveal values inferior to theoretical ones.

    One possible reason for higher thickness of their beams (compared to a theoretical UD one) could be the fact that, as I understand it, they use polyester resin with glass. So they might be using a layer of cfm between each load-carrying UD layer, or a combined woven-cfm fabric.

    Cheers
     
  5. groper
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    groper Senior Member

    Ok to clarify, UD fibers are anything under the NCF or non-crimp fiber umbrella. whether 0degrees, +-45degrees or any other direction, a non-crimp continuous strand fiber is still a uni directional fiber.

    A specifically engineered beam using fibers oriented in the stress direction is far more efficient than a chop strand fiber beam which is designed as an isotropic material so that its properties can be published for loads in any direction. Id say the latter appropriately describes this manufactured section, for its commercial marketability...

    Once again this illustrates the inefficiency of some professional approaches. The only professional approach to this problem becomes expensive because the exact laminate and complete and total design needs to be detailed. Hardly worth it for a set of small davits on a small private use vessel... see my previous contributions :) This has been the subject of quite a few heated debates ive had with Adhoc and it stems from our different backgrounds, Adhoc designs commercial vessels where his *** is on the line everytime, any mistakes lands him in a lawsuit. My background is very different, i have noone to answer to, no lawsuits to worry about, and if something breaks, only my pride is damaged and a go around to pick up a lost dingy... Such commercial thinking doesnt do anything for me except land me a more expensive material cost, a heavier end result, and higher fuel consumption from that point forward.

    There is no right and wrong here, only shades of grey... choose your colour and accept living with it...
     
  6. daiquiri
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    daiquiri Engineering and Design

    You are in a privileged situation. Sadly, the army of corporate lawyers and the liability system has put the engineers' heads in the box.

    But it's not only a legal matter. Having to design davits and CAE systems every day, and having seen cases where their mechanical or structural failure has crippled or killed persons, or damaged the ship, I just don't want to suggest any solution which is not sufficiently safe according to either official or my personal standards. Want to sleep well at night. :)

    Cheers
     
  7. oceannavigator2

    oceannavigator2 Previous Member

    There may also be something to be said about this particular application when it comes to safety.

    There are not your everyday davits. Normal davits protrude from the aft beam, lifting the tender just above the water. If a person climbs in the suspended boat to fiddle with the outboard or refill fuel, a break results in a tiny drop to the water below. I've had hardware give out and drop me in this case. It didn't hurt. ;) The side with the outboard broke so i slid into the outboard and was mostly just startled.

    But these davits are different. These will be able to lift the tender just above the water, but also to a height of 3 meters above the water. A drop from that height could kill someone as well as damage the boat and tender.

    I'm sure it could be made from composite. After all, I can build a mast and a chain plate from composite. But, if from composite, the davits need to be engineered to act more like steel, or be so stiff and strong that the will not fail. This probably eats away at the weight advantage significantly?

    For argument's sake, how would Groper's laminate design look if it were to act more like the lightest steel solution?

    PS: Maybe if i make them wide and out of composite, they can double as 3 meter high dive boards. ;)
     
  8. rxcomposite
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    rxcomposite Senior Member

    AH is correct.

    Material properties available in the net is identified as UTs, not the yield point. Yield point start from 50-60% of UTs depending on resin choice.
    Refer to my link provided earlier.

    Class rules formula use the elastic portion of the laminate to ensure you are working on the fracture free laminate. Yield (fracture) is somewhat higher at 8 to 10% more.

    Don't use the Class recommended stress or strain ratio at this point. It is 0.3 or 0.33. That will leave you with almost nothing to start with. It looks like a safety factor of 3 when computed to yield point it will be much higher proportioned to UTs.

    I have a different data sheet and it seems the UTs is consistent with my spreadsheet.
     

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  9. daiquiri
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    daiquiri Engineering and Design

    Good one, RX. :)
     
  10. rxcomposite
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    rxcomposite Senior Member

    It can be made out of composite but as you admitted, you dont have the tools. General FRP is too flexible to use as a lifting apparatus. Consider this. Aluminum is 2.8x stiffer than GRP, Steel is 10x stiffer.

    When advanced composites is used, high fiber volume is used, therefore, the laminate approches that of the fiber. When it breaks, it is catasthropic. Steel although lower in tensile strength, bends first, stretches (or yield) then break. It has a long (high) modulus of fracture.

    It is possible to engineer it and here is the approach to optimize the properties of the materials. It is also possible to taper it instead of a constant cross section but the math is tedious. You will need to use steel or alloy inserts at shear point and compression limiting bushings at bolt joints. Composites are poor in shear and compression.

    Note that I am using a rectangular section to increase section modulus. Note also that the bottom cap is thicker as Unis are poor in compression than tenstion. You have to compensate for the loss in mechanical strength.
     

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  11. oceannavigator2

    oceannavigator2 Previous Member

    This drawing looks suspiciously like my infused, cored aft beam, which makes a lot of sense! :idea:
    Same forces! :idea:


    [​IMG]

    And yes, I am limited to hand lamination and vacuum bagging at the moment. No more infusion resin. But... I do have quite a supply of zero void BS1088, 12mm okoume. That stuff is good for shear webs, right?

    It is looking like painted mild steel is the best at this point though, I see why.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Plywood can be used as shear web but nothing beats a 45/-45 bd. Core can be low density if the outer fibers take up the load but it can also be engineered to take part of the shear load. The figure can be transformed into an I beam profile for analysis.
     
  13. daiquiri
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    daiquiri Engineering and Design

    I don't get this in your example, RX - why do you use cored upper and lower flanges? A beam works through the Section moduli (= I/y_max), so the objective is to maximize that quantity and to bring all the walls of the section as much outwards as possible. A cored flange doesn't do that. A solid one is less stressed for the same weight, and lighter for the same stress.
    Unless you do it as a way to prevent buckling of compressed areas.
     

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  14. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed. The "legal" matter is the end result though.

    But it is initiated by a lack of of "professional" approach that is clearly seen by those that do not work in such an environment on a daily basis and no appreciation of the ramifications.

    Also consider this, although some will consider this as being an inefficiency and an expense approach.

    The tender is propelled by what...an outboard...what feeds it...fuel...where is the fuel..in a tank..where...on the tender.

    Thus if there is an accidental spillage or a burst/ruptured tank...what will occur? Yup..a fire. Even though the fire may not burn long, but then again it may (depending how quickly the fire is addressed), it creates heat. What happens when you place heat next to composite??...yup...you got it. But i'm sure some may argue it is a one in a million chance...hmmmm.

    Now back to D's point about sleeping at nights.

    When sat in a courtroom and being questioned by the lawyer of the family of the victim that was seriously injured, or worse, you could simply say in your defence that my pride was not hurt in the design and it is all a shade of grey anyway. One can only imagine the response that would garner from the opposing side!

    Cost cutting and ignorance costs lives, what ever the type of boat:
    http://www.maib.gov.uk/cms_resources.cfm?file=/HooliganVSynopsis.pdf
     

  15. rxcomposite
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    rxcomposite Senior Member

    Daiquiri- The example is a pure Uni in upper and lower flange to take up the tension and compression load. The core I did not identify because the inside can be hollow, low density core, or foam sized to handle the shear stress.
     
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