Cantilevered Davits - Engineering Problem

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

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  1. rwatson
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    rwatson Senior Member

    That's a fair thing, everyone should appreciate the processes involved.

    Also, I find from a lot of reading, that the engineering calculations for composites are one end of the problem, the other is getting the material put together correctly by the glasser. For hi-stress areas like this, there is, perhaps, more re-assurance in getting it done with arguably less error-prone industrial methods.
     
  2. oceannavigator2

    oceannavigator2 Previous Member


    That is the side I am coming from, actually. No problems creating perfect laminates - open or resin infusion.

    I have plenty of glass and reinforced, 100% solids resin just sitting here. Ready to whip up a davit if composites were worthwhile from the engineering side. Since they are evidently not worthwhile here, I have to go out and buy steel. This is why i leaned toward composites to start with. Composites are free or already paid for and lying about the shop.


    Originally, I was thinking I could use a core and a ton of glass down the upper and lower faces to have a stiff and strong set oif davits without needing to go buy more things.

    I just didn't quite understand how materials qualities are used properly to caculate strength and loading. Getting much closer now with some help here.
     
  3. daiquiri
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    daiquiri Engineering and Design

    If you have all that material, want to use it and feel confident that you can make a good job with it, then you can opt for a compromise solution between the two beam examples given in the post #40.

    A square laminate beam 115x115, wall thickness 8 mm and a good fiber fraction (f=0.5) will give you around 20-25 mm deflection when statically loaded and around 30-35 mm deflection with dynamic seaway loads. Should weigh around 20-22 kg each.
     
  4. Rastapop
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    Rastapop Naval Architect

    Yes it will...

    Why wouldn't aluminium have been preferable?
     
  5. oceannavigator2

    oceannavigator2 Previous Member

    Whew... I've created a can of worms here. Sorry about that. I will have some thought as to if the composite davits make sense. The low cost and low weight is the driving factor. I will close the thread with my decision and also take some pictures of the finished davits. As soon as they are installed.

    Thank you very much for helping me to understand the engineering side.
     
  6. Rastapop
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    Rastapop Naval Architect

    Low cost and low weight are generally negatively correlated.
    Steel is cheap, but heavy for a given strength.
    Carbon fibre (for example) is very light but very expensive.
    Aluminium is a compromise of both, in the middle somewhere.
     
  7. oceannavigator2

    oceannavigator2 Previous Member


    Read the thread a little more closely.

    Low cost and low weight are easily achieved when you have a lot of epoxy, fiberglass rolls and structural foam already purchased and just lying about the shop. ;)

    Aluminum is out here because of the expense.
     
  8. Rastapop
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    Rastapop Naval Architect

    Of course free is the cheapest option! :)

    I saw you'd decided on steel, and wondered why not aluminium instead (the cost will be what, tripled?)
    But I didn't know which had the higher weighting in your decision: cost or weight.
     
  9. oceannavigator2

    oceannavigator2 Previous Member



    It was cost.
     
  10. philSweet
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    philSweet Senior Member

    Painting galvanized steel was mentioned way back in this thread. Just wanted to put up a link on that.

    http://www.galvanizeit.org/specific...ifying-duplex-systems/preparing-hdg-for-paint

    There are quite a few ways NOT to do that. Sometimes the galvanizing can get confused in the dark and protect the paint by sacrificing the steel, which usually isn't what was intended. Or the galvanizing chemically reacts with the paint and it flakes off. Plan early and get the galv parts done to paintable specs if you plan to go that way.
     
  11. groper
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    groper Senior Member

    ON2. .. I would also make them from glass.

    If you can, increase the vertical dimension of the beam. Stiffness of beams is proportional to depth cubed. So if you go upto 150mm or even better 200mm, then you will need only a thin laminate. You could sculpture some foam and wrap with glass. Mostly uni top and bottom, wrap the sides and around top and bottom with dbias . This gives optimum Fibre orientation. Weight could be very light. ...

    Beam size 150mmx100mm and a 3mm laminate thickness, i get a 7.2mm deflection on static load of 150kg on a single beam. (all of the weight on a single beam not both)
    Laminate stress is extremely low at 24Mpa.

    Alternatively you could mold them from a 2 peice mold and bond the halves together if you prefer finished molded surfaces. Use a core material with additional inner laminate to prevent buckling issue and structural core like coosa or plywood etc with high compressive strength under the support post area. Also internal shear web in this location. Simple plywood will do.

    Im not sure what daquiri did with his calculations, but his laminate seems very excessive, must be a mistake somewhere?

    The cantilever distance of 1m (provided by you) also seems in error as the dingy is 2m beam. This means no thickness of the vertical support posts right to floor level and the dingy is touching them. Can you hoist the dingy without it fouling on the way up? I think you need some clearance and also allow for thickness of support post, so a cantiveler distance of 1.5m is probably more "real world"... If so, then the deflections i mentioned earlier will change...
     
  12. rwatson
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    rwatson Senior Member

    Aluminum is lighter, but you need a lot more to get the same strength.

    In trailers for example, the main beam in aluminium is 2/3 weight, but 250 mm deep, and 150 ,, wide. A nusisance when you are trying to get a structure as low to the ground as possible, or reduce windage

    Also, aluminum is prone to fatigue in welded joints, and davits will get a lot of pushing, pulling, impact and vibration of structure.

    Steel will deliver the least bulky solution, with only a modest weight penalty, and last a lot longer against all the stresses and strains of a highly exposed structural element.

    Then, as OC pointed out - the cost.

    When push comes to shove, and you have to have a competently engineered structure, both glass and aluminium have a high calculation component.

    Then, say you have a glass solution designed- imagine the time involved in making the either foam or moulded framework to layup the glass. Then the hassle of the hi stress sections where you need pulleys and controls embedded.

    Having a lots of welded Aluminium joints is problematic. Ideally, you need to have each one x rayed - because in a davit, each joint is critical.

    Compare that to getting the MIG to create a 5 tonne joint in a critical join - in 30 seconds. In one minute you have a joint that will last for years with compete confidence.
     

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  13. daiquiri
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    daiquiri Engineering and Design

    As I said in the post #40, I have simply took the real-world data from Fiberline Composites: www.fiberline.com . They produce prefabricated structural grp beams and have a very straightforward calculator for their structures. If there is a possibility to use a calculation tool based on standardized data, I prefer using that rather than calculating theoretical laminate properties and plugging them into beam equations. Since their square tubes are industrially made with a volumetric fiber fraction of 0.6, I doubt that Oceannavigator will manage to make a stiffer one with manual methods.

    Also (though you probably know it), please take care of flexural buckling when such thin sections are used, especially when cyclic loads are present.

    Cheers
     
  14. oceannavigator2

    oceannavigator2 Previous Member


    Ah, aesthetics!

    The forgotten factor here. It is true that a thicker davit, positioned as indicated in the first post, will look worse than a thinner one.

    So now.... mild steel painted? To avoid the galvanized pain issues?
     

  15. oceannavigator2

    oceannavigator2 Previous Member

    Without infusion, yes, it would be difficult for me to achieve a higher glass to resin ratio, especially given the required thickness, which would need multiple layers of glass.

    These could probably be produced nicely at close to a 50/50 glass to resin ratio if I infused them, but... then it does become a bit of a project and expense since I have no infusion resin handy at the moment (plenty of infusion hardener if anyone wants to buy it though!).

    Flexural buckling - is this when the skin buckles in compression, delaminating from the core? For example, the underside of the davit beam would have flexural buckling when the tender is suspended, the thing is full of rainwater and there is a failure by the bottom skin buckling in compression?
     
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