Supports for equipments

Discussion in 'Boat Design' started by vinceUK, Nov 17, 2008.

  1. vinceUK
    Joined: Feb 2006
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    vinceUK Junior Member

    Hi all,

    I am not sure but I hope I have posted this in the right section.

    I am actually designing supports for quiet heavy equipments (e.g sewage treatment plant, around 250kg dry) and I wondered how to treat that. I know all about the equipment and the geometry of the support. I am just trying to work out the ideal scantlings (composites, certainly plywood cored sandwich).

    Actually, I use as the worst case a 90° knockdown( as for keel bolts basically) but this is for a catamaran (110', so it seems quiet unlikely to capsize...).

    I can calculate motions, accelerations and g's taken by the machine but how would you translate this in terms of stress for the material? Is there any constant to find and then to apply to the static stress formulae in order to take into account the dynamic loads created by pitch, roll...?

    Or maybe is it worst just using more basic formulae or rule of thumb?

    Any help would be much appreciated. Thanks in advance. Vince
     
  2. daiquiri
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    daiquiri Engineering and Design

    What kind of boat will it be mounted on - displacement or planing one? If the hull is planing then you will have high impulsive vertical accelerations to take into account, roughly 5 or 6 g's.
    Boats designed to operate with significant wave hights can be pounded by even higher accelerations, up to 9 or 10 g's.
    I can link you to some interesting readings on that subject:
    http://www.davidson.com.au/products/force/ist/pdf/High-speed-watercraft.pdf
    http://stinet.dtic.mil/cgi-bin/GetTRDoc?AD=ADA446789&Location=U2&doc=GetTRDoc.pdf

    By the way, they contain some data about percentage of injuries among Navy SEALS during the high-speed transportation to the battleground, which are simply impressive.
    A displacement hull will be subject to much lower loads, for obvious reasons.
    I have seen that 4 g's accelleration is a general practice among my clients when they want to take account of pounding loads in heavy seas. Looks like they don't sell their stuff to Navy SEALS. :D
    Translating these into dimensioning of supporting structures is an ordinary structural engineering task.

    Is that what your question was?
     
  3. vinceUK
    Joined: Feb 2006
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    vinceUK Junior Member

    Thanks Daiquiri.

    It is very interesting. This is part of the answer I'd like to have.

    The boat is a 110' sailing catamaran. Regulations (like BV that is in charge of this design) give formulae to calculate these accelerations and I am happy with this part. The values given in your link are within the range I expected.

    The other part of my question is: Is it usual to bother going into these dynamic calculations for cruising vessels or do people tend to use static formulae (much quicker) with a worst condition (e.g, the 90° knockdown)?

    I know there is not only one way to do this and every architect may have his own technique but for a cruising vessel, how far would these kind of analysis go? Maybe there are shortcuts with high factor of safety for this kind of cruising yachts. If not, I will then jump into this and spend a bit more time calculating. This is just about getting some tips and advises from experienced people for these kind of questions.

    Thanks in advance for all your opinions. Vince
     
  4. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    The classification society rules, like BV's, usually prescribe you the laminate thickness to use.
    The other way, apparently very en-vogue in this forum, is to use scantling rules from Dave Gerr's book "The Elements of Boat Strength", which are very simple, quick and straightforward. Though the method implicitly assumes that we trust mr. Gerr did his calculations properly. ;)
    Then you also have ISO 12215-1, ISO 12215-2 and ISO12215-5 rules...

    For important structural parts or those with complex geometry I personaly prefer using FEM (with proper and robust safety factors) or some of classic empirical or semi-empirical engineering methods - but first I need to understand were they come from and what assumptions are they based on.
     
  5. vinceUK
    Joined: Feb 2006
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    vinceUK Junior Member

    Daiquiri,

    last question and then I stop disturbing you. Can i ask oyu to check that the methodology I will use (if validated) is good?

    1) Determine acceleration (links given above, BV, etc)
    2) Determine force (mass * acceleration)
    3) Determine bending force (height cg x force) for two of the four bolts
    4) threat this as a canlitever and it is likely that the mode of failure is the tearing out of the washers through the plate as long as the plate can whistand the load of the device laid on it.

    Does this make sense? Would you use this kind of methodology? This would mean that the out of plane shear properties of the plate are the main determinants of the non-failure.

    Thanks for answering in advance. Best regards, Vincent
     

  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    You know, it is difficult to give an answer (and be sure of it's correctness) without knowing the constructive details of what we are talking about.

    Item 3 is not correct as a general approach.
    You need to consider your system (body+support) as if it was a rigid system subject to static loads only, in which one of static forces will be the force due to accelleration F=m*a (D'Alembert's principle).
    Then you apply the cardinal equations (force and moment equilibrium) to this static system and find the reaction forces at the suports.
    The reaction force at a support will generally be axial, tangential (shear or perpendicular) and bending. Whether you can ignore one or more of them depends on the mutual order of magnitude of these components.
    Usually only the tangential (perpendicular or shear) force can be ignored for cantilever beams, because very small compared to the other two - but this has to be verified too. It again becomes important when calculating the bending moment.

    One more thing. You say that the moment due to inertial force will be F*Ycg. It sounds correct if the force component is horizontal. But when considering the slamming load (which is vertical) it's moment will generally be written in a form F*Xcg.

    If it is not clear enough just ask. A sketch of your assembly would help. You can PM me.
     
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