Grounding loads...

Discussion in 'Boat Design' started by ErikG, Oct 18, 2003.

  1. ErikG
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    ErikG Senior Member

    Well I know Henrik might feel like I'm taking a shortcut here (internal)...

    When running aground Accord. to L&E PoYD, there are a number of things to consider. But having a modern bulb on the keel and running aground (badly) how does deformation and so on affect the loads? As far as I understood it from L&E all the energy caused by the total displacement speed and stopping time constant must be transferd into the boat as load through the hull/keel joint. But is that so? the above mentioned deformation does take its part how big depends on the amount of deformation obviously.

    I guess my real question is... does the energy really only have to do with the above? On a lightweght keelboat where the keel stops violently [BAM] won't that energy just "stay there" hmm guess not but why and how...

    If anytbody understands my question (not sure I do myself) please feel free to respond.

    :D :confused: :D :confused:
  2. SailDesign
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    SailDesign Old Phart! Stay upwind..

    If I understand the question (?!?) correctly, you will need to take into account a certain amount of damping from the boat tucking it's nose violently downwards immediately following the impact. It is part of what causes the hull/keel joint loads, but it does extend the time factor somewhat, leading to lower accelerations.
  3. PaulCoffin
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    PaulCoffin Junior Member

    I'm going through the same thought process. This is what I've got, but I don't claim that it's correct:

    Assume a bulb keel boat, with all the keel's mass in the bulb (negligable mass keel strut)

    If a moderm boat hits a rock with the keel bulb, the keel will stop immediately with all the keel's energy going into deformation of the contact point on the keel and rock. The high loads occure from the keel slowing down the rest of the boat, so I would use total mass - keel mass as the critical mass.

    The instant after the keel contact, the keel will rotate about the contact point with the rest of the boat continuing at its initial velocity. The high loads occure as the keel sole (right word? the contact surface of the keel and hull) tries to rotate the hull. The hull will resist the effort due to primarily its moment of inertia and to a lesser extent due to the hydro forces resisting the plunging of the bow.

    Now how you turn this into loads is the hard bit. like L&E, assuming a time might be a good first approximation.
  4. henrikb
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    henrikb Senior Member

    This is exactly my thoughts on this subject! The rotation is stopped by the bow diving into the water and increasing displacement (in a math model a spring load?)
  5. ErikG
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    ErikG Senior Member

    So now that we have a guess, we just need to create a mathematical model and compare to real life :D

    The question in my mind (not being the smartest guy around) is how does the energy of the moving body translate to Nm's?

    I'm quite sure that not all of the impact moment will be translated into deformation, so how much and how does it transfer to the hull. And can we really free the moving weights from each other theoretically, my brain says no but *** says yes, in way perhaps... ;)

    Do we run with the old school calculation as in L&E and adjust the time constant and use the whole displacement, or do we try to invent something new?

    Could we use our existing swedish example here Henrik? Try to get the answer about how big the deformation really was at the speed of impact and
    what it was that he went on, a loose rock will give a different resul than a large piece of concrete that is a part of a structure. Because if we know haw much the bulb deformed and can check how "deformable" the lead used was (level of antimony) we should get a force that can be deducted from the normal calc as stated in L&E as well as getting an idea of how much the time constant might change during the deformation time.

    As we say in sweden "am I out riding a bike?" or in english "am I stupid or what?"...

    :D :D :D
  6. PaulCoffin
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    PaulCoffin Junior Member

    The attached file shows the aproximate model I was thinking of. It probably needs work, and the following description is far from rigerous because I am doing this rather quickly since I should be working on something else.

    The dynamics of the impact could be modelled in a spreadsheet using small time increments.

    K1 is a spring constant aproximating the stiffness of the aft area of the hull where the keel attaches.

    K2 is similar for the forward area.

    K3 models the change in bouyancy as the boat tilts.

    D3 is the damping effect due to the bow plunging, sort of a CApV^2/2 quantity.

    These numbers will take some effort to determine.

    In the spreadsheet at the beginning of the first time increment the hull is at rest and the keel tip begins moving aft at the imact velocity V.

    At the end of the first time increment the springs K1 and K2 have deformed a certain amount (can be calculated) that creates forces that accelerate the hull over the next time increment.

    At the end of the secong time increment the hull is at a new position and now has an angular velocity, allowing the forces from K3 and D3 to be calculated, etc etc.

    In the end, forces and deformations of K1 and K2 can be tracked over the impact event, and, hopefully, this could lead to useful design information.


    Attached Files:

  7. ErikG
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    ErikG Senior Member

    Thanks Paul

    Paul gave me a bit to think about there...
    Are there any books that discusses geoundingloads in any other way than the above mentioned Principles of yacht design by Larson & Eliason?

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