Canting Keel reactions @ Cylinder connection!!

Discussion in 'Boat Design' started by GuestR01312011, Oct 26, 2005.

  1. Hi,
    need help getting the reaction at the end of a canting keel that attaches to the Hydraulic Ram.
    The Bulb weighs 4500kg, The Fin weighs 1820kg.
    The complete length of bulb+Fin upto pivot is 3.5metres.
    The Ram will be connected to the fin be means of a con rod that connects to the pivot shaft and is 150mm long(into the hull after pivot).
    The max canting angle is 40 degrees both sides.
    Could someone tell me how to resolve these forces at 40degrees into a single reaction at the conn. point.?
    Also since the Hyd. Ram will only be pushin/pulling horizontally will it have to be resolved also since as the keel cants the angle of force of the ram changes???
    Much Appreciated all,
    Ronan Currid.


    The Mass `C` is 4500Kg @ length 3.5metres from the Pivot `B`. The Ram con-rod point `A` is 150mm from `B`.
    The angle of |BC| is 40 degrees.


    I was wondering if you could tell me how to figure out the reaction needed at A to keep this assembly @ 40 degrees?

    I have treated is as a horizontal beam and taken moments about B and found a reaction for A but would it not need a lesser reaction if the angle is 40degrees and not 90?
    Also the Hydraulic Ram will be effecting the point `A` only in a horizontal direction, (from both sides-2 rams) will a ram producing a reaction equal to the above query be sufficient(including safety factor)??

    Much Appreciated,
    Ronan.

    The Mass `C` is 4500Kg @ length 3.5metres from the Pivot `B`. The Ram con-rod point `A` is 150mm from `B`.
    The angle of |BC| is 40 degrees.


    I was wondering if you could tell me how to figure out the reaction needed at A to keep this assembly @ 40 degrees?

    I have treated is as a horizontal beam and taken moments about B and found a reaction for A but would it not need a lesser reaction if the angle is 40degrees and not 90?
    Also the Hydraulic Ram will be effecting the point `A` only in a horizontal direction, (from both sides-2 rams) will a ram producing a reaction equal to the above query be sufficient(including safety factor)??
    Much Appreciated,
    Ronan.
    File attached shows assembly
     

    Attached Files:

  2. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    Ronan,
    Forget the whole 40 degree thing, and concentrate on the worst case scenario - that of the boat lying such that the keel itself is parallel to the water. This will put the most load on the ram - don't forget to deisgn the ram and cylinder to take the compression loads. :)
    Steve
     
  3. Karsten
    Joined: Jun 2004
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    Karsten Senior Member

    You have to get your had around which load case you want to analyse. The 40 degree case or the load case that produces the maximum load in the ram? As you have drawn it the moment arm of the bould is only the horizontal distance between A and B because gravity only pulls down. The load in the ram also depends on the angle of the ram. Is it horizontal or at an angle? If it's horizontal only the vertical distance between B and C is the moment arm for the ram and that decreases with the canting angle.

    In the worst case cenario (keel horizontal) you also have to apply acceleration loads! Usually about 3g is used. So you have to multiply the weight of the bould and fin by 3! In your case I would design the ram for 315000 kg!
     
  4. nemo
    Joined: Apr 2002
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    nemo Naval Architect

    Wow 3g?? Isn't it a bit too much?
    I designed a canting keel for a mini transat, and I calculated a vertical acceleration of about 12 m/s2, supposing a wave spectrum with a wave height of 4.5 m. Since the keel is near the boat centre, there is no pitching effect, just heave, and 3g sounds too much on the conservative side to me, but I would like to know if that is really what designer use for the calculations.
     
  5. Pelle
    Joined: Oct 2005
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    Pelle Naval Architect

    My feel is that 3 g is probably a too small acceleration load. When you dimension keel bolts and supporting structure 5g is often used. IMO the safety factor is choosen based on fatigue considerations so you must work out a probable acceleration load spectrum. If one looks at the failures that have occured one can fairly certtain that the loads invlolved is not very well understood so a large safety factor should IMO be used..
     
  6. SailDesign
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    Location: Jamestown, RI, USA

    SailDesign Old Phart! Stay upwind..

    3g is not excessive as factors of safety go, especially for a keel. If you don't have it, especially on a Mini in the middle of the race, you are basically forked. You have to balance factors of safety with the expected conditions and locale. America's Cup boats can use a minimal 1.01 if they desire, since the money is available and the conditions are known. Mid-Atlantic on a 21-footer, I would like my keel to stay on, and don't mind weight down there :)
    Steve
     
  7. Karsten
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    Karsten Senior Member

    It depends for what load you are designing the structure. 3g is used for an ultimate load. This means that the structure is going to break at this load. Composites are usually designed for a limit load as well. This is the load where first cracks start to appear in the resin. You want to make sure that the cracks don't appear under normal working loads.
     

  8. nemo
    Joined: Apr 2002
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    nemo Naval Architect

    I think we should separate loads from safety factors. IMO loads should be as much realistic as possible, and then, in the design process, we should decide safety factors, depending on the mission of the boat or the part we are designing.
    My keel fin is made of solid stainless steel, and, for the design load, its safety factor is 3, compared with the yeld strenght, not the ultimate strenght.
     
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