Marine air filled roller haul out force calcualtions

Discussion in 'Boat Design' started by Annode, Sep 22, 2019.

  1. Annode
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    Annode Junior Member

    These are used in the east a lot. Anyone care to take a stab at the winch load required to move say 100 tons of boat like this factoring in slipway angle, roller resistance and friction?

     
  2. gonzo
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    gonzo Senior Member

    I think that the roller manufacturer probably has that information. It will depend on many factors. For example, the physical characteristics of the material and the air pressure in them.
     
  3. Annode
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    Annode Junior Member

    manfr not any help and I dont trust them anyway. They make rubber air rollers. They are not qualified engineers.
     
  4. Waterwitch
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    Waterwitch Junior Member

    May I ask what sort of old steel vessel, and length you are dreaming about restoring? You have tossed around the displacement tonnage of 100 to 300 tons in your previous post but you show videos of much larger vessels than that as examples.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    The force required to pull it up the slope = Force to overcome gravity + Friction force.

    F = W.sin(theta) + Fric.Wcos(theta)

    The friction of that arrangement would be i'd say around 0.25 on a slope of around 10 degrees.

    F = 100xsin.10 + 0.25x100xCos.10 = 17.4 + 24.6 = 42 tonne, up the slope, parallel.

    That's why using air bags is only good for launching, not so for recovery, as the force required for the winch to pull up is not small!
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You would need in excess of 10 tons of cable pull to move your 100 ton boat up a 10 degree slope, neglecting the frictional losses. Its the slope that kills it, the frictional losses are just added pain.
     
  7. Annode
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    Annode Junior Member

    Thank you!! I assume you are just taking the vertical component of gravity?
     
  8. Joakim
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    Joakim Senior Member

    0.25 sounds a lot. The ship on the video seems to weight 410 tons: http://kodiakboatyard.com/wp-content/uploads/2012/04/Kodiak-Lifts-Historic-Vessel.6.8.11.pdf

    The video says "Using our 1995 Hitachi Ex550LCE Excavator and a D8". EX550 weighs 55 tonnes and Catepillar D8 36 tonnes. Likely they can't push and pull more than their own weight. Their weight is about 25% of the ship and they don't seem to have much troubles moving her with some slope (5 degrees?).
     
  9. gonzo
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    gonzo Senior Member

    If the force required is 43 tonnes, and both machines combined weight 91 tonnes, they have to pull 47% of their weight. I don't know what they can tow, but the arm digging force is 27 tonnes for the Hitachi, so that is how much it can pull.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed. But it is not just a random number of finger in the air.
    I did a quick check of my old Kempe's Engineers ref book. They quote:
    Rubber on Asphalt - wet - 0.25 to 0.75
    I know the ground is dirt...but that makes it worse...so in the whole scheme of things and without proper tetsing of the bags the soil etc etc...i thought 0.25 seems reasable.

    Thus:

    Yes, most likely much less of an angle.

    But 5 degrees still gives around 33 tonne.

    If 5 degress and say 0.10 friction it becomes 18 tonne. Those machines can really pull though!!

    Thus it is most likely less of an angle, and the rolling friction of the set up is much less.

    I've done more launching calculations than I can shake a stick at, and design our shipyards launching system with a rail and rollers too......so, it always comes down to the angle and reducing the friction as much as possible.
    We hauled up vessel up to 700 tonne with very little winch power; because we reduced the angle and used high quality bearings in the rollers. But, as for the above, in the absence of any hard data...who knows, it is just a finger in the air calc?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess the digging arm can pull it forward a little at a time, but how do you stop it slipping backwards between grabs !
     
  12. Waterwitch
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    Waterwitch Junior Member

    In times past, wooden sailing ships were hauled to the shore for launching with oxen and log rollers. So a team of 50 oxen, assuming small ones at 1000 pounds each, capable of pulling their own weight at a walking pace, would yield 50,000 pounds of pulling force...
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd have thought a fixed winching point would be the way to go, the limiting factor might become your cable breaking strain.
     
  14. DCockey
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    DCockey Senior Member

    The 1993 edition of the "Automotive Handbook" gives rolling resistance coefficients (absolute) for pneumatic car tire of 0.013 on concrete and asphalt, 0.02 on rolled gravel, 0.05 on unpaved road, and 0.1 to 0.35 on "field". For "track-type tractor in field" it gives a rolling coefficient of 0.07 to 0.12. Obviously the coefficients for fields and dirt surfaces will vary by a very large amount depending on the surface conditions. Rolling resistance on a dry, hard packed surface will be much less than on a muddy surface or a surface with loose material.
     

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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks DC..handy figures to know.
    So, a value circa 0.10 is probably more like it then. :D
     
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