Help designing simple jib crane

Discussion in 'Boat Design' started by DogCavalry, May 8, 2021.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    So I want to put a crane on my boat.

    I envision a steel tube with a small section 6½' 2m long I beam welded on at the top. Probably a reinforcement triangle welded in. Bottom to drop into cup. Upper bearing to rotate in, mounted about 45", or 1.1m higher. Maximum expected load on the fnd of the jib would be 800#, 350kg.

    Aluminum would be much lighter, but I can't weld aluminum. I'm sure I could sketch up something that would do, but I don't have the engineering math to properly proportion the components. What outer diameter tube? Wall thickness, reinforcement detail. Like that.

    John

    ps- I envision a future where I listen to my spell checker.
     
    Last edited: May 8, 2021
  2. Tiny Turnip
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    Tiny Turnip Senior Member

  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Thanks Tiny Turnip! I'll check it out.
     
  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Autodesk "Inventor" CAD Student Edition FREE software will do "real" stress, bending, etc simulation. You can see which parts are under what stress and run loads to bending/breaking points.
    You just gotta find someone officially enrolled in legit 2 or 4 yr college (any major, or non-major) to activate.
    The kind of crane you describe would be easy to sketch. Or you could pay some starving CAD student a couple bucks to run some sims at local Junior College.
     
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  5. TANSL
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    TANSL Senior Member

    I do not think it is necessary to resort to any type of CAD / CAM. The basic formulas for the strength of materials and calculation of beams would suffice.
     
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  6. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.


    Unfortunately I don't know these formulas TANSL.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Can you post a sketch arrangement of what you would like and the max lift you wish to have...
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Absolutely. Within the hour.
     
  9. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    20210508_153105.jpg 20210508_153617.jpg
    This location.

    These proportions. 20210508_155301.jpg
    So the collar at the 1m mark would be a bearing to take side load from the couple. Cup at the foot takes the other half of the couple, plus all vertical load.
     
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  10. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Port side be okay instead?
    Less intrusive to the drivers view (especially to strd, the right-of-way side and all).

    I was thinking more along the lines of a schedule 40 Al pipe inside another, stationary pipe so the whole assembly could be lifted out and be removed.
    A locking pin could keep it in place in a seaway.
     
  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    I'm thinking mounts both sides and both sides of the back deck too, and 1 post and jib that can be lifted out and moved around for maximum convenience.

    The Al tube is preferable to steel, but I can't weld Al.
     
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  12. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    If you do up a good design (or find a better one) have someone weld it up in Al.

    Did you talk to your Al contact in Alaska (the Sea Sled guy)?

     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Haven't talked to Marcus in months. I expect AdHoc or TANSL, or other forum members could improve on my sketch. Realistic scantling certainly. Anything I drew would be ridiculously heavy, because I'd do my usual "when in doubt, overbuild" thing.
     
  14. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Just a few random thoughts.
    If the top section of your crane / davit is an 'I' beam, would it be worthwhile having the lifting point able to slide along on a trolley that has it's wheels on the lower flange? Or is the range of travel not worth such complexity, as the arm is only 1.85 m long anyway?

    Would you have a chain block hoist, or just a simple block and tackle system?
    I am thinking that if you want to be able to hoist 350 kg, a chain block would probably be easier and safer than a block and tackle (although it would take longer to lift up and put down the load with a chain block).
    There is a mind boggling array of chain blocks for sale on Amazon, at pretty cheap prices. Here is a typical example.
    https://www.amazon.com/SPECSTAR-Manual-Chain-Block-Capacity/dp/B07TVGZSTN/ref=sr_1_9

    As an alternative to having a horizontal cantilevered jib, would it be feasible to have a derrick instead?
    One advantage of a derrick is that the end can be 'luffed' up and down. The base of the derrick could be pin-jointed, attached to the mast just above the collar at the 1 m. mark on John's sketch.
    And if the derrick is designed so that the maximum load of 350 kg can be lifted at 1.85 m. outreach, then you still have the possibility of lifting a lighter load at a further outreach, or a heavier load with the derrick 'luffed up' a bit more, so that the outreach is less.
     

  15. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Most excellent questions! The I-beam is standard, because a traveller that locks onto the lower flange is an off the shelf item.

    A derrick is an appealing idea. I need clearance under the horizontal component for a quad, or a large refrigerator, but the joint isn't necessarily fixed. The engineering for a jib crane would also apply to a derrick. No doubt there are many improvements available. I'm trying to minimize complexity, because I need to grasp it all, without dropping any balls.
     
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