Help designing simple jib crane

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Ok, so with those dim's, there are several things going on here....
    1. The bending moment on the tie bar is 31.76kNm.
    2. This give s a modulus required of 276cm^3, which means a tube of circa - 5" 1/2" x 1/2", or going metric of 160mm x 10mm wall thickness....or any another with the req'd modulus.
    3. Deflection check shows, the imperial one with a deflection of 94mm, the metric one 70mm.
    This is clearly too high!

    So, you really need to add a diagonal jib to reduce that huge bending moment!
     
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  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    How large is your boat? Have you considered its stability? Having the equivalent of 4 stocky men 6 feet outboard will be a significant overturning moment.
     
  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    She's not a particularly large boat, being just 26 by 10 feet, but the hull form is very stable, like a barge/catamaran hybrid. And in normal operation she should weigh about 4200#, so I expect her to heel some, but not dangerously.
     
  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Thanks Ad Hoc. Any suggestions on the jib dimensions?
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Here is a rough sketch with my thoughts - I have not done any calculations on it, I just wanted to see what yourselves think about it.

    I think it would be useful to have a luffing ability on the jib, so that you can bring the cargo a bit closer to the mast if required.

    If need be, extra aluminium plate webs could be welded to the boom and to the forward side of the mast, to increase the second moment of area of the section, if the deflections are still excessive.

    Dog Cavalry sea sled crane 1.jpg
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Depends how you wish to arrange it.

    As your sketch but with a fixed jib from the support to the end
    or
    as per Bajansalior's sketch above, with luffing?

    I suspect more of yours since whilst B's sketch is an excellent idea, but, it doesn't give you much clearance for bulky items once raised, they may not get over the bulwark! Unless you reverse the wire/jib/boom

    Thus how do you wish to go?
     
  7. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    It is precisely the bulwark clearance issue that had me thinking fixed jib. Bajansailor's notion has a lot more flexibility, and I may graduate to that. At the moment I'm getting ready to build on small islands, some without even a dock. So a quad, or 4 wheel atv is an invaluable resource. The largest is around 350 kg. And the height above bulwark to swing one out defines my jib clearance.
     
  8. bajansailor
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    bajansailor Marine Surveyor

    If you are happy to keep this crane / derrick fixed in one place, the proposal in my sketch above could be made taller, and you could then even have an additional support bearing in way of the cabin roof (?)
    This would have a major advantage in considerably reducing the load on the mast, as it will be supported (near to) the top as well, rather than being cantilevered over the top 1200 mm or so as shown in my sketch.
    And if the mast is taller, then the hinge location where the boom attaches to the mast could be raised a bit higher as well - if DC can determine the maximum cross-section area (height and breadth) of the most bulky cargo item (eg an ATV) that needs to be lifted over the cockpit coaming, then this could be incorporated into the sketch, to determine what height the boom has to be, and what angle it has to be used at, in order to load the ATV easily from the shore.
    I am guessing (very roughly) that it might have sufficient clearance if the hinge for the boom is perhaps 0.5 m above the cockpit coaming / gunwhale (?)
    How high is the coaming above the cockpit sole?
    I have shown the boom as 2 metres long; how long is the forward cockpit?

    Edit - an extra thought - if the mast height was increased some more, to (say) 500 - 750 mm above the wheelhouse roof (assuming that it has a support bearing in way of the roof), then the angle between the block & tackle (for luffing the jib) and the boom is increased, which means that it will be easier to luff / hoist the boom with a load on it (you can resolve the forces using simple vectors).
     
    Last edited: May 9, 2021
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  9. BlueBell
    Joined: May 2017
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    BlueBell WOT

    Seems like this thread has merged with your other Sea Sled thread, John.
    Perhaps it should be merged?
    Your call.
     
  10. DogCavalry
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    DogCavalry I aim to misbehave.

    Oops! Cross contamination. Sorry guys. I'll insert Marc's preliminary work here, because whatever genius stuff Bajansailor, Ad Hoc or TANSL might come up with, it will be valuable to other forum members who are too smart to get Sea Sled Madness.
     
  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    JH connectin.png
     

    Attached Files:

  12. DogCavalry
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    DogCavalry I aim to misbehave.

    20210524_162832.jpg
    So here's my tentative post notion. Thick walled aluminum tube drops over this post.
     
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  13. bajansailor
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    bajansailor Marine Surveyor

    Would you plane down the corners of the post above the deck to give it a circular cross section?
    If so, what sort of clearance would you aim for between the post and the inside of the tube?
     
  14. bajansailor
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    bajansailor Marine Surveyor

    I had previously posted this note on DC's Sea Sled thread here -
    Sea Sled madness. It’s in my brain. https://www.boatdesign.net/threads/sea-sled-madness-it%E2%80%99s-in-my-brain.63079/page-93#post-908361

    Here is a sketch showing a section through the forward cockpit looking forward, based on various dimensions given to me by John. Including outline dimensions for the all terrain vehicle (ATV) - which is a heavy beast, weighing around 400 kg.
    I was optimistically thinking that I could try to arrange a luffing davit arm rather than a fixed horizontal cantilevered jib (as per my earlier post above), but I don't think this is possible - there simply would not be room.
    I have shown the outlines of the ATV resting on the cockpit sole and when lifted such that the bottom of the wheels are 100 mm / 4" above the deck when it is being swung ashore or recovered.
    I think that the whole assembly is going to have to be bigger than what John initially envisaged.
    I have not done any calculations - this sketch is just to see how everything fits.
    Re the 200 mm deep 'I' beam, I am thinking it could have a little trolley running along the lower flange to which a chain block hoist is attached. The position of the hoist will have to be movable along the length of the beam as the davit is rotated, especially so for cargo that is loaded / discharged over the port side rather than over the bow.
    On the drawing I mentioned the pillar as a 6" square hollow section, but it should ideally be able to rotate happily about the massive tentative post in the photo above, so a tube would be better in retrospect.
    And I have shown the length as 2 m. but I think it could probably be brought down to perhaps 1.6 m.
    Similarly it might be possible to reduce the overall height a bit, depending on how much vertical depth is required for the hoisting device.
    It should be possible for the pillar to rotate 360 degrees, as the jib will be comfortably above the wheelhouse roof.

    To all of John's loyal followers, please feel free to offer lots of constructive criticism! And any other alternative ideas that you have for getting a 400 kg ATV on and off the boat.

    I also had a crazy idea involving a gantry crane...... the loads on this would be easier to resolve in some ways, but the whole structure would be a lot more complicated.

    Edit - I started off drawing this on an A4 sheet of paper, and quickly ran out of room, so I glued another sheet on, to make it a bit smaller than A3. No scale is mentioned, as it is now not to scale (as I took a photo of it), so I have put a few dimensions on for reference.

    Dog Cavalry fwd cockpit - section looking forward 1.jpg
     

  15. BlueBell
    Joined: May 2017
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    BlueBell WOT

    If it's going to be this big, I wonder if a gantry wouldn't win out here.
    We used both styles on the ships, each had their merits.
    Have you considered it John?
     
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