Help designing simple jib crane

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

  1. bajansailor
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    bajansailor Marine Surveyor

    I can see advantages with a gantry - however I don't think that slewing (like a pillar crane in my sketch above) would be possible, hence the gantry would only be able to operate over the bow, or over the side, but not both (?)
    And I think that ideally you want to have the capability of loading / discharging both over the bow and over the side, for maximum flexibility.
     
  2. fallguy
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    fallguy Senior Member

    I have been on some p-northwest unimproved shorelines.

    I find the entire exercise too limited by the bulwark, lack of docks, weight, existing boat structures.

    So much easier to make a capable small barge, winch on front, power or manual and cable it up to a tree and voila', instant dock. Drive on, drive off.

    I get the challenge aspect. Keep in mind, most of these are made for lifting a small boat and the mounting positions are much, much higher. Now and then I see a specialty version, but typically those are on platform barges with no bulwark.

    The decision to say no here seems best to me. I comment as a friend more than anything. Too many reasons against.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    I think John needs to stop... go back to basics....
    Then assess, what he wants.

    Too many 'solution's looking for a problem.

    So...what is the objective and what constraints, if any, are there ...?
    Can't do much without that ..
     
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  4. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    I had not, although mostly because I don't know what that is.
     
  5. bajansailor
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    bajansailor Marine Surveyor

    John, what is a typical sort of distance that you would have to travel to reach your construction sites- and what might be the occasional maximum range?
    I can see the advantage of having the cargo on board Serenity travelling at say 21 knots, compared to if you are towing it on a barge at perhaps 7 knots.
    For a work location 42 miles away, that would be 2 hrs vs 6 hrs.
    However Fallguy makes very valid points above re using a barge instead of trying to load the heavy cargo onboard Serenity.
     
  6. fallguy
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    fallguy Senior Member

    You have to determine what sort of cargo is going on the jib crane. The crane is not only subject to a weight limit, but also a height limit and a pretty serious height limit at that. If you want to haul a quad, aren't those about 48" high with handlebars? Then is there any clearance requirement for the lift mechanism? A cheap chain lift needs at least a foot. Now, aren't we say 60" over the bulwark? And woth a 3' bulwark, a 9 foot post?

    Conversely, sort of, a pallet of sheetrock mud might be say 24" high, but we still need 3' boat clearance and a foot chain mechanism and a 7' post.

    In Bajan's remarks on speed; don't forget a quad onto the boat and off of is much longer than drive on/off a barge. I bet at least 30 minutes of securing the lift and cargo each way. And will you really run at 21 full of cargo in your beautiful boat? I bet you won't. You'll start to pound in some chop and drop back to barge speeds.

    Anyhow, I don't want to beat my opinion horse. I have just seen these cranes and don't think it fits well here as anything massive. I'd tame it down to a bucket lifter. Something where the mechanism requires no clearance, and a 2' height limit. It'll still be handy for certain things, but not ridiculous. You could build one on a barge and not face the bulwark issue.

    Also, adding.. I have landed a LOT of boats on shore. Everytime the boats are draft affected (say camp gear) we end up moving cargo or persons aft so we can get close enough to shore. On a rocky shoreline or a 3' wide dock, how in the H do you expect to offload cargo if the shoreline prevents it or the dock is too narrow for a quad? You would literally need an absolute perfect landing site each time. Last fall, I put a 90 pound ice block in the boat and even that left me short of shore coming in with my 16' boat. What is an 800 pound forward load going to do? Your boat may be load capable, but draft will be affected.
     
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  7. fallguy
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    fallguy Senior Member

    Can the luffing get the lift higher than the post?
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Just a thought - a small knuckleboom crane would be a lot more expensive than a home made davit with a chainblock hoist, but it would be a lot more versatile.

    Here is a link to the smallest crane in the Palfinger range - in the technical data they mention that it can lift 440 kg at 3.1 m. outreach.

    PC 1500 Compact https://www.palfingerusa.com/products/knuckleboom-crane-compact-crane-series/pc-1500-compact

    However Serenity would most probably need to have a generator running to provide the power for the crane, which makes things even more complicated.
     
    Last edited: May 25, 2021
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  9. Tiny Turnip
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    Tiny Turnip Senior Member

    Late to the party I know. And I think Ad Hoc is quite right about thinking through actually what's needed. But a couple of thoughts anyway.

    1) Just welding up an I section on a tube, with a gusset to make a cantilever is structurally ardous and means big heavy sections. Using (cable, possibly rod) stays, as Bajan Sailor has suggested and drawn earlier, can relieve the bending and cantilever stresses, and mean much lighter sections, as they are then mostly only dealing with compression in the mast and jib, and tension in the stays (some bending in the lower part of the mast between the top of the deck tube and where the jib joins).
    2) The stays could be used to raise the jib, as Bajan Sailor has also suggested.
    2) Having to lift a tube up to drop over a post is extra work, when the tube could just drop into a tube in the deck. This can be braced with additional bulkhead and/or shelf if necessary.
    3) a round tube allows the crane to revolve, which has got to be a huge advantage.
    4) The stayed solution can fold or break down for easy storage.

    Oh, and I've not followed the main Sea sled thread closely, so I may well have missed something, but I don't understand the mentions of a height restriction, apart from not wanting to get top heavy.

    A sketch to summarise. Regards all.

    sea sled crane.png
     
  10. bajansailor
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    bajansailor Marine Surveyor

    Nice sketch TT!
    Your arrangement for the davit would be much more efficient structurally (and lighter / simpler as well) than a cantilevered jib as shown in my previous sketch.
     
  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    I agree, Martin. Nice sketch TT. I may have to concede Fallguys point, but not yet. There's just so much practical utility in having a crane. Although it is starting to grow, as the farmer's daughter said. If it's too big it's just not practical.
     
  12. DogCavalry
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    DogCavalry I aim to misbehave.

    She said that too btw.

    Maybe a gin pole seated on the bottom step, with stays back to the port and starboard corners of the ram, and a third to the side deck at the wheelhouse, opposite whatever side the cargo is going.
     
  13. fallguy
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    fallguy Senior Member

    The height restriction is only air draft and strength, and aesthetics and weight. The real issue is making it high enough to swing the load and it gettin big, that is something for John. But if you load a quad into the boat; you do need to consider it coming off in the San Juans. It won't be easy like a barge kept on a trailer.
     
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  14. DogCavalry
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    DogCavalry I aim to misbehave.

    Might be no choice about the barge.
     

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

    upload_2021-5-25_16-26-27.png upload_2021-5-25_16-26-27.png
    Maybe an A frame derrick. Frame seated at the forward corners of the cockpit. Mast lands on the steps. That assembly is already completely tied into the boat structure.

    Certainly some HiTest brainstorming here.
     
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