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.

    20210527_230917.jpg No, no hydrostatics.

    But here is the sketch. To scale, within reason: 1 square = 30cm/1'. The gantry is hinged on the side decks over bulkhead 1. I have not shown the blocks and tackle controlling the position of the gantry. In this sketch I drew the gantry 8' high, because standard dimensional lumber, but 10' high is hardly any heavier, and gives more clearance and reach.
     
    Last edited: May 28, 2021
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  2. dreamingbarrierreef
    Joined: Oct 2018
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    Location: Colorado

    dreamingbarrierreef dreamingbarreef

    Something I thought about a while ago. Just want to share. The triangular form shall offer more reach outboard. Hinged on the outside edge, and hydraulics lift the inner edge. There is also the cradle for loads.
    Screenshot_20210528_141121.png Screenshot_20210528_183615.png
     
    Last edited: May 28, 2021
  3. dreamingbarrierreef
    Joined: Oct 2018
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    Location: Colorado

    dreamingbarrierreef dreamingbarreef

    Sorry, not hydraulic. It's winch. Can lift the edge with a winch; or use pulleys & rope. Been a while since that idea.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Hmmm.
    Ok..can you send me the Lines and your frame spacing. Because i think you're gonna need a set of hydrostatics. Because what you are showing, needs to be looked at properly, as it could end in tears if not done correctly.
    I'll create a set of hydrostatics for you, from the Lines you send.
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I'm still nervous about weight forward. Almost any planing hull has a limit where they do nothing but plow when loaded heavily forward.

    I have been in more than one boat where I've said, we've got too get some weight aft to plane. Maybe I am just doubling down on ad hoc's remark here, but a boat tends to become like an engine on a bobber when so.

    The perfect example is me on 14' Lund skiffs with a bow mounted trolling motor. I weigh about 215 and sitting on the bow to run the motor and cast results in the complete loss of helm when alone. I call it corking. There is probably a better name for it. But what happens is you try to steer the boat and every steering action results in oversteering. It becomes impossible to take a straight course and becomes a bit comical.

    If you get too deep forward, you will also have issues.

    But you may find after doing all the work to place a 300kg load forward that the boat is so adversely affected, you have a truer limit of say 200kg on the bow, etc. That weight may include the lift mechanism itself.

    Perhaps this is what @Ad Hoc is going to look for..
     
  6. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    That may be so, Fallguy. But extensive conversations with Marcus Lee of Leecraft in Sitka suggest that, at least for his sea sleds, it was not possible in practise to load the foreword deck enough to make getting on the plane difficult.
    That won't help when idling up onto a beach though. At extremely slow speeds, with the quad in the cockpit draft should be around 11"/280mm. Putting a quad in 11" of water is fine, but a lot of cargo can't take that.

    Ad Hoc, I'll get some updated offsets. The ones I provided at the beginning changed slightly. The planking was breaking when bending around B1, so I took the makita planer to it.
     
  7. Tiny Turnip
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    Tiny Turnip Senior Member

    Is it worth considering a larger cabin door, so on those occasions when you have large heavy cargo, you can load it further back? fewer passengers that trip, obviously.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Draft
    So, 11" fwd but the water depth is less ahead of the bow.
    A truck could be backed to the boat when unloading keeping items out of the water.

    Gantry Frame
    Great sketch.
    You and The Misses look terrific!
    I have never seen a square cornered gantry.
    I would suggest a flat top A-frame, stronger, less material, sexier.
     
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  9. Tiny Turnip
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    Tiny Turnip Senior Member

    the two arms could unbolt at the top and the hinge points and potentially store flat on the cabin roof if you wanted.
     
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  10. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    That is the vision, so far.

    Re Tom's suggestion. I could build it a little narrower at the top, but not much. Probably 10' at the bottom, 8' at the top. Narrower and a bulky load can't be hoisted high enough to clear the bow when swinging forward.
     
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  11. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    I mught make it a little wider, but a big forward facing door into the cockpit seems vulnerable.

    This assembly should be easy to relocate to the transom. Set bulky loads on the back deck, and make the back door wider.
     
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    Shall await... :cool:
     
  13. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Indeed.
    It is rarely a simple case of 'lifting'.

    On such a small boat lifting any appreciable weight (as a high percentage of the vessel's weight) will influence the structure - what is lifting it and where is is cited, and the stability, in terms of how much trim/list will occur and the temp virtual rise of the KG during the lift which could end up a capsizing moment. But until one investigates such, it is all just... well....guesses and assumptions.

    Far better to deal with facts...
     
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  14. fallguy
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    fallguy Senior Member

    Don't forget, at some point your scuppers may be unter wasser?
     

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

    The bottom of the freeing ports are 18" above the keels. And even when they are flooding, they are under the cabin sole.
     
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