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Extreme block & tackle

  1. Oct 19, 2009 #1

    DaveC426913

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    Gold Member

    [ backstory ]
    Yesterday I finally, and sadly, put my Matilda http://davesbrain.livejournal.com/315479.html" [Broken] to bed for the winter.

    I want to lift her off her trailer so I can work on her hull. She's 1500lbs. The barn she's sleeping in has a massive travelling lift system (18" steel I-beam on a track) but no winch.

    I can't afford to rent a winch for the whole time I have it hanging. (Yes, I know I can build a cradle to sit her on, but...) So I've been thinking about building a manual winch using block & tackle.
    [ /backstory ]

    So I started wondering how big a block & tackle system I'd need. A rope with 1500lb test?

    Question 1 : Correct me if I'm wrong: for a block & tackle with, say 4:1, my line's strength need only be 1500/4=375lbs, right?

    What if my block & tackle were 100:1? I'd only need a line that could support 15lbs.
    I know that 100 pulleys may sound like a lot but each wheel would only have to support 15lbs. so they could be compact.

    What if I had 500:1? 3lb test line. I could lift my boat off its trailer with sewing thread tied to my pinkie!

    (Of course, to lift it a mere foot, I'd have to run a tenth of a mile away!)

    Question 2 : Is there a flaw in my logic?

    I'm thinking the friction might do me in, but I'm also thinking that the friction would be distributed among the 500 strands so it shouldn't be a problem. Or is the friction cumulative?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 19, 2009 #2
  4. Oct 19, 2009 #3

    DaveC426913

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    Gold Member

    Really. :bugeye:

    That little <$100 device will allow me to hand winch my boat into the air. Wow.

    Thank you.

    Then my next question is going to be how to design the cradle so that it supports and balances the boat.
     
  5. Oct 19, 2009 #4
    Unless you can get a cradle that has the pivot point above the boat's center of gravity, I suggest two comealongs, or two straps lifting the boat tied to a crossbeam.
    Bob S
     
    Last edited: Oct 19, 2009
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