Extreme Block and Tackle: Building Manual Winch with 4:1 to 500:1 Ratios

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Discussion Overview

The discussion revolves around the construction of a manual winch using a block and tackle system to lift a 1500lb boat off its trailer. Participants explore the necessary specifications for the block and tackle, including the required rope strength and the impact of friction on the system's efficiency.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the required rope strength for a block and tackle system, suggesting that with a 4:1 ratio, only a rope with a 375lb strength is needed, and further calculations for higher ratios imply even lower strength requirements.
  • Another participant warns that friction will significantly affect the lifting capability of the system, suggesting the use of a comealong instead.
  • A later reply emphasizes the importance of the cradle's design, particularly the positioning of the pivot point relative to the boat's center of gravity, recommending the use of two comealongs or straps for better balance.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of the proposed block and tackle system, with some supporting the idea while others recommend alternative solutions like a comealong. The discussion remains unresolved regarding the effectiveness of the block and tackle system versus other lifting methods.

Contextual Notes

Participants have not fully explored the implications of friction in detail, nor have they reached a consensus on the optimal design for lifting the boat.

DaveC426913
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[ backstory ]
Yesterday I finally, and sadly, put my Matilda http://davesbrain.livejournal.com/315479.html" 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 traveling 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?
 
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Bob S said:
Get a comealong:
http://www.cvfsupplycompany.com/hanpowpulcom.html
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.
 
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
 
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