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  Pulley size actually doesn't matter
  Posted by: guideboatguy on Jun-21-13 2:29 PM (EST)
 

-- Last Updated: Jun-21-13 2:31 PM EST --

For any single pulley, the length of "take up" of rope on one side is always exactly equal to the distance of "pull" on the other side. The pulley is just a wheel, and rope passes over its surface exactly the same as the pavement passes by the surface of the wheels of your car. There's no "gearing up or down" via that interface. This should be easy to visualize since people often use eye-bolts and carabiners instead of pulleys and they work exactly the same, but with more friction (people even rig up compound pulley systems with multiple carabiners for mechanical advantage, and you get the same mechanical advantage as you would with pulleys, not counting loss due to the extra friction).

It's the number of pulleys you put into the system (and therefore the number of lines going directly between the lifting point and the load) that determines mechanical advantage, not the sizes of the pulley wheels.



In the case of this particular hoist, the portion of the rope that lifts the boat is free to slide back and forth through the system independently of the part that's outside the system (the part that the operator pulls on), and that's why the boat won't stay level. Put a hand on one end of the boat to steady things and you can keep it level, but it would be better if that weren't necessary. Using the two-rope method described later in this thread, you end up with the same setup that's used to raise and lower Venetian blinds. To get Venetian blinds to "tilt" requires that you pull the two lines unequally.


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