Reeving a Tackle to Advantage -- What am I Missing?

[Lodesman]

Originally Posted by StuM

If the line is pulled at 90 degrees, only half of the force from the last fall on the last pulley is moving the object forward. The other half is trying to pull it sideways.

Edit: to clarify

The pulley is a lever, as someone recently pointed out. A sideways pull acts on that lever to change the direction of the force to that which brings the blocks together.

[Lodesman]

OK I concede. Conachair's badly drawn image has me convinced. Enjoy your scotch.

[Blue Reef Cafe]

Provided the free end of the rope is pulled parallel to the other lines there is no difference between the two scenarios - they are both a 3:1 purchase. If there were, then the force at either end would be different which is in direct contradiction to Newton's third law - every action has an equal and opposite reaction.

[Ian R]

I had to try this for real without the theory with spring scales and a weight. The results are indeed 2 to 1 and 3 to 1 ,but I think many of the explanations are wrong.
In both cases pulling the rope 2 meters will move the pulleys 1 meter closer together.
Thanks Dockhead .

[2Hulls]

Originally Posted by Seaworthy Lass

Dave, if a tackle is "rove to disadvantage", the angle the tail is pulled makes no difference. However, if it is "rove to advantage" it certainly does. As do the angles of the rest of the lines that act directly on the moveable object.

I really hate to belabor this, but no, you're wrong. The angle the "tail" makes exiting the final block has no bearing on the MA for moving the load directly to the fixed block, no matter which block the rope exits.

If this is not intuitive to you, visit any authoritative source on block and tackles and notice the absence of any mention of the exit angle. Please let me know if you find any scholarly support as I stand ready to learn.

That said, if you pull a rope to the side at 90* angle at the mid point of the rope suspending a weight, you have a tremendous MA to lift the weight small distances even without any tackle. This is due to trigonometry at small angles.

Dave

[Canuck Sailor]

In the first picture the loop around the block is just a change of direction, no advantage other loss of friction. Set up 2:1
In the second picture the boat (movable) block is being pulled up and you end up with 3:1 advantage.
I have six strands on my dinghy davits and get a 5:1 advantage as the last turn of the fixed block is just a change in direction. It is easier to handle that way.

[conachair]

Originally Posted by Lodesman

OK I concede. Conachair's badly drawn image has me convinced. Enjoy your scotch.

Hurrah!

[conachair]

Originally Posted by Blue Reef Cafe

Provided the free end of the rope is pulled parallel to the other lines there is no difference between the two scenarios - they are both a 3:1 purchase. If there were, then the force at either end would be different which is in direct contradiction to Newton's third law - every action has an equal and opposite reaction.

Off we go again....

Wrong ...

Newton rests peacefully, but you've actually nailed it. Rope exiting the fixed end, force acts on the strong thing the fixed pulley is attached to.2:1 Rope exiting the moving end, the force acts on the piece you want to move and helps you. 3:1.

[conachair]

Originally Posted by 2Hulls

I really hate to belabor this, but no, you're wrong. The angle the "tail" makes exiting the final block has no bearing on the MA for moving the load directly to the fixed block, no matter which block the rope exits.

If this is not intuitive to you, visit any authoritative source on block and tackles and notice the absence of any mention of the exit angle. Please let me know if you find any scholarly support as I stand ready to learn.

Count the forces....2.86 x the tension in the rope acts vertically on the bucket to lift it, 0.5 x the tension pulls the bucket sideways..
Prove this wrong if you can..... (you can't )

[Chuck Hawley]

Originally Posted by charliehows

I'll be damned! Re-rigged the blocks on my mainsheet a while ago and have had difficulty sheeting in to gybe - never occurred to me that I'd reeved my sheet to disadvantage (wouldnt have even known what that meant). Still learning, maybe I'll know it all just before I die...

With any luck, we're all still learning!

Chuck

[Paul Elliott]

Another way to look at this is to measure the distance you have to pull the rope vs the distance the load moves. Here are the 2:1 and 3:1 cases:

[Seaworthy Lass]

Originally Posted by Lodesman

OK I concede. Conachair's badly drawn image has me convinced. Enjoy your scotch.

Originally Posted by conachair

Hurrah!

Lodesman, I think my cheer could have been heard all the way to Athens .
Very glad the penny finally dropped. So many members have put in effort to help you understand. Wish the scotch could have been passed around - great work guys .

[Seaworthy Lass]

Originally Posted by 2Hulls

I really hate to belabor this, but no, you're wrong. The angle the "tail" makes exiting the final block has no bearing on the MA for moving the load directly to the fixed block, no matter which block the rope exits.

If this is not intuitive to you, visit any authoritative source on block and tackles and notice the absence of any mention of the exit angle. Please let me know if you find any scholarly support as I stand ready to learn.

That said, if you pull a rope to the side at 90* angle at the mid point of the rope suspending a weight, you have a tremendous MA to lift the weight small distances even without any tackle. This is due to trigonometry at small angles.

Dave

Hi Dave
No, it certainly isn't intuitive to me, but I am thirsty for knowledge, so I glad of any input that corrects any false impressions I have.

I haven't found any "authoritative sources" on the internet (anyone have anything to recommend that I can read?). I find these concepts interesting.

If I give you my reasoning regarding the angle of exist of the tail from the moveable object's end influencing the force required to move the object, could you please let me know where it is flawed?

Take these two situations:
1. The tail exists parallel to the direction of movement of the object at the object's end (rove to advantage) along with the other lines.
If you pull the tail x units, all the lines applying acting directly on the object effectively shorten by x divided by the number of these lines (this includes the tail - it lengthens by x less the movement of the object) and the object moves by this amount.

2. The tail exits perpendicularly to the direction of movement of the object at the object's end (rove to advantage).
The tail is is no longer exerting any force in the direction the object is moving. If you pull the tail x units, the object now moves x divided by the number of lines less than the tail ie it moves a greater amount.

So, you have pulled the tail the same amount in both cases, but the object has moved different amounts. No physicist here, but surely if the object moves different amounts in the two cases, the force applied has been different?
If not, could you please explain why not?

SWL

PS If the tackle is rigged so that it emerges at the fixed end (rove to disadvantage) then it doesn't matter which direction you pull the tail - the tail is not acting directly on the object. These two situations are very different.

[jkindredpdx]

Dang... I had my climbing rope attached incorrectly resulting in more effort lifting this portly old body up the mast

[conachair]

Handy site.. https://awwapp.com/
Draw up your latest force vector masterpiece. Click on menu|post then click on full size, then copy the web address and paste it into the insert image dialog box.