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

[Lodesman]

Originally Posted by Departing2017

Attachment 100287

Does this look right?

This is not the same situation - there are two parts at the moving block and one of those parts (the angled one) is longer than the vertical part, so the effective purchase is greater than 2:1.

[conachair]

Originally Posted by Lodesman

The third drawing is only 2:1

Don't know how to put it any easier - sorry but you're just plain wrong.

[Seaworthy Lass]

Originally Posted by Lodesman

Lass, you have a special way of misinterpreting what I write and passing that creation off as my opinion.

In the image you reference there are two parts at the moving block - so it is 2:1. I don't see how that is so hard to understand.

Lodesman, this is not just a matter of personal opinion, this is a case of simple principles, hence I am persisting at a risk of sounding 'shrill' .

It is worth it if the light bulb switches on for you . This is not just a case of being hard nosed about being right, I am anonymous here, it matter not a scrap to me if I am deemed right or wrong (and I have been wrong plenty of times and admitted so freely when this has occurred ). This is about the concept being understood correctly, not just by you, but others who are reading the tread and possibly agreeing with you.

OK, I am glad we both agree my first example below is 2:1.

But I take it you still believe the second example is 2:1 also?

I will see if I can explain it mathematically if diagrams are not relaying the message well.

SWL

[monte]

Back to the original drawing for some clarification. Stand next to the left block in the top pic and pull the line. How much line do you need to pull if the blocks are 2m apart to make them block out? ...
A= 4m, ie 2:1
Now stand next to the right block in the lower pic and repeat...
A = 6m ie 3:1

How much load is on the lines in the top pic
First two runs from the block 1/4, 1/4
Standing part 1/2

And on the bottom pic
All runs 1/3

Maybe this makes sense, happy to be corrected if not

[monte]

The pic

[CHAZ]

Originally Posted by Seaworthy Lass

Lodesman, this is not just a matter of personal opinion, this is a case of simple principles, hence I am persisting at a risk of sounding 'shrill' .

It is worth it if the light bulb switches on for you . This is not just a case of being hard nosed about being right, I am anonymous here, it matter not a scrap to me if I am deemed right or wrong (and I have been wrong plenty of times and admitted so freely when this has occurred ). This is about the concept being understood correctly, not just by you, but others who are reading the tread and possibly agreeing with you.

OK, I am glad we both agree my first example below is 2:1.

But I take it you still believe the second example is 2:1 also?

I will see if I can explain it mathematically if diagrams are not relaying the message well.

SWL

SL,
I think that you are wrong here
A single block as in your first example gives you no advantage, you need block and tackle to get a 2:1 advantage.

Fair winds and good anchoring
Martin

[Departing2017]

Originally Posted by Lodesman

This is not the same situation - there are two parts at the moving block and one of those parts (the angled one) is longer than the vertical part, so the effective purchase is greater than 2:1.

Actually, I believe the effective (theoretical) purchase in my drawing is around 1.414:1, double the force vector applied to the load. The tension that goes into a pulley applied in the direction of the load (.707 in this case) will theoretically be equal to the tension that comes out of a pulley. That's what pulleys do; they double tension.

And you are right. This is not the same situation. My drawing was merely intended to clarify a way of illustrating the concept mathematically. (My firefighter students are going to be so thrilled at the opportunity to absorb a little trigonometry). I didn't mean to muddle up the main discussion here. Sorry about that.

[conachair]

Originally Posted by CHAZ

SL,
I think that you are wrong here
A single block as in your first example gives you no advantage, you need block and tackle to get a 2:1 advantage.

Fair winds and good anchoring
Martin

It does, maybe if you turn it the other way up it will make more sense.. the same as the centre example here...

[monte]

I think she's got it right. Pull 1m on the line and the moving block comes .5 m closer......2:1. Pretty magical after a hard night on anchor watch too

[CHAZ]

I concur that I was wrong
Thank you all for the information and entertainment

Fair winds and happy anchoring

Martin

[Seaworthy Lass]

Originally Posted by CHAZ

SL,
I think that you are wrong here
A single block as in your first example gives you no advantage, you need block and tackle to get a 2:1 advantage.

Fair winds and good anchoring
Martin

Martin, we are regressing here . Even Lodesman agrees the first example in post #78 illustrates a 2:1 purchase.

Lodesman, can you explain why the first example is 2:1 please while I think of how to explain to you that the second example is 3:1? Although my magic seems to increase with a glass of wine my explaining abilities seem to deteriorate

SWL

[Lodesman]

Originally Posted by monte

Back to the original drawing for some clarification. Stand next to the left block in the top pic and pull the line. How much line do you need to pull if the blocks are 2m apart to make them block out? ...
A= 4m, ie 2:1
Now stand next to the right block in the lower pic and repeat...
A = 6m ie 3:1

You won't be able to pull it 6m out. Diagram might help.

[Lodesman]

Originally Posted by Seaworthy Lass

Martin, we are regressing here . Even Lodesman agrees the first example in post #78 illustrates a 2:1 purchase.

Lodesman, can you explain why the first example is 2:1 please while I think of how to explain to you that the second example is 3:1? Although my magic seems to increase with a glass of wine my explaining abilities seem to deteriorate

SWL

Your second example is 2:1 also. See diagram in post above

[conachair]

Originally Posted by Lodesman

You won't be able to pull it 6m out. Diagram might help.

That works for a 2:1 as drawn, now turn the ropes upside down so the hauling part exits at the load, now you have the same 4m of travel plus the 2m the load has traveled, you need to pull 6m to move the load 2m - 3:1.

[monte]

Originally Posted by Lodesman

You won't be able to pull it 6m out. Diagram might help.

Correct. Nice drawing btw! So in that example the load moved 1m and you hauled in 2m of line. 2:1