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Old 11-20-2018, 10:07 PM   #21
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Originally Posted by Rain Dog View Post
I think his proposal is to tie all 5 knots to a padeye of infinite strength. In this case, there is no single part line, but all lines in the system are attached independently to the load. The knot could weaken things, but in the spirit of the question, I think we can assume the knot does not weaken the line.

In this case, 2 lines, each with a 5000 lb breaking strength, could lift a 9999 load.
Yes, thatís what I meant, all four lines attached to the padeye.
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Old 11-20-2018, 10:23 PM   #22
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You are all confusing the effort required to lift a weight with the breaking strength of the line, in this case we are assuming 5000# breaking strength. Forgetting for a moment that lines are rated lower than their actual failure point, you could lift 5000#. The purpose of the ropes and pulleys (on a boat, that is a "purchase") is to lower the amount of work that is required to be expended to lift that weight, by a factor of the number of pulleys and the number of "lines," making it possible for one person to lift a much larger load than they could directly. The earlier poster was correct, the breaking strength of the rope must be less than the load or you will drop it.
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Old 11-20-2018, 10:26 PM   #23
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I didnít design this paravane system, the original owner did and I bought the boat from him. Ive always wondered what the pull or stress on the poles, pulleys etc. are when in use. Any of yíall ever built one of these from scratch?
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Old 11-20-2018, 10:33 PM   #24
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I didnít design this paravane system, the original owner did and I bought the boat from him. Ive always wondered what the pull or stress on the poles, pulleys etc. are when in use. Any of yíall ever built one of these from scratch?
You need to talk to Richard on Dauntless. He installed a set of them while his boat was docked behind my store.
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Old 11-20-2018, 10:42 PM   #25
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You need to talk to Richard on Dauntless. He installed a set of them while his boat was docked behind my store.
I remember reading that thread, very interesting. Didnít he then sail off to Europe? That was a hell of a ride, sure got everybodyís attention.
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Old 11-20-2018, 10:47 PM   #26
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Two things are true here. First the breaking strengh of the line never changes. What does change is the pull on each section of lines. In a simple scenario, with a single pulley. Diagrams make this easy but using the >. One end attached to the load, goes around the pulley (skinny end of the > the other end to a solid anchor. The pulley is also fixed to a solid anchor.

The load on each line is reduced. So if pulling 2000lbs, you reduce the load on the rope is half(in a perfect wold of course, depends on the angle and friction).

So the breaking strengh is the same, but each end sees a lower load. When the load is static you can calculate safely, but if the load is unkown, you can get in trouble quickly.
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Old 11-20-2018, 11:28 PM   #27
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Read this: https://en.wikipedia.org/wiki/Block_and_tackle
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Old 11-21-2018, 02:49 AM   #28
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I remember reading that thread, very interesting. Didnít he then sail off to Europe? That was a hell of a ride, sure got everybodyís attention.
Yes Richard crossed the Atlantic both ways, passed through the Panama Canal, andi think heís in California now.

He did have a failure of one of his rigs while single handed in the middle of the ocean. I suspect he is a real expert on paravane rigs after that little scare.
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Old 11-21-2018, 08:35 AM   #29
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...

In this case, 2 lines, each with a 5000 lb breaking strength, could lift a 9999 load.
But not if they had zero stretch!

If you had zero stretch and exactly the same length, you will have a zero/zero equation of indeterminate value.
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Old 11-21-2018, 09:09 AM   #30
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Without measuring angles and or lengths of your setup I would use 2 to 2.5 as a measure of mechanical advantage. The problem with the design is that the loads are actually higher with the booms deployed than when they are parked. Also the load will vary with the size of the bird and the boatís speed through the water. Lastly that lineís working load is 1000 pounds, so expect it to stretch 2 % or about 18Ē when it reaches that load. When Richard was trying to figure all of this stuff out I suggested he talk to a professional sailboat rigger. In the end he let him design and build him a custom system.
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Old 11-21-2018, 11:31 AM   #31
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You are all confusing the effort required to lift a weight with the breaking strength of the line, in this case we are assuming 5000# breaking strength. Forgetting for a moment that lines are rated lower than their actual failure point, you could lift 5000#. The purpose of the ropes and pulleys (on a boat, that is a "purchase") is to lower the amount of work that is required to be expended to lift that weight, by a factor of the number of pulleys and the number of "lines," making it possible for one person to lift a much larger load than they could directly. The earlier poster was correct, the breaking strength of the rope must be less than the load or you will drop it.
The purpose of a tackle is also to be able to lift something heavier than the line's load rating or breaking strength. A 5000 lb strength line on a 4:1 tackle will fail at 20,000 lbs load on the tackle, not 5000, if the blocks are rated for 20,0000. If the safe load for the 5000 lb line is 2000 lbs, then a 4:1 tackle using it will lift 8000 lbs safely (provided the blocks are up to it) - much higher than the 5000 lb breaking load of the line.
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Old 11-24-2018, 01:21 PM   #32
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Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.
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Old 11-24-2018, 01:49 PM   #33
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Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.


But if the block is below the array of lines, the story changes.
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Old 11-24-2018, 01:52 PM   #34
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A basic statics course will show all line segments see the same force when only threaded around free pulleys
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Old 11-24-2018, 05:18 PM   #35
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Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.
But the force on the line does change - If you only apply 2500 # to lift it that's all the force there is on the line at any point

Solly
Read and study xsbank's link - the weight of the item being lifted is NOT the load on the line (or force req'd to lift the wt) when a multi line / pulley system is used - That's the magic and physics of the basic design... just ask the Eqyptians who built the pyramids... mechanical advantage is a wonderful thing.

The other way to find the advantage by empirical means vs theoretical is to pull in a length of line and measure the distance the load is lifted
- if you pull 2X as much line as the load is lifted the force on the line is 1/2 of the wt of the load - 2:1 mech advantage
- If you need to pull in 4X the distance the load is lifted the mech adv is 4:1 and the force req'd is 1/4 the wt of the load.

That's just the way it is
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Old 11-25-2018, 10:56 AM   #36
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Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.
Yep. If that weren't the case, oil tankers could tie up to the wharf with 1/4" clothes line and a bunch of 1/4" pulleys.
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Old 11-25-2018, 11:03 AM   #37
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But the force on the line does change - If you only apply 2500 # to lift it that's all the force there is on the line at any point

Solly
Read and study xsbank's link - the weight of the item being lifted is NOT the load on the line (or force req'd to lift the wt) when a multi line / pulley system is used - That's the magic and physics of the basic design... just ask the Eqyptians who built the pyramids... mechanical advantage is a wonderful thing.

The other way to find the advantage by empirical means vs theoretical is to pull in a length of line and measure the distance the load is lifted
- if you pull 2X as much line as the load is lifted the force on the line is 1/2 of the wt of the load - 2:1 mech advantage
- If you need to pull in 4X the distance the load is lifted the mech adv is 4:1 and the force req'd is 1/4 the wt of the load.

That's just the way it is
And the "way it is" with any lifting tackle is that you have a single lead of line either on the hoist side or both sides. And that single lead doesn't gain magical powers just because the line is doubled, tripled or whatever elsewhere.
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Old 11-25-2018, 11:07 AM   #38
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Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.
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Yep. If that weren't the case, oil tankers could tie up to the wharf with 1/4" clothes line and a bunch of 1/4" pulleys.
You are absolutely correct pulleys don't/can't change the strength or any other property of the line.
What pulleys / blocks do is reduce the load on the line so multiple weaker lines are capable of lifting a heavier load that would break a single line of the same size / strength.
In theory the tanker example would work if enough smaller lines were employed AND the pulleys & their attachment were capable of handling the loads.
It seems like folks are reluctant to read and understand the link xsbank provided or think it's a hoax???
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Old 11-25-2018, 11:14 AM   #39
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And the "way it is" with any lifting tackle is that you have a single lead of line either on the hoist side or both sides. And that single lead doesn't gain magical powers just because the line is doubled, tripled or whatever elsewhere.
That's true with one single pulley but not with multiple pulleys / blocks.
Have you looked at the link?
Don't understand it or just don't believe it?
You don't have to take my word for it.

Why would companies ever be able to sell blocks/tackle if they don't provide a mechanical advantage?
Have you ever used a block & tackle, chain fall or come along to lift or pull something?
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Old 11-25-2018, 11:35 AM   #40
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That's true with one single pulley but not with multiple pulleys / blocks.
Have you looked at the link?
Don't understand it or just don't believe it?
You don't have to take my word for it.

Why would companies ever be able to sell blocks/tackle if they don't provide a mechanical advantage?
Have you ever used a block & tackle, chain fall or come along to lift or pull something?
The question was whether a tackle increased the tensile strength of the line, so I have a simple question for you Bacchus. If you have a lifting tackle, is there a single line that you pull on or that attaches to the thing you are lifting somewhere in the tackle? Pro tip: If there isn't, you don't have a tackle, you have a continuous line that is doubled, tripled, etc. around a bunch of pulleys.

Stick as many pulleys into a tackle as you like. You won't change the tensile strength of the line, although you will reduce the load on the line when in use. Assuming, as you seem to, that the tensile strength does change, then if a car has enough horsepower to get to 100 mph, dropping it off a cliff increases the horsepower.

Perhaps you didn't understand the OP's question, or prefer to answer one he didn't ask?
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