Line Strength

[Gulf Comanche]

I just replaced the old lines that control the paravanes. New line is 3/8” double braided polyester, roughly 75’ in length per pole. According to one strength chart I checked, the tensile strength of this line is 5,000 lbs. There are 5 pulleys that control one pole, thus 4 loops from the crane (?) to the near top and mid point of the pole. Does this translate to 20,000 lbs tensile strength?
If you look at the top right of the attached photo you can see the pulleys.

 

[Gulf Comanche]

Photos of stabilizer.

 

[MRRiley]

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.

 

[Gulf Comanche]

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.

I agree with that. Four lines working together can lift a heavier weight than one line, yes? So no tensile strength for one line but four together increases tensile strength for the bundle?

 

[micheln]

Not quite. Strengh remains the same, but the force in the line is reduced by the pulleys.

 

[Comodave]

The strength of the line does not change, just with the mechanical advantage you don’t have to lift as hard to raise a given weight.

 

[Rain Dog]

That looks to me like a 3-1 system, and the bend radius will reduce the load a bit. I would guess the breaking strength at 10-12k lbs. However, I would guess the weakest link in that system is the pulleys, not the rope. I bet if you tested it to failure one of the pulleys would fail.

 

[Gulf Comanche]

Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?

 

[Gulf Comanche]

That looks to me like a 3-1 system, and the bend radius will reduce the load a bit. I would guess the breaking strength at 10-12k lbs. However, I would guess the weakest link in that system is the pulleys, not the rope. I bet if you tested it to failure one of the pulleys would fail.

Yeah, I would agree with that point. The pulleys are top notch equipment but also 25 years old.

 

[Rain Dog]

Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?

They will lift the block. Even 2 lines would probably lift 7k.

 

[Portage_Bay]

Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?

You still have a single part (line) at the final attachment to the load. Mechanical advantage of the block and tackle does not matter here. The final part has a tensile strength of 5,000 lbs. The load is 7,000 lbs.

 

[Rain Dog]

You still have a single part (line) at the final attachment to the load. Mechanical advantage of the block and tackle does not matter here. The final part has a tensile strength of 5,000 lbs. The load is 7,000 lbs.

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.

 

[Portage_Bay]

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.

Post #2 clearly slows blocks and the lines forming a block and tackle arrangement.

 

[JDCAVE]

The bigger issues are the forces on the fore and mast stays that connect to the tops of the poles and also those on the chain/cable to the paravanes. The forces on the rigging that raises and lowers the poles is minor in comparison.

Jim

 

[psneeld]

Go to an engineering web site.

I think block and tackle do allow a line to lift more than it's normal working load, but what percentage...???? I would have to look it up.

 

[Rain Dog]

Post #2 clearly slows blocks and the lines forming a block and tackle arrangement.

I think you need to be looking at Post #8, not post #2 if you want to answer OPs question.

 

[HopCar]

If I understand the setup correctly, in a perfect frictionless system, you could pick up 20,000 pounds without exceeding 5000 pounds load on the rope.

In the real world you want to design your system using “Safe Working Loads” not Tensile Strength or Breaking Load. Don’t forget the pulleys and attachments need to be strong enough as well.

 

[Rain Dog]

If I understand the setup correctly, in a perfect frictionless system, you could pick up 20,000 pounds without exceeding 5000 pounds load on the rope.

I still think the system is 3-1, so in a perfect system you could pick up 15k.

 

[DDW]

There are four lines going to the moving part (the boom). All four get shorter when you pull on the tail. Therefore it is a 4 part system, if 5000 lbs pull is put on the tail the boom will pull 20,000 in an ideal world. In this world, figure 20% loss for friction, plus those blocks will let go at probably something less than 5000 lbs and certainly less than 20,000.

 

[kchace]

Also keep in mind that rope has a working maximum that if exceeded will damage the rope. I don't know the numbers for polyester but for nylon it's about 20% of breaking strength.

Ken

 

[Gulf Comanche]

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, that’s what I meant, all four lines attached to the padeye.

 

[Xsbank]

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.

 

[Gulf Comanche]

I didn’t 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 y’all ever built one of these from scratch?

 

[HopCar]

I didn’t 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 y’all 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.

 

[Gulf Comanche]

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. Didn’t he then sail off to Europe? That was a hell of a ride, sure got everybody’s attention.

 

[micheln]

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.

 

[Xsbank]

Read this: https://en.wikipedia.org/wiki/Block_and_tackle

 

[HopCar]

I remember reading that thread, very interesting. Didn’t he then sail off to Europe? That was a hell of a ride, sure got everybody’s attention.

Yes Richard crossed the Atlantic both ways, passed through the Panama Canal, andi think he’s 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.

 

[diver dave]

...

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.

 

[cafesport]

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 boat’s speed through the water. Lastly that line’s working load is 1000 pounds, so expect it to stretch 2 % or about 18” 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.