Block and tackle breaking strength - Page 4 - SailNet Community
Old 01-30-2011
Senior Member

Join Date: Aug 2009
Posts: 208
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8

If a rope is holding an 80 pound weight up, then the link needs to hold 80 pounds, no matter where it is in the rope.

As a fun experiment in rope tension, do this...

Put a spring scale on both ends of a rope. Fix one spring scale to a wall, or a very large, cornfed-type man (one which should not move, you see). Pull on the other scale until it reads 80 pounds. At that point, both scales will read 80 pounds. Every time. Honest.

Now, hook a third spring scale anywhere you want on the rope. Somewhere in the middle would be fine, of course. Pull on _that_ scale until it also reads 80 pounds. [edit: for clarity, you would be pulling 'away' from the rope.. not in line with it. so if somebody were watching all this from above you, they would see a triangle of sorts.] At this point, all three scales will read 80 pounds. Every time. Honest.

Tension is always evenly distributed over the span of a rope.

... or I'm wrong.

Living aboard, currently in the Chesapeake
O'Day 37, still new to us
CapTim is offline

Old 01-30-2011
Telstar 28

Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 18 Times in 14 Posts
Rep Power: 16

Now don't pick on SWAY or SAILORTIM that way....

Quote:
Originally Posted by CapTim View Post
If a rope is holding an 80 pound weight up, then the link needs to hold 80 pounds, no matter where it is in the rope.

As a fun experiment in rope tension, do this...

Put a spring scale on both ends of a rope. Fix one spring scale to a wall, or a very large, cornfed-type man (one which should not move, you see). Pull on the other scale until it reads 80 pounds. At that point, both scales will read 80 pounds. Every time. Honest.

Now, hook a third spring scale anywhere you want on the rope. Somewhere in the middle would be fine, of course. Pull on _that_ scale until it also reads 80 pounds. [edit: for clarity, you would be pulling 'away' from the rope.. not in line with it. so if somebody were watching all this from above you, they would see a triangle of sorts.] At this point, all three scales will read 80 pounds. Every time. Honest.

Tension is always evenly distributed over the span of a rope.

Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

sailingdog is offline
Senior Member

Join Date: Nov 2007
Location: SF Bay area
Posts: 1,940
Thanks: 9
Thanked 7 Times in 6 Posts
Rep Power: 10

The last couple of posts are reinforcing my thinking about tension.

Regarding breaking strength of soft shackles, while you are using a knot, the knot is made up of two lines loaded in one direction, and then hooked onto a loop, essentially made up of two lines loaded in the other direction. So on the side of the shackle that has the knot/loop hookup, you've got double the strength. So I would say that that side of the shackle has a breaking strength approximately equal to the strength of the underlying rope (doubled because you've got two lines equally sharing the load, half because of the knot).

The other side of the shackle is stronger, because no knot, but who cares, weakest link and all. So the breaking strength on either side of the shackle is the strength of the underlying rope. But now the shackle, when closed, is again folded in half, like a line going over a sheave. The load, hanging from the bottom of the shackle, is supported equally by both sides of the shackle, so we again get to double.

So it seems, more conservatively (taking the 50% hit from the knot into consideration), the breaking strength of a soft shackle is twice the breaking strength of the underlying rope.

s/v Laelia - 1978 Pearson 365 ketch

Old 01-31-2011
Old as Dirt!

Join Date: Nov 2008
Location: Tampa Bay Area
Posts: 3,212
Thanks: 21
Thanked 155 Times in 147 Posts
Rep Power: 9

Quote:
Originally Posted by AdamLein View Post
The last couple of posts are reinforcing my thinking about tension.

Regarding breaking strength of soft shackles, while you are using a knot, the knot is made up of two lines loaded in one direction, and then hooked onto a loop, essentially made up of two lines loaded in the other direction. So on the side of the shackle that has the knot/loop hookup, you've got double the strength. So I would say that that side of the shackle has a breaking strength approximately equal to the strength of the underlying rope (doubled because you've got two lines equally sharing the load, half because of the knot).

The other side of the shackle is stronger, because no knot, but who cares, weakest link and all. So the breaking strength on either side of the shackle is the strength of the underlying rope. But now the shackle, when closed, is again folded in half, like a line going over a sheave. The load, hanging from the bottom of the shackle, is supported equally by both sides of the shackle, so we again get to double.

So it seems, more conservatively (taking the 50% hit from the knot into consideration), the breaking strength of a soft shackle is twice the breaking strength of the underlying rope.

WRONG Boyo. Any bending in a line reduces the strength of the line due to the unequal loading of the fibers with the fibers on the outside curvature of the bend carrying most of the loading and those on the inside virtually none. That is why one can tear a towel apart by starting the tear at an edge but not by simply pulling on opposit ends of the towel's middle. Knots/sharp bends reduce a lines load capacity by anywhere from 20% to 60%. Based on your comments thus far you are in desperate need of some lessons in basic physics. I suggest you leave engineering matters to others.

"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."

Last edited by svHyLyte; 01-31-2011 at 06:54 AM.
svHyLyte is offline
Old 01-31-2011
Senior Member

Join Date: Dec 2008
Posts: 716
Thanks: 1
Thanked 7 Times in 7 Posts
Rep Power: 9

Speaking of tension on a block and tackle. This last summer I had occasion to use the 4 to 1 block and tackle that normally is on the boat. (used for the running back stays which I've never had occasion to use) Anyway I take this back to the house to use on a project in which mechanical advantage is needed. When I went out to the dock that prior morning I noticed that our stairs were missing that went down to the small beach. A couple of lots down on a neighbor's jetty were the stairs all still intack. The size of the stairway was about 20 feet long and about 5 feet wide and being water logged the only way to move it off the jetty was with a breaker bar. My whole estimate was that the thing maybe weighed about 500 lbs, but that was totally off considering that with that 4 to one block and tackle myself and a neighbor was barely able to raise the end of it up the 10 feet to reattach it to the dock with bolts rather than nails which the contractor had used. My neighbor and I are both pretty strong and with one of us looping the end of the rope around our hip and the other heaving with all our might we finally got the end up to the dock with the other end remaining in the sand. I had a half inch line in the block and tackle and a 5/8 inch line on the stairs as a bridle and that 5/8 bridle was stretched a lot more than I thought possible. I'm not sure how much more margin I had before breaking, but you'll see noticable stretch way before breakage espically with 3 strand. I estimate that we were putting about 400 lbs of pull on the block and tackle and that would result in lifting maybe 1600 lbs. With one end still on the sand the whole thing must have weighed around 3000 lbs.
lancelot9898 is offline
Old 01-31-2011
Telstar 28

Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 18 Times in 14 Posts
Rep Power: 16

Unless that 5/8" line was in really rotten shape or a really weak fiber, like polypropylene, I seriously doubt you were anywhere near the breaking strength of it since 5/8" nylon three-strand anchor line has a breaking strength somewhere in the neighborhood of 10000 lbs.

Half-inch polyester double braid, which is likely what was in the block, has a breaking strength of 8000 lbs or so.

Part of the issue you may have had is friction. If the blocks were twisted, then there may have been significant friction in the tackle, making it less effective than it should have been. Also, if the line was the maximum diameter for the blocks in question, you would have seen more friction than if the line were thinner.
Quote:
Originally Posted by lancelot9898 View Post
Speaking of tension on a block and tackle. This last summer I had occasion to use the 4 to 1 block and tackle that normally is on the boat. (used for the running back stays which I've never had occasion to use) Anyway I take this back to the house to use on a project in which mechanical advantage is needed. When I went out to the dock that prior morning I noticed that our stairs were missing that went down to the small beach. A couple of lots down on a neighbor's jetty were the stairs all still intack. The size of the stairway was about 20 feet long and about 5 feet wide and being water logged the only way to move it off the jetty was with a breaker bar. My whole estimate was that the thing maybe weighed about 500 lbs, but that was totally off considering that with that 4 to one block and tackle myself and a neighbor was barely able to raise the end of it up the 10 feet to reattach it to the dock with bolts rather than nails which the contractor had used. My neighbor and I are both pretty strong and with one of us looping the end of the rope around our hip and the other heaving with all our might we finally got the end up to the dock with the other end remaining in the sand. I had a half inch line in the block and tackle and a 5/8 inch line on the stairs as a bridle and that 5/8 bridle was stretched a lot more than I thought possible. I'm not sure how much more margin I had before breaking, but you'll see noticable stretch way before breakage espically with 3 strand. I estimate that we were putting about 400 lbs of pull on the block and tackle and that would result in lifting maybe 1600 lbs. With one end still on the sand the whole thing must have weighed around 3000 lbs.

Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

sailingdog is offline
Old 01-31-2011
Senior Member

Join Date: Dec 2008
Posts: 716
Thanks: 1
Thanked 7 Times in 7 Posts
Rep Power: 9

Quote:
Originally Posted by sailingdog View Post
Unless that 5/8" line was in really rotten shape or a really weak fiber, like polypropylene, I seriously doubt you were anywhere near the breaking strength of it since 5/8" nylon three-strand anchor line has a breaking strength somewhere in the neighborhood of 10000 lbs.

Half-inch polyester double braid, which is likely what was in the block, has a breaking strength of 8000 lbs or so.

Part of the issue you may have had is friction. If the blocks were twisted, then there may have been significant friction in the tackle, making it less effective than it should have been. Also, if the line was the maximum diameter for the blocks in question, you would have seen more friction than if the line were thinner.
That 5/8 line was an old anchor line and I tied it off with a bowline. It would be interesting to see how much the strength degrades with age and knots. And to also compare the stretch prior to failure. I use to subscribe to Practical Sailor, but most of their tests seemed useless and many flawed. Maybe they have done some testing of lines and breaking strengths that I missed since to me that would be some useful knowledge rather than just some emperical numbers from the rope manufacturers using brand new lines. My son during his college days was an EMT in mountain rescue and they routinely got rid of lines if not properly handled.

The block had nylon double braid and not polyester.
lancelot9898 is offline
Old 01-31-2011
Telstar 28

Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 18 Times in 14 Posts
Rep Power: 16

Quote:
Originally Posted by lancelot9898 View Post
That 5/8 line was an old anchor line and I tied it off with a bowline. It would be interesting to see how much the strength degrades with age and knots. And to also compare the stretch prior to failure. I use to subscribe to Practical Sailor, but most of their tests seemed useless and many flawed. Maybe they have done some testing of lines and breaking strengths that I missed since to me that would be some useful knowledge rather than just some emperical numbers from the rope manufacturers using brand new lines. My son during his college days was an EMT in mountain rescue and they routinely got rid of lines if not properly handled.

The block had nylon double braid and not polyester.
Even if the line was 10 years old with the corresponding UV degradation, I doubt the breaking strength was less than 5000 lbs...

Your son was wise, since improperly handled lines are far more likely to break unexpectedly.

Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

sailingdog is offline
Senior Member

Join Date: Nov 2007
Location: SF Bay area
Posts: 1,940
Thanks: 9
Thanked 7 Times in 6 Posts
Rep Power: 10

Quote:
Originally Posted by svHyLyte View Post
Knots/sharp bends reduce a lines load capacity by anywhere from 20% to 60%.
How sharp of a bend are we talking about? I'm happy to consider that the knot reduces the strength of the line by 60% instead of 50%.

By about how much, would you say, does the amount of bend in an eye splice without a thimble reduce the strength of the line? In the soft shackle, other than the knot, that's the only bend in the line.

Quote:
Based on your comments thus far you are in desperate need of some lessons in basic physics. I suggest you leave engineering matters to others.
Thanks for your help with the basic physics of nonrigid mechanics. svHyLyte's law: you can rip towels from the edge. I'm not sure that that's basic enough for me, maybe you could condescend a little further?

s/v Laelia - 1978 Pearson 365 ketch
Senior Member

Join Date: Nov 2007
Location: SF Bay area
Posts: 1,940
Thanks: 9
Thanked 7 Times in 6 Posts
Rep Power: 10

Quote:
Originally Posted by lancelot9898 View Post
with that 4 to one block and tackle myself and a neighbor was barely able to raise the end of it up the 10 feet
What angle were you pulling at? Were you standing on the dock and pulling sideways, or standing on the stand and pulling down, or at an angle?

Regarding nylon's stretch, I once rigged a nylon line to use as a jib downhaul. It was single braid, I think. I won't do that again; with the line run aft to the cockpit and the jib at full hoist, I could pull the line probably two feet without the jib budging. It was surreal.

s/v Laelia - 1978 Pearson 365 ketch

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

## Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
OR

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

 Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)