Physics question - which line method is stronger?

[Wdeertz]

All things being equal when tying to a piling which method provides the strongest lateral breaking strength:

Method A - line around piling and back to boat cleat (variation double wrap around piling), or

Method B - line through eye and back to boat cleat.

Attached is a crude drawing showing both methods.

If I remember correctly from my high school physics class Method A spreads the load along the back radius of the circle while Method B has the force concentrated at the eye. Is my thinking correct?

 

[Benthic2]

I suspect the drawing on the right is weaker because there is only 1 length of rope between the boat and the piling and you have a potential stress/heat point where the rope passes through the eye.....However, you should be so far below the operating limits of your line that it shouldn't matter.

 

[O C Diver]

There are other factors in play.

The loop with the line passing through would probably put the most strain on the line where it passes through the loop, especially if it makes a sharp turn at that point.

If the loop on the left returns to the boat, then the strain is shared by 2 lines, however the shock mitigation is halfed

Ted

 

[Wdeertz]

Ok thanks for the responses. This was more of a theoretical question than anything as my wife and I were debating which method was stronger.

I think both your replies confirm my belief that method A is the strongest. Method B has the lateral force more concentrated than Method A.

 

[O C Diver]

In a perfect world, I prefer a loop formed with a splice held up on the piling by a line hook. Using a loop as a choker isn't the best.

Ted

 

[wkearney99]

What circumstance are you thinking about that would involve needing "strength"? Or are you talking about just a 'thought experiment'?

In the real world you'd also need to factor wear. A line around the piling is going to see constant wear back/forth of the line against the piling material. This may be worse than the single line coming through it's own eye.

 

[Dougcole]

What circumstance are you thinking about that would involve needing "strength"? Or are you talking about just a 'thought experiment'?

In the real world you'd also need to factor wear. A line around the piling is going to see constant wear back/forth of the line against the piling material. This may be worse than the single line coming through it's own eye.

My thoughts as well. It's surprising how much a line works against a piling over time, even when it doesn't noticeably slip while observing it. I've seen them wear grooves in a piling, which can't be easy on the line.


But yeah, short term, the doubled line seems stronger to me.

 

[sunchaser]

What circumstance are you thinking about that would involve needing "strength"? Or are you talking about just a 'thought experiment'?

In the real world you'd also need to factor wear. A line around the piling is going to see constant wear back/forth of the line against the piling material. This may be worse than the single line coming through it's own eye.

Yes. Add to this, using a larger diameter line than ideally necessary will compensate better for wear and support a bigger load.

 

[John R M]

Is a clove hitch around the piling a suitable alternative?

 

[RT Firefly]

Greetings,
IF all else fails there's always...

 

[ABfish]

Method A doubles the breaking strength of the line (or chain or cable).

 

[GoneFarrell]

Method A will be stronger, and it's easier to retrieve. O C's tip about a line hook can save a lot of frustration too.

 

[psneeld]

Or a single turn around the piling will greatly reduce wear...been doing it for years with a little wear and tear.... if used as long term dock lines...UV deterioration may be on pace for normal replacement.


I like the loop with no (overnight/short term use) or one piling round turn ( for longer periods) as I can uncleat and retrieve the line easily.

 

[Mako]

If you're really concerned about strength (upcoming storm for example) then take 4 full wraps around the piling. That is considered full strength of the rope.

 

[Bacchus]

There are other factors in play.

The loop with the line passing through would probably put the most strain on the line where it passes through the loop, especially if it makes a sharp turn at that point.

If the loop on the left returns to the boat, then the strain is shared by 2 lines, however the shock mitigation is halfed

Ted

Ted
I was with you until your last sentence re shock.
My thinking was the left diagram would be better for mitigating shock as with 2 lines returning to the boat the effective length is doubled and longer should be better for shock than shorter lines.
What am I missing or interpreting incorrectly?

 

[rslifkin]

Ted
I was with you until your last sentence re shock.
My thinking was the left diagram would be better for mitigating shock as with 2 lines returning to the boat the effective length is doubled and longer should be better for shock than shorter lines.
What am I missing or interpreting incorrectly?

The effective length to stretch is unchanged, as both ends are attached to the boat. But you now have half as much load on each leg, so half as much stretch.

 

[Bacchus]

The effective length to stretch is unchanged, as both ends are attached to the boat. But you now have half as much load on each leg, so half as much stretch.

Doesn't that arrangement also reduce the shock load on each leg by half?

 

[rslifkin]

Doesn't that arrangement also reduce the shock load on each leg by half?

It does. But it reduces shock absorption because of that. 2x lines each loaded to 10% breaking load will stretch half as much distance as 1x line loaded to 20%. So ideally, if using a single line around a piling and back, you'd downsize it to compensate.

 

[O C Diver]

Ted
I was with you until your last sentence re shock.
My thinking was the left diagram would be better for mitigating shock as with 2 lines returning to the boat the effective length is doubled and longer should be better for shock than shorter lines.
What am I missing or interpreting incorrectly?

The strain is cut in half by 2 lines. Shock absorption doesn't start immediately. Until the line starts to stretch, there is no absorption (not counting the piling flexing). I'm guessing if you graphed it out, there's little or no stretch till you get near 1% of the line's breaking point. With 2 lines, the force required for any stretch (shock absorption) doubles.

Ted

 

[O C Diver]

My thoughts as well. It's surprising how much a line works against a piling over time, even when it doesn't noticeably slip while observing it. I've seen them wear grooves in a piling, which can't be easy on the line.


But yeah, short term, the doubled line seems stronger to me.

I'm having a hard time Imagining a properly secured boat moving so much for and aft in a slip, that the rotation of the lines are wearing groves in the pilings.

I'd be looking for a new slip if that was happening.

Ted

 

[rslifkin]

Any time I run a line around a piling and back to the boat without wrapping it, I slide a long chafe sleeve onto the line first and adjust it so that's in contact with the piling. I figure that should reduce the abuse on the line.

 

[skyhawk]

the only thing I can add is this...
the image on the right...the eye splice.... ideally, I think that one should be positioned so that the working end of the line comes off the pile on a tangent straight to the load...so that it doesn't bend through the eye splice
ditto if that was a tied knot instead of an eye splice....

 

[JWellington]

In Method A, it seems like there are two ropies attached to the piling. But at the back of it, where the force is concentrated, there is only one rope taking the force. So I dont think its any stronger..

 

[Peter B]

I think both your replies confirm my belief that method A is the strongest. Method B has the lateral force more concentrated than Method A.

The question is in a way, unanswerable without the knowledge of the intended reason for having the line around the pile. Assuming it is not for longterm mooring, then the strength becomes secondary to the ability to leave easily, so the left hand option wins.

What circumstance are you thinking about that would involve needing "strength"? Or are you talking about just a 'thought experiment'?
In the real world you'd also need to factor wear. A line around the piling is going to see constant wear back/forth of the line against the piling material. This may be worse than the single line coming through it's own eye.

This is the real issue, not so much the strength, but again, the intended use. Long term mooring would make both ideas less than ideal.

In Method A, it seems like there are two ropies attached to the piling. But at the back of it, where the force is concentrated, there is only one rope taking the force. So I dont think its any stronger..

See both comments above. But assuming it is for a short term moor, the left quick release method would be the way to go for mine...the dead pull strength being almost irrelevant.

 

[psneeld]

Sharp bends in rope like in knots are one of the major things that weaken line strength.


Around a piling hardly does at all.... a choker loop in most situations probably has at least one or more points of tighter bends.


This info is pretty well documented in knots reducing line strength...a quick google search will show numerous articles.

 

[ssobol]

You'd may be surprised at how much a knot reduces the strength of a line. Depends on the knot and the type of line.

In the pictures, arrangement A (loop around a pole) you only have one line, same as B. Whatever the strength of that line is what you get. The only advantage to A over B is that there are no knots.

 

[psneeld]

You'd may be surprised at how much a knot reduces the strength of a line. Depends on the knot and the type of line.

In the pictures, arrangement A (loop around a pole) you only have one line, same as B. Whatever the strength of that line is what you get. The only advantage to A over B is that there are no knots.

And that could mean a lot in overall line performance between the 2 diagrams.

 

[ABfish]

But the original question regards breaking strength, not the ease in leaving the dock or propensity for chafe.

Method A would have twice the breaking strength of Method B. If the boat pulls with a force of 50 lbs, each leg of the line will see 25 lbs. of force (25 x 2) and the piling will oppose with a force of 50 lbs. All of the forces must be balanced, assuming the boat is not moving.

Assume you pull on a rope, with one end in each hand. If your right hand pulls with 10 lbs of force, your left hand must oppose with 10 lbs. The tension in the rope is 10 lbs., not 20.

 

[BruceK]

Greetings,
IF all else fails there's always...

Yeah? Got something against zip ties?

 

[JWellington]

But the original question regards breaking strength, not the ease in leaving the dock or propensity for chafe.

Method A would have twice the breaking strength of Method B. If the boat pulls with a force of 50 lbs, each leg of the line will see 25 lbs. of force (25 x 2) and the piling will oppose with a force of 50 lbs. All of the forces must be balanced, assuming the boat is not moving.

Assume you pull on a rope, with one end in each hand. If your right hand pulls with 10 lbs of force, your left hand must oppose with 10 lbs. The tension in the rope is 10 lbs., not 20.

Im not sure thats right. On the opposite side of the piling there is only one piece of rope, that takes all the strengh. It would be easy to do an experiment on this , with say a fishing line that we know the breaking strengh of. Im on holiday so cant do it here. Lets say its a 20lb line, we wrap it over a pipe in the ceiling, mimiking the piling. Do you think we could put a 40 lb weight on it, on the bottom?