Breaking Strength of Tow Line?

[cfolster]

As a new hammock owner (yeay), I've just discovered Amsteel and am wondering why we're not using it for tow lines. It's incredibly light weight and compact for it's strength and floats. http://www.westmarine.com/buy/samson-rope--amsteel-blue-dyneema-as-78-single-braid--P014895700

What would the recommended breaking strength of a tow line be?

[jason]

Just about every tow line we have has a much higher breaking strength than we need. The one I use has a spectra core. They could make it much smaller but it will tend to tangle and won't be very easy on the hands.

One could make one out of the same line as we use as deck line to save space, Having it thicker makes it easier for one to work with and pull by hand when it has some tension.

[Brian Nystrom]

Just about every tow line we have has a much higher breaking strength than we need. The one I use has a spectra core. They could make it much smaller but it will tend to tangle and won't be very easy on the hands.

One could make one out of the same line as we use as deck line to save space, Having it thicker makes it easier for one to work with and pull by hand when it has some tension.

Exactly! The actual forces on a tow line in use are in the tens of pounds, so virtually anything could be used. A size and texture that's easy to handle, along with a small amount of stretch to act as a buffer (if you don't have a bungee shock absorber in the rig) are the most important characteristics. Anything that fits that bill will be strong enough.

[josko]

kayak tow lines are all about handling, floating, and overall ease of use. That Dyneema is very slippery and has virtually no stretch. I've seen Kevlar/spectra lines do some horrible things to ships and people by their lack of stretchability.

[leong]

Obviously when you’re towing someone the average force on the towline is probably less than 20 pounds. However, this doesn’t account for changes in momentum from quick jerks (perhaps due to wave action). If the towline is not stretchy enough a jerk can create a very large force on the towline. For a given jerk, the physics says that each towline (stretchy and stiff) must handle the same change in momentum. But the stretchy towline does it over a longer amount of time (during the longer stretch). Thus a stiff towline requires a larger tensile strength (the force required to break it) than a stretchy towline.

Moreover, towlines are sometimes used for more than simple tows. For example, towlines are sometimes used for rock rescues where you pull someone up a cliff. For that reason I think towlines should have a large tensile strength. Here’s an example from the whitewater world (I couldn’t find a good video from the sea kayaking world but I know many exist).

That said, I think towlines should probably have tensile strengths of over 200 pounds.

-Leon

[jason]

Obviously when you’re towing someone the average force on the towline is probably less than 20 pounds. However, this doesn’t account for changes in momentum from quick jerks (perhaps due to wave action). If the towline is not stretchy enough a jerk can create a very large force on the towline. For a given jerk, the physics says that each towline (stretchy and stiff) must handle the same change in momentum. But the stretchy towline does it over a longer amount of time (during the longer stretch). Thus a stiff towline requires a larger tensile strength (the force required to break it) than a stretchy towline.

Moreover, towlines are sometimes used for more than simple tows. For example, towlines are sometimes used for rock rescues where you pull someone up a cliff. For that reason I think towlines should have a large tensile strength. Here’s an example from the whitewater world (I couldn’t find a good video from the sea kayaking world but I know many exist).

That said, I think towlines should probably have tensile strengths of over 200 pounds.

-Leon

White water lines get used for a lot more critical applications. They tend to be even thicker (easier on the hands). Lines in throw bags get put into use as emergency climbing gear and are (in the case of what I have) is much more stout than what I use on the ocean.

My ocean tow belt has rope that's rated for 3000 pounds (less where it's tied). It's as big/strong as it is to make it not tangle and easier on the hands.

[leong]

White water lines get used for a lot more critical applications. They tend to be even thicker (easier on the hands). Lines in throw bags get put into use as emergency climbing gear and are (in the case of what I have) is much more stout than what I use on the ocean.

My ocean tow belt has rope that's rated for 3000 pounds (less where it's tied). It's as big/strong as it is to make it not tangle and easier on the hands.

Agreed. The point of my post was to warn people not to use weak towlines even for sea kayaking. Being easier on the hands and the floating ability are all important. But because situations happen it's also important that a sea kayak towline has a high enough tensile strength for emergency use; for example, pulling someone up onto the rocks during a landing. There are thick floating lines that are easy on the hands but have very low tensile strength; for instance, some clothes lines.

-Leon

[Brian Nystrom]

While I understand your premise, I think you're way overestimating the strength required for sea kayaking situations. Even if you actually do get into the situation you describe, exactly how much force do you think you can apply to a rope? Even if you could brace both feet and get a perfect grip on the rope (effective doing a horizontal "squat"), how much force can you generate? 200lbs? 250? 300 (if you're a stud)?

Obviously, one needs to consider the loss of strength at knots and the possibility of working over an edge. I'm not suggesting that one should use shoe laces as a tow rope, but strength is really the last thing that needs to be considered. Handling characteristics are much more important and any rope that meets those needs will be more than strong enough.

[Dan Foster]

Here's a good article on why a rope that's rated for 3000 lbs may not be overkill (and may actually be pushing the limits of safe use) in a situation where there's a person on the end of it:

http://whitewatersolutions.net/Rope_Breaking_Strength.html

The Neptune's Rangers once had to haul themselves and their boats 800 feet up a steep hill to a road to escape the surf, using their tow lines.

http://neptunesrangers.blogspot.com/2012/12/trapped-inside-at-devils-slide-by-bill.html

[Dan Foster]

Yikes, I just looked at the specs for my tow belt. I can safely lower 65-pound kids off a cliff, given a 1:10 safety ratio.

http://www.nrs.com/product/1829/nrs-kayak-tow-line

Rope Length: 50' Rope Material: Polypropylene Rope Diameter: 3/16" Load Max: 650 lbs. (2.9 kN)

http://www.engineeringtoolbox.com/polypropylene-rope-strength-d_1516.html

[leong]

Brian,

I don’t think we disagree. My posts on this topic were more in the way of a public service; i.e. to warn people that the forces on a towline might be a lot more than “the average force on the towline is probably less than 20 pounds. The purpose was mostly to warn DIY-ers that fairly strong ropes should be used when building towlines.” I also said, “I think towlines should probably have tensile strengths of over 200 pounds.” I never quantified the required tensile strength beyond that. I’m pretty sure any commercially available sea-kayaking towline has a tensile strength much greater than 200 pounds.

-Leon
PS
You said:

>>how much force do you think you can apply to a rope?

In a prior post I said,
“If the towline is not stretchy enough a jerk can create a very large force on the towline. For a given jerk, the physics says that each towline (stretchy and stiff) must handle the same change in momentum. But the stretchy towline does it over a longer amount of time (during the longer stretch). Thus a stiff towline requires a larger tensile strength (the force required to break it) than a stretchy towline.”

Here’s another example using a rope bridge. Suspend a 50-foot rope very tightly between two anchor points about 6 feet high. Now pull down on the center of the rope. The tension on the rope will be inversely proportional to the angle of sag of the rope. If the rope is non-stretchy the sag angle will be small. In that case, you can cause a very large tension in the rope, much greater than the force you pull with.

PPS
Dan F, good posts.

[leong]

While I understand your premise, I think you're way overestimating the strength required for sea kayaking situations. Even if you actually do get into the situation you describe, exactly how much force do you think you can apply to a rope? Even if you could brace both feet and get a perfect grip on the rope (effective doing a horizontal "squat"), how much force can you generate? 200lbs? 250? 300 (if you're a stud)?

Obviously, one needs to consider the loss of strength at knots and the possibility of working over an edge. I'm not suggesting that one should use shoe laces as a tow rope, but strength is really the last thing that needs to be considered. Handling characteristics are much more important and any rope that meets those needs will be more than strong enough.

Brian,

For the fun of it I computed the tension force on a rope after stopping a 175 pound person after he falls just 2 meters before the rope stretches and tightens to stops his fall in 10 cm. I think my assumptions are reasonable and the example is a realistic use of a towline for rock landings (see the links provided by Dan F).

The answer I got is 3500 pounds of tension in the rope. Obviously, a longer distance fall would increase the tension accordingly.

-Leon

PS

To validate my answer I went to an on-line program that gave almost exactly the same answer. Attached is the output of their calculator using my assumptions.

Energy of falling object.pdf

Edited January 7, 2015 by leong

[Brian Nystrom]

As a former ice climber, I'm well aware of the impact forces created in falls. How does that in any way relate to the use of tow ropes in paddling? I have no intention of ever using my tow rope for climbing!

Stepping back another post, when would one ever rig a tow rope between two anchor points and pull in the middle? Are you planning to shoot a kayak like an arrow?

If I was so inclined, I could cite all kinds of situations where ropes are put under high stress, but if they're not ever going to happen in a kayaking context, they're irrelevant to the selection of a tow rope. Also, if you're concerned about stretch and impact forces, for a given type of rope, the heavier and stronger it is, the lower the stretch and the higher the impact force it generates.

Again, this is all extreme overkill - and over-thinking - in the context of a kayaking tow rope, which is unlikely to ever see a force much greater than 50 pounds placed upon it.

[leong]

As a former ice climber, I'm well aware of the impact forces created in falls. How does that in any way relate to the use of tow ropes in paddling? I have no intention of ever using my tow rope for climbing!

Stepping back another post, when would one ever rig a tow rope between two anchor points and pull in the middle? Are you planning to shoot a kayak like an arrow?

If I was so inclined, I could cite all kinds of situations where ropes are put under high stress, but if they're not ever going to happen in a kayaking context, they're irrelevant to the selection of a tow rope. Also, if you're concerned about stretch and impact forces, for a given type of rope, the heavier and stronger it is, the lower the stretch and the higher the impact force it generates.

Again, this is all extreme overkill - and over-thinking - in the context of a kayaking tow rope, which is unlikely to ever see a force much greater than 50 pounds placed upon it.

>>I have no intention of ever using my tow rope for climbing!

Then you don’t need an extremely strong towline.

>>when would one ever rig a tow rope between two anchor points and pull in the middle?

Never (although it would make a good “Stupid Kayak Trick” for launching a kayak at Walden Pond). It was just a thought experiment on how to break a rope.

>>Again, this is all extreme overkill - and over-thinking - in the context of a kayaking tow rope, which is unlikely to ever see a force much greater than 50 pounds placed upon it.

Again, I agree with you in general. The points of my posts were to:

1. To address the ‘unlikely” cases such as when the Neptune's Rangers hauling themselves up a cliff.

2. As a public service to DIY-ers who make their own towlines (so they choose ropes with tensile strengths much greater than the average pulling force needed for towing a kayak).

3. To have a little fun calculating impulse of momentum forces.

Peace, my friend.

-Leon

[josko]

I think it's all about hand-friendliness, ability to float, nice tangling properties low volume and low drag in water. In theory, one could tow with 200# fishing-line spectra fiber. It would be tiny, a tangled mess, and would slice through arms and kayaks equally easily, with the facility of a bandsaw blade.