Forces in White Water Rescue

I participated in an Aquatic Rescue course last week and it had me wondering what the various forces involved are. After a bit of internet research here is a collection of bits of information that I found interesting. I can now apply some of this to optimising the equipment I carry and how I use it on the river.

Warning this is a pretty nerdy and long post. Grab a brew and settle in for while.

Throwbag ropes – there is a wide discrepancy in quality of  rope used in throwbags. The old adage of “cheap, light & strong – pick 2” applies. As will be shown later the actual force that can be applied by a rescuer directly to a throwline going to a swimmer is relatively small so bags used just as throwbags do not need to be strong at all. Low weight and cost effectiveness can be prioritised. If the throwbag is going to be used in any rescue using mechanical advantage then simple throwlines are not up to the job.

There appears to be 3 main types of floating rope available plus climbing style, non floating, low stretch rope is often found in wrap kits. All numbers are for 8mm which seems accepted and an industry standard hitting a sweet spot of weight, packsize and grip.

Coreless Polypropylene Rope – MBS of around 4kN or lower

Kernmantle Polypropylene Rope – MBS of 8 to 10kN

Spectra Polypropylene Rope – MBS 16kN

Non floating ‘climbing style’ EN1891 A Nylon Low Stretch Kernmantle Rope –  22kN without terminations, 15kN with fig 8 terminations

Most climbing gear like karabiners and slings have a MBS of 22kN or greater. This is because of the way the EU PPE (Personal Protective Equipment) standards have been set up

Of course check the specs given by the manufacturer on any ropes you have or are buying. Without good information assume the lowest rating.

Things to note:

A knot in a rope reduces its strength by 30 to 50% depending on the knot type.

MBS – Minimum Breaking Strain – the minimum force that equipment can be expected to hold before breaking. Normally rated by the manufacturer as a result of extensive testing.

kN – a measure of force. In layman’s terms 10kN = approx 1000kg or a tonne.

Working load limit (WLL) or safe working load (SWL). Normally rated by the manufacturer after extensive testing this is the load the item can take all day every day without detrimental effect. (If you’ve ever seen a video of a karabiner breaking you will see it starts to deform significantly before it actually break). Items that have been overloaded should not be used again. In the absence of a manufacturers WLL or SWL rating then a 1:5 (WWL = 1 fifth of MBS) is often used but this varies between industry types.

1:10 Safety Factor – When working out what forces may be encountered in a rescue environment it is common practice to factor in a 1:10 safety factor for the equipment used in a system. This gives the wiggle room needed to prevent damage to equipment and potential dangerous failure.

At this point its worth noting Paddle Australia’s guidance throw lines in mechanical advantage systems (from here)

The rope that should be used for mechanical advantage systems must be strong enough to cope with the loads being generated. If we work with a 10 to 1 safety factor to generate a force of 200 kg on a rope we must use a rope with a minimum breaking strain of 2000 kg. The majority of polypropylene ropes used in throw bags are rated at between 800 to 100 kg and are not suitable to be used as rescue ropes other than throw ropes. Ropes such as Spectra with polypropylene sheaths tend to be the only floating ropes that are suitable for these tasks. Many boaters carry static kernmantel climbing rope that can be used for rescue systems. It should be noted that this type of rope does not float and care needs to be taken not let this rope get near a victim as it will sink, with the potential to tangle with the victim’s body and limbs.

So we know how strong our equipment is/should be so lets look at what forces can be generated on our on it in a white water environment.

With a bit of interneting and asking some daft questions I’ve found some good data from people who know what they are doing.

First off I wondered how much force can be applied using a 3:1 Z-drag and some folks to pull on it. I asked the question on Rope Test Lab (here) and got the following answers.

First of all from Chris Onions:

Check out the rule of 12 for high line operations; if the no of people pulling x MA = 12; then track line tension is equal to or less than 3kN. In practice I have always found it to be less when checking with load cells.

Chris is an experienced North Wales based R3 SWR Instructor and has done some really interesting research some of which I’ll link to later.

What this rule of 12 means is a person pulling on a rope can typically generate approx 0.25kN of force (3kN divided by 12 people = 0.25kN). This is a great piece of info. This means in a 3:1 Z-drag it would take 4 people to create a 3kN force. This is an easy principle to remember while dealing with a rescue on a river bank.

But, I’ll be honest, this surprised me in that its quite low. It means that while you’d be over loading and permanently damaging a standard throwline you’d not be breaking it.

Gethin is another North Wales based instructor (and one I’ve worked with) who has been doing some interesting research with a load cell. Check out his HeaveHo research on Tyroleans for underground use. Lots of interesting results with progress capture devices like Petzl Stops and Rigs. He makes an interesting comment:

The portable load cell has been amazing at gathering real time information, really highlighting (in my mind) that it’s the simple things we should be focusing on (ie edges, good rigging, well maintained kit etc) rather than getting to worries about excess forces. That said the stand out finding we came across was the limitations of a toothed jammer if used during a haul and the potential of damaging ropes if toothed jammers were used.

On the subject of progress capture in white water rescue it’s prusiks that are mainly used. I’ve seen a few instances where these are used as ‘clutches’ with the aim they will slip at around 4kN and therefore protecting the gear in the system from exceeding its WLL. Unfortunately it seems that they can be a bit unreliable in this with a very wide variation in where they will slip and fail. In research I’ve read I’ve seen a huge variation between 4 and 10kN where they slip and then fail. Moral of this story is that if you find your prusik is slipping you are creating a lot of/too much force on your system and you should back it off and try something else plus you’ve probably exceeded WLL’s so should look at retiring gear that may have been overloaded.

For another Rope Test Lab rabbit hole here is a link to my question. Some really interesting research papers linked (if your a nerd obviously!)

With this good info I was then wondering what forces are actually needed in rescue scenario’s. In an internet search I came across Chris Onions Thesis looking at the forces created on a raft off a high line. Here is a link to the paper.

Again I was surprised by the low forces involved. In the testing they do a “Worst Case Event (WCE)” where they submerge the front of a raft into the fastest flow tested. My assumption is this would be similar to a wrapped raft, one of the most common use cases for mechanical advantage pulls in white water scenarios. Even this WCE only generates a 3.5kN force. While this would need a big pull, exceeding the rule of 12, this is within the WLL of the higher rated rescue spec throwlines and just within the MBS of a non rescue spec line.

The last set of forces I was wondering about was those generated in a V-Lower.

Again its Chris Onions who came up in my google internetting. This time its a paper in conjunction with Loel Collins (someone I’ve had some great days on the river with)

The paper is looking at the way chest harnesses release and can be found linked here

As part of the testing they put a load cell on the pfd harness attachment point and put folks into the Tryweryn multiple times with some good results. 234 time to be precise, thats a lot of swimming!!!!

Interestingly the CE standard for rescue spec PFD harnesses is broad range between 0.25 and 2.5kN and the problems around this are discussed in the paper.

In the V-lower the forces generated by a person on a line were 0.5kN in a passive body position and 0.725kN in a purposeful high drag star position. Again less than I thought it would be but still very strong. E.g. if a person can pull approx 0.25kN on a rope then a person grabbing a throwline in fast moving water and creating a pull of up to 0.5kN is going to easily over whelm them. This information puts numbers against my experience of needing to ‘play’ swimmers on throwlines into the bank by dynamically letting the rope slip through my hands.

So with all this interesting but nerdy information what are my conclusions.

Firstly that in many instances the forces while feeling powerful and being sustained aren’t actually that high a number, less that I was expecting when I started looking at this.

The most force a person can pull on a rope is somewhere between 0.25kN and 0.5kN. This is relevant to the pull a rescuer has to deal with when dealing with the pull that comes when the rope comes tight to a swimmer that has recieved a throwbag. Its also relevant to how much force can be applied to a mechanical advantage system.

If you are planning on using throwbag ropes for mechanical advantage to stay within WWL/SWLs you need a rescue spec spectra type throwline with a MBS in the region of 20kN.

If you find yourself needing to do a strong pull using a non rescue spec throwline you’ll probably have exceeded its WLL and should consider replacing the line. By strong pull I’m thinking as little as 1 person maxing out the effort on a 3:1.

Prusiks are extremely useful but can be inconsistent in their slip/failure points so don’t rely on them as a clutch.

The rule of 12 creates around the most force you should be applying in a rescue scenario.

There is really good information out there and good information the foundation for good decision making. Hopefully this post will save you digging around for some of it.

If you’ve got this far I hope you’ve found the info useful despite its pictureless, wordy nerdiness. Please comment if you have any thoughts or further useful info



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