Some interesting observations re. hammocks & webbing

[HuBBa]

Hi all...

decided to give an assymetrical hammock design a try today so i sewed one up and headed down to the forest to test it out aswell as loadtest my rig.

First impressions is, pretty good comfort in it.. though i can't say it felt that extremely different to my normal thai setup. This may be because my thai hammocks are 150 cm wide so they allow for diagonal resting aswell.

However.. i did notice one interesting thing with the webbing. I'm using 2cm webbing which should have several hundred kg of load strength... and it may have that.. however when pulled into a tight knot and loaded repeatedly with a pulsating load (like me jumping onto the hammock to test it =), the webbing around the knot warmed up with friction and thus became brittle and subsequently broke.

Now you may say "sure, but we don't usually swing and jump on our hammocks." and true. You don't. However, several factors will fall into the same category. Extreme cold will make the webbing more brittle aswell, elongated use of the webbing will weaken it by small ammounts of friction force wherever you put knots, creating a small weakened spot there. Eventually it will most likely break.

2cm might be thought of as too small. But this is load-tested to 800kg with normal pressures. But in this case, the knot actually weakened this by quite a lot. So it might be interesting to have a close look on what rigging you use for the hammock.

Just thought i'd share my observations. I think i'm going to change my rig to a rope based instead.

On the positive note.. the cordura hammock didn't even flinch. So the fabric is pretty damn strong.

 

[Bardster]

Its one of those things that a knot weakens a length of rope/webbing by up to 70%/80% depending upon the knot. this is why knot enthusiasts are always looking for the perfect knot - the one which weakens the cordage least. There are some knots that only weaken by 10 or 20%
One of the reasons why a loop or join is best laid-in or spliced rather than knotted. with webbing your best bet would be sewn rather than knotted.

 

[HuBBa]

Yepp. was definately a good thing to find out while testing rather than out in the field while sleeping

i think i'll change the connectors to either two carabiners (or metal rings depending on what price i can get =) and use sewn webbings between the hammock and the carabiners. Then use climbing rope (non-giving) for between the carabiner and the tree.

 

[arctic hobo]

Also, the force exerted down the length of the straps is many many times your weight, because it's pulling almost horizontal. As a sort of analogy, think how tight you have to pull a length of string to support even a small weight.
And jumping puts a lot of strain on the webbing - this is partly why climbing rope is rated to such great strength, despite the fact that few climbers are heavier than 150kg - because if they fall, a huge amount of stress is placed on the rope
PS The Hennessy has straps rated to I think 1600kg

 

[HuBBa]

Good point Arctic. Time to start hunting down retail prices of climbing rope Anyone have any business 2 business suppliers handy?

 

[swamp donkey]

Hold on chaps,

I will try to keep this as simple as possible for the moment.

Climbing rope is designed to stretch a single rope is designed to stretch up to 40% depending on rope grade .(not diameter) even low stretch rope stretches 4% .
The reason is that climbing ropes need to asorb the load of a falling climber. shock load is expressed in fall factors which in basic terms equate to distance traveled before the load comes on the rope at a fixed point.
Most cord brought of of the roll in climbing shops will be classed as assecory cordand as such will be low stretch.(so not to be used for climbing)

Very few knots reduce the strength of a rope less than 50% most are around the 30% mark (bowline ,fig 8 ) However these test are almost always carried out on a sraight pull. Shock loading brings different results as they are rope (material ,construction from) specific.

Webbing tape particulary, made made fibre ones have basically no stretch and are not therefore designed to recieve shock loading. .
Failure will depend on diameter of object tied around , rest time between shock loads, height from which load is dropped etc ,etc, etc. More importantly what the webbing is constructed from and they are definatly not all the same.
I suppose the question is do we shock load our hammocks if they are suspended by tape? I can see no reason why we should.

Over to you

 

[Motorbike Man]

So, what would be good to support a hammock with then? In simple terms please, I'm a newbie to hammock camping

 

[MagiKelly]

So, what would be good to support a hammock with then? In simple terms please, I'm a newbie to hammock camping

Climbing Tape!

19mm , will hold your weight and then some and kinder to the trees etc. It is what the Ecosystem uses and can be bought for about £1.50 a metre from your local climbing shop. Knots easily and undoes easily too.

 

[swamp donkey]

Well there lies a dilema

What ever I use for suspending my self. I apply a safety factor of 10 to 1 so if you are going to put a total load on of say 90kg then the cord, rope webbing should be have a braking strength of at least 900kg.

The type of hammock will in some respects push you one way or another.and indeed your perferred rigging method I would happily use webbing or cord but then I know not to take a running jump on to the hammock if using webbing but let my self on gently

I am using 8mm cordage at the moment because it was free

with a canvas tree protector

 

[arctic hobo]

As Magikelly says, climbing tape/slings are best. I think Leon said it's called tubular nylon at the meet, but there isn't really any reason why you can't just get slings and slit the stitchings.
The actual load on the rope is an equation dependent on your weight and the angle of the rope to the horizontal - it's equal to half the load (your weight times 9.8) divided by the sine of the angle with the horizontal.
For example, say I pitch my hammock with the ropes at 6 degrees or so (a rough guess is all that we need). I weigh 70kg, which in half is 35. 35 times 9.8 equals 343 Newtons. I then divide this by sin 6 (which is 0.104) to get 3281N. This is the strain I put on the webbing if I pitch it at 6 degrees to the horizontal. Obviously it would be very hard to pitch it so tight, but it's just an illustration to guide you.
However it is best to have the strength up to of 1.5x this, because hammocks are not always slung flat. Plus a safety margin which is as large as you want it to be.

 

[Motorbike Man]

As Magikelly says, climbing tape/slings are best. I think Leon said it's called tubular nylon at the meet, but there isn't really any reason why you can't just get slings and slit the stitchings.
The actual load on the rope is an equation dependent on your weight and the angle of the rope to the horizontal - it's equal to half the load (your weight times 9.8) divided by the sine of the angle with the horizontal.
For example, say I pitch my hammock with the ropes at 6 degrees or so (a rough guess is all that we need). I weigh 70kg, which in half is 35. 35 times 9.8 equals 343 Newtons. I then divide this by sin 6 (which is 0.104) to get 3281N. This is the strain I put on the webbing if I pitch it at 6 degrees to the horizontal. Obviously it would be very hard to pitch it so tight, but it's just an illustration to guide you.
However it is best to have the strength up to of 1.5x this, because hammocks are not always slung flat. Plus a safety margin which is as large as you want it to be.

Nooooooo!!!!!! not maths!!!!! not with the state my heads in after last night

 

[wentworth]

Ahem. I think we're all still waiting for some piccies of this asym hammock Hubba!

 

[HuBBa]

hehehe.. damn.. and i thought i'd get away with it

Actually, i'm waiting for some nicer weather. It's been raining for 2 days and i'd rather not take the camera out in that.

But rest asured.. pictures are coming

 

[HuBBa]

Now with pics! =)

First a disclaimer. I suspect i've probably done this all wrong but remember. i've never seen a asymetrical hammock, except for pics. And they all pretty much show 0 details and just a big overview. Still this is my first attempt at making one

Closeup of the side-flap connectors (to stretch out the hammock flaps)

Closeup of my new connection. works much nicer, thanx for the tip about sewing the webbing instead of knotting it.

It's comfy, but imho not that much different from the normal straight hammock. But as i said, it could be because my straight thai style hammocks are 150 cm wide. so they give plenty of room.

 

[Motorbike Man]

Looks good Hubba, but I'm still having problems picturing what makes it an asym. Is it the fact that it is supported at opposite corners when you tie it up? I know it's asking a lot, but any chance of a pic of either laid out flat or hung up flat froma washing line or something?



Pretty please

 

[Stew]

Some interesting thoughts. I had never really thought about the angle o load, even though I'm a climber. I guess it's thinking of things in different situations.

Why do you want to use 2cm wide tape? To keep the weight down? Has anyone looked into using dyneema tape? That's 1.2cm wide and should be plenty strong enough. The cost will be higher and isn't available off the reel (a far as I know) but if anyone is retiring a climbing sling...

I'm still having problems picturing what makes it an asym.

That's essentially how the Hennessy works. Yuo have to make sure you sleep as though it's straight though else it makes no difference.

 

[HuBBa]

I'll get you some better pictures at how it's made but basically each tie end is not cut in a 90 angle to the long sides but at an angle, creating a sort of a paralellogram instead of a rectangle. This means that when bunched together, it will strive to make the two short sides (tie-ends) parallel towards eachother, which will make the two long ends offset. This creates the flaps that you can then tie out to create the asymetrical shape.

Stew: I used 20mm webbing because it would be easier to tie into a knot (i assumed) than say 50mm webbing. That and the fact that i got it pretty cheap
I noticed the same thing with having to sleep "straight" over the load line (ie. the line between the two load-bearing points. if you slide to the side, you will end up in a sort of enveloped U shape, not comfy at all.

The load setup as it is now works pretty nice. I might switch to better quality webbing though if i can find a supplier.

 

[swamp donkey]

Now Mr Moterbike , I deliberatly leave out the maths .

Whilst Mr Artics math is correct what he failed to point out, Is the fact that almost all of the load is concentrated on the anchor point and is not spread evenly though the webbing / rope ,plus leverage of course. .
So what does that mean for us when rigging our hammocks?

Well, the tighter we string our hammocks the more load is transfered to ,what ever we have tied it to. So choose a sturdy tree and if you suspect your tree/ anchor put more sag in your hammock and tie on lower down .

Tubular, flat, dynema,spectra climbing tapes will all do the job , as long as you know the breaking strain. . cold does not really effect nylon tape significantly. A properly seated tape knot (water knot) is equally as strong as a stitched tape .

The 10 to 1 safety factor, allows for 30% reduction in strength due to knots and allows enough extra residual strength should shock loading occur.

 

[Powderburn]

Try polypropylene webbing instead of nylon. That is what the commercial hammock makers like Speer use and is readily available in any fabric shop. Nylon webbing can melt together when knotted and placed under load. Polypro can too, but not as much. Use an Ed Speer 4 Wrap knot or Risk's Improved Speer 4 Wrap knot to tie the webbing off to the tree.

To save money, especially if you plan on making more than one hammock, check online sources for polypro webbing. I bought 100 yards of 1 inch webbing (rated at 660 pounds / 300 kg) for about the same price as it would cost for 10 yards in a local store.

If you are using rope, it is preferable to have the rope secured to the hammock and use webbing to attach everything to the trees. That's the method Hennessey uses with their Tree Hugger setup. Webbing mars tree bark much less than rope will.

 

[swamp donkey]

Without being to perdantic,
The melting point of poly prop is much lower than nylon and size for size it is weaker.
Under load it has greater elongation but less ability to recover unlike Nylon /polyester .Which means it eventually fails under ess load

The stuff that was supplied with my hammock was a hollow braided polypropelyne. which is fine as it gives the poly prop extra strength. and better ability to take a shock load