Calculating hammock load?

[Wayland]

OK it's nearly 30 years since I did any real maths.

I'm trying to work out what the static load is, per suspension line, with a hammock secured to trees 4.5m apart when I put 100kg in the middle of it.

I know it's not as simple as 50kg each but I cannot for the life of me remember how to work out what it is.

 

[wattsy]

it depends on the angle you end up with from the tree when you load the hammock. if you end up with an angle of 20 degrees from horizontal then the equation is force = (0.5x100kg) / sin20 = 146kg (rounded up)

 

[Mojoracinguk]

it depends on the angle you end up with from the tree when you load the hammock. if you end up with an angle of 20 degrees from horizontal then the equation is force = (0.5x100kg) / sin20 = 146kg (rounded up)

I take it this is then multiplied by factor 'X' to give a margin of safety and take into account the shock loads of sitting/falling into the hammock...and swinging
What factor of 'X' would you apply?

Mojo

 

[Tadpole]

Interesting post on hammock forces

 

[Wayland]

Right, that's helpful.

So if I wanted to apply one of these to my hammock set up it would need to be able to handle over 150kg?

Rats, looks like they're only rated to 136kg

 

[SimonM]

How about a pair of rap rings on each Wayland?

After seeing a guide on here, I've just started using them and can now set up without knots in under a minute.

Simon

 

[Wayland]

Rap rings?

 

[Shewie]

Why do you want to use the fig-9s Gary, just for ease of adjustment ?

 

[SimonM]

Rap rings?

I got mine from Needle Sports...
http://www.needlesports.com/Catalog...lay-Abseil-Ascent/Abseil/Rap-Ring-OME-RAPRING

EDIT:
Seen here...
http://www.bushcraftuk.com/forum/showthread.php?t=59239&highlight=garda+hitch
Simon

 

[Shewie]

Have a look at this vid Gary, at around 3 mins in Shug demonstrates the whoopie slings. If you're interested in trying them you have to shop across the pond at the moment, I can send you some links though if you want to give them a try.

http://www.hammockforums.net/forum/video.php?do=viewdetails&videoid=189

 

[FGYT]

Have a look at this vid Gary, at around 3 mins in Shug demonstrates the whoopie slings. If you're interested in trying them you have to shop across the pond at the moment, I can send you some links though if you want to give them a try.

http://www.hammockforums.net/forum/video.php?do=viewdetails&videoid=189

cheers for that just ordered a set to see what they are like

 

[snozz]

Use a ratchet strap and have done ;-)

I tie one end using tape, a krab then more tape to the hammock. At the other end I just attach the krab to a ratchet strap and tighten to required load.

Snozz

 

[ged]

So if I wanted to apply one of these to my hammock set up it would need to be able to handle over 150kg? ... Rats, looks like they're only rated to 136kg

It's only 146kg if the angle of 'droop' is 20 degrees. At 30 degrees it drops to 100kg. For a 136kg SWL rated component you should be all right at about arcsin(50/136) = 21.6 degrees.

As you can see, as the anges get smaller the stresses start to rise very quickly.

Don't forget that when you get in the hammock, everything stretches and the trees bend so they get closer together. If you set it up UNLOADED at 20 degrees you'll find that the angles are quite a bit greater when you're in the hammock, meaning there will be less tension than you calculated.

It's the angle between the lines and the horizontal that matters, not between the lines and the trees, which may not even be vertical.

Load-carrying components rated to some value will usually have a breaking load of at least twice that value for safety.

 

[Gailainne]

Gary

The fig 9's tho good, (I still use them for my tarp guide ropes) eat rope mate, the teeth especially on the big ones are really aggressive.

I would go with the descender (rap) rings. I tried and failed to explain my set up on another thread, I'll try again using links from hammockforums...here we go

webbing around the tree as normal, one end looped, take the free end thru and cinch. You can use a biner to make life easier.

Next I make a marlin hitch in the free end, (I have some small sections of dowel I use) Linky Grizz's video shows both the marlin hitch and the garda hitch.

I have a continuous loop made from amsteel, but a loop made with a double fishermans serves just as well, take 2 descender rings and make a prusik containing both rings, its in the video above,the opposite end from the rings gets looped over the marlin hitch and settles BEHIND the knot, NOT on the dowel, (the knot takes the load, the dowels..no way).

I have 7 or 9mm static rope on my DD, a bit ott but shrug its what I had. This is garda hitched thru the descender rings, and will lock with no additional knots required, but is very easy to adjust, just take the load off, and it will release.

NB if you use hollow weave rope, amsteel etc, put in a slippery hitch to lock it or you will probably over the course of the night end up with your butt on the ground.

This is a very quick, easy system to use, more important, its very secure.

NNB I used to use welded steel rings 1 1/2" dia as part of my garda set up, and they worked fine, heavy tho, I got some whoopies and a suspension rig for my diy bridge made by Opie on hammockforums, he very kindly sent me over a set of SMC descender rings, you don't get them over here, they are quite a bit smaller tube dia then the rap rings.

Hope this helps

Stephen

PS GED not being funny, but the trees I hang from wouldn't bend if an elephant was hanging from them. Totally agree on everything else, I normally try to set my lines at a minimum of 30deg, ie neutral. I've never understood people wanting to set their hammock tight in the first place? the whole point is to hang it slack, so that you can sleep on a diagonal, and so get a fairly flat lay.

Only other point is, a lot of ropes the rating is breaking load, not safe working load, please be aware of that, for instance 550 paracord has a breaking load of 550 lbs, NOT a safe working load of 550 lbs, have a look at the graph above to see how quickly a normal sized guy would reach that limit, if his hammock is strung to tight.

 

[Mojoracinguk]

Don't forget that when you get in the hammock, everything stretches and the trees bend so they get closer together. If you set it up UNLOADED at 20 degrees you'll find that the angles are quite a bit greater when you're in the hammock, meaning there will be less tension than you calculated.

Yup...just about to say that
but to repel the thought of trees not bending....it does not matter, the system will still give under your body weight quite a bit (unless you are using steel rope and turnbuckle tensioning on some imovable object to make your hammock as tight as the proverbial drum) so the strain on the lines is not going to reach maximum for the given system until an equilibrium is reached...as it happens the more the system gives due to the load 'falling' vertically down the more the lines will follow and the angle from horizontal will increase wich in turn decreases the effective load encountered by them. This is all countered by the hammok fibres resissting being pulled apart...they keep the lines from reaching 90 to horizontal and some where in all this mess an equilibrium is reached therefore it is time to put down the ghillie kettle and go to sleep

night night...happy hanging...
Mojo

p.s....on the 'exped' slotted lines I used some £2.90 ish 'D' shackles from B&Q ....I was not sure they'd see the night out but did a fine job and i'm happy to use them again.