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post #192 of Old 01-02-2014
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Re: Do you wear a life jacket?

Originally Posted by capt vimes View Post
Well... The impact energy calculates 1/2.m.v^2... (m is the mass, v is velocity)
That means that a cubic meter of water traveling at 2 m/sec (7.2 km/h) results roughly in 2000J or 2000 kg.m^2/sec^2 of kinetic energy which gets transferred fully onto a solid, immobile object...
A sailor standing on the foredeck is now not immobile, which results in the object of say 80 kg of mass getting accelerated to the speed of the wave because the mass of the man is rather small compared to the mass of the water...
That in result means he will be pushed into the tethers by 2100 J or 2100 kg.m^2/sec^2 plus additional pressure from the water still pushing the sailor...
If the tether now has no stretch, all of that energy is transferred via the harness to the sailor in an instant... If the man is stopped in a tenth of a second the force on the harness and therefore the sailor equals to something of 21 kg.m^2. Now divide this with the area of the harness in m^2 and you should get the actual pressure on your body from the harness in kg...

I hope i did not make any mistakes here...
I think you have been right, the forces are really not that high as i initially thought...
I'm with you on the first part. Any mistakes were in the guessing about the scenario, which is all we can do. Since the impact part has been studied to death by the UIAA (climbing gear regulatory authority), I think it is simpler to detour to fall test data.

2000 jules is about equivalent to a UIAA test fall on a standard length tether (1.8 * 2 M * 50 kg * 9.8 = 1764 Jules. It is well known experimentally that 1" webbing cannot withstand that (only about 1100 joules for 2 meters of webbing), that 8 mm climbing rope can withstand that once, and that 10 mm climbing rope can withstand that ~ 10 falls, including a moderately sharp edge.

The impact force (experimental determined) if using a rigid object will exceed 4000 pounds with webbing and will be 1200-1600 pounds with climbing ropes. A harness and human body (which will deform a few inches) will reduce the impact about 100-500 pounds, depending on the example.

Since sometimes the harness fails, this further supports that the forces are very high. The military has found injuries become common at about 10 Gs in a full body harness; sailors have much less harness and more force.


Previously I had suggested that a Screamer (Yates) would be a good candidate for force reduction, but I withdraw that idea. The problem is that a Screamer is single-use and that lost MOBs were often during rollovers with repeated high impacts. An elastic tether would be more rugged in actual use. Screamers are fine for industry where only a single fall is contemplated; after that the guy kisses the ground, goes home and kisses the wife, and takes the rest of the week off.

(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber

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Last edited by pdqaltair; 01-02-2014 at 12:47 PM.
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