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Subject: Re: [world-cruising] A serious question re Jack lines
JAXAshby@XXX.XXX
Date: Sun Feb 09 2003 - 13:07:28 EST
Let me say this again. 3 inches stretch gives 100 times (that's 100x) times
LESS G-shock load than 3/100ths of an inch stretch.
How hard on your body is a two foot drop? Depends on how quickly you stop.
Hollywood stunt men (and women) sometimes "stunt" a 70 foot (or more) fall
and walk away without problems. Oh course they fall into an airbag that
slows them down for a couple seconds. Not much G-shock load there.
1 G acceleration is about 32ft/sec/sec, or about 20 mph increase in speed
each second. (If I recall the formula correctly), a 2 foot drop takes about
sg rt 1/8 seconds -- about 1/3 of a second, which gives a speed of about 7
mph.
But speed is not important, how quickly one stops is. *IF* one stops in
about 1/3 second from a 2 foot fall, one experiences about a 1 G
deceleration;1/30th of a second, about 10 G's; 1/300th of a second, 100 G's.
FWIW, 10 G's will hurt you, and is more than enough to break your back or
neck. 3 G's can easily knock the wind out of you, and can break bones under
the right conditions.
You don't need 45 feet of stretch, but you -- if you fall -- you do very much
want stretch.
OSHA goes into this in some detail somewhere. I first found out about the
OSHA angle when I asked an industrial worker wearing an fall arrest harness.
Damned expensive looking that harness, and the guy said nobody but nobody
complained about wearing one, once they saw the videoes.
btw, the center of my chest to my feet is way more than two feet (I imagine
yours is as well), and if I were to fall off the side of my boat (going say 7
knots) it would be quite some fall before my tether came up short.
I'll say it again. 3 inches stretch has 100x LESS G-shock load than 3/100ths
of an inch stretch.
As the FAA says, "Safety is no accident".
In a message dated 2/9/2003 12:46:14 PM Eastern Standard Time,
capella@XXX.XXX writes:
> Don't you have to *Accelerate* before you *Decelerate*? If you're held
> fast, you'll never have to decelerate, right?
>
> Now, if you're walking around with a 50 foot tether, I can understand how
> someone could accelerate to a significant speed before the slack was gone.
> But six feet??? If my six-foot tether is attached at my chest level and at
> my foot level, that leaves a bit over 2 feet of slack. How much maximum
> acceleration can I experience with two feet of slack? How much "stretch"
> must I build into my arresting gear to prevent injury?
>
> Which is the greater risk: injury from non-stretch jacklines, or
> injury/loss
> of life from being swept overboard as the nylon stretches? Keep in mind
> that most people I know - including myself - are incapable of pulling
> themselves aboard a typical sailboat (approx 3 feet of freeboard) that's
> anchored in calm water. Don't even consider one that's underway in stormy
> conditions.
>
> BTW, "non-stretch" jacklines are only non-stretch along their length. If
> you put a perpendicular load on them, as every tether will do, they stretch
> considerably.
>
> --
> Best,
>
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