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Subject: Re: [world-cruising] A serious question re Jack lines
From: Bryan Genez (capella@XXX.XXX)
Date: Sun Feb 09 2003 - 12:14:16 EST
JAXAshby@XXX.XXX wrote:
> With all due respect to Alvah -- a far finer sailor than I might ever hope to
> be -- he knows diddly about the physics of deceleration.
>
> I suggest anyone installing jacklines they think they might need check on the
> requirements OSHA has for fall arresting gear for industrial workers. I
> understand even with that gear, people get hurt.
>
> Remember, stretch of 3 inches has 100x less G-shock load than a stretch of
> 3/100ths of an inch. Remember, it's not the fall that hurts you, it's the
> stopping.
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,Bryan (KB3HMZ) "Capella" Valiant 40 #158 Annapolis, MD
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