Re: [Paddlewise] Tethers, hydraulic forces

Whitesavage & Lyle wrote:
> 
> "Does anyone have any feel for how much load such tethers must take?"
 [snip]
> With either paddle or personal tethers the loads will equal whatever
> force is required to suddenly accellerate the mass of your body, moving
> against water resistance, pulled by a lunging object weighing 100 to 200
> pounds (for a loaded single).  The lunging kayak could have hundreds of
> pounds of water pressure acting on it.  Because of the hammer-blow like
> nature of the forces that might act on this tether if you and your boat
> are thrown in differrent directions I would make a wild guess at
> thousands of pounds of force (momentarily).  I would not be surprised if
> the forces involved can be similar to the forces involved in leader
> falls.  Certainly it would not be overdoing it to use gear as strong as
> climbers use to make a personal tether.  Sailors, expecting to be
> dragged through the water by a multi-ton boat, use very strong tethers.
> This kind of tether could snap a light paddle shaft in half easily.
> 
> Does this sound right to someone who knows more specifics about the
> physics of this problem?

Well, yes and no.  In a leader fall, the climber will have fallen a distance
twice the distance from the last point of protection to the climber before
he/she falls, before the rope begins to arrest his/her descent.  Because
modern climbing ropes elongate as they are loaded, the total energy of the
falling climber is dissipated over quite a bit of rope elongation.  This has
the effect of reducing the peak forces generated in the system, a feature
older ropes (Goldline, manila, etc.) did not have.  There is nothing
comparable to this amount of energy (the falling climber) in a yakker-paddle
boat system under the likely scenarios sea kayakers would encounter in water
(excluding big surf).

If you used a modern climbing rope as part of the leash system, it would
perform similarly to help reduce the peak forces.  In addition, if you are in
a water environment, and your body is *not fixed* (anchored) to something
solid, then your body will respond to the tug of a leash by moving, thus
spreading out the dissipation of the energy of the (moving) boat over more
time, and making the peak forces much less than those in a leader fall, in
which the belayer's end of the rope is "anchored" through a belaying device. 
("Anchored" except in the sense that the belaying device is designed to allow
more rope to slide through it when the force reaches a specified figure,
making the belayer's end of the rope *not* rigidly anchored.)

Nick, I suspect "thousands of pounds of force" is probably way too high an
estimate even for the force generated in a worst-case "over the falls"
scenario for a surf kayaker, because there is nothing rigid to "anchor" the
yak or the paddler to, and the kayaker can only fall a few feet.  Is "over the
falls" the scenario you are envisioning, or do you have something else in
mind?

In any event, I believe it is widely agreed that leashes on kayaks used in
**big** surf constitute a death wish (by entrapment or strangulation), and
that leashes in even moderate to small surf are a very bad idea, *for
kayakers.*  Board surfers, of course, use bungie systems on their ankles in
big surf, and do OK, because the dynamics of the surfer-bungie-board system
are much different than the dynamics of a kayak-bungie-paddler system.

I believe most of us are concerned with the forces generated if we capsize in
a tide rip or in gnarly wind waves.  Under these conditions, the
yakker-paddle-boat system will not generate anything like the forces in a
climbing rope in a leader fall.  Mainly, we are dealing with the forces when
somebody capsizes or falls out of the boat.  This is because the yak is never
moving faster than a few knots, and the yakker falls only a foot or two into
the water, so falling overboard can not generate huge forces, unlike with the
falling climber, who might be travelling 50 - 60 mph (!!) in a leader fall
before the climbing rope begins to arrest his/her fall.

Finally, a bungie or "telephone cord" leash, such as those discussed for a
tether or paddle leash, will elongate lots at very low load (way less than a
hundred pounds of tension), further reducing the peak force.  OTOH, if a leash
of wire cable is used, all bets are off -- wire does not elongate.

-- 
Dave Kruger
Astoria, OR

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