Re: [Paddlewise] floating tow rope

Julio MacWilliams wrote:
> 
> That is a good argument in favor of braided polypropilene lines.
> They are light and bulky, float, and are easy on the hands.
> 
> The initial 300+lb shock, should be absorbed by a shock cord, not
> the paddler nor the boat.
[snip]
> 
> Let us see (boring discussion ahead).
> 
> 3 knots = 1.5 meter/second (+or-).
> 50lb kayak + 180lb paddler = 230lb = 104Kg equivalent
> 
> Energy = 1/2 * m * v^2 = 0.5 * 104 * 1.5^2 = 117 Joules to dissipate
>                                              ^^^^^^^^^^^^^^^^^^^^^^^
> Energy = Force * distance; therefore; Force = Energy / distance
> 
> Hence,
> 
> For 25 cm stretch, (if the rope stretched with constant force, which
> does not; at some intant, therefore, there are even higher forces)
> 
> Force = 117 / .25 = 468 Newtons, equivalent to 103lb of shock.
>                                                ^^^^^ not too bad.
> 
> For 4 cm stretch (no shock cord)
> 
> Force = 117 / .04 = 2925 Newtons, equivalent to 644lb of shock.
>                                                 ^^^^^ not me!, thanks :-{
[snip]
> 
> If my physics are too rusty, please let me know ASAP. ;-)

Well, the physics is OK in terms of energy transfer, but I think you have
assumed a stationary boater (the towee?) attached to another moving at 3
knots (the tower?).  Probably not realistic unless the tower gets up to
full speed just as the rope gets tight.

Another thing which you might consider is that part of the shock
transferred to the tower goes into accelerating the tower (in the reverse
direction).  This diminishes the average force somewhat, but your analysis
is not affected much.  I suspect that peak forces anywhere near the 103 lbs
in the bungie version will do damage to the tower's torso, if the tow rope
is attached at his/her midriff.  Oww!

-- 
Dave Kruger
Astoria, OR

***************************************************************************
PaddleWise Paddling Mailing List
Submissions:     paddlewise_at_lists.intelenet.net
Subscriptions:   paddlewise-request_at_lists.intelenet.net
Website:         http://www.gasp-seakayak.net/paddlewise/
***************************************************************************