good luck
If you think the engineers are bad, just wait until the attorneys find this thread.
that’s an interesting point gbg …
… the part about the occupant(s) and gear shifting forwards .
That does seem as if some of the weight in motion might not be fully applied into the shock loading on the tow line . Seems occupant(s)/gear deceleration rate would be taking place at a bit slower rate than the fixed tow boat .
Sounds interesting to me . Yet I’m not certain at the moment the actual shock load is imparted that the full weight of boat/occupant(s)/gear aren’t still fully realized on the tow line . I can see the forward momentum of occupant(s)/gear is a resultant reaction of a body in motion and all that , I’m just not sure how it all plays out in the time span of the instant .
Besides , I’m still not sure how much shock load in lbs. force could be generated to the tow line in the scenario I offered of a 250 lb. boat moving at 3 mph. being stopped abruptly .
If some of the 250 lb. weight can be displaced prior to the moment of shock loading , that should make a significant difference . Also as you mentioned if the boat rolls slightly (but what if it rolls the other way in favor of the shock loading ??) , and some of the other things you mentioned … all possibly contributing to a form of cushioning effect (energy absorption) . I think they are valid points worth considering . How they would play out in the generated force equation I don’t know , but it seems they could reduce it to some degree , “if” they took place .
I really think the main factor here in the abrupt stop scenario I suggested is the weight of the towing vessel (magnified by it’s velocity) , always have .
Flags
Of course, to avoid the power boat situation described above, you would want to fly your “vessel in tow” flags and lights.
This is the longest thread for what seemed to me a very simple question that I have seen. It’s not even really debatable, like the rudder/skeg or British/North American fiascos.
yeah , wish I knew how to do the calcs
– Last Updated: Jun-16-11 10:16 AM EST –
...... thanks if you can find the time or motovation to show me how it's done , if not don't sweat it no big deal really just curious .
Out of context with OP's situation but perhaps not out of context with using a 1/4" poly rope for towing a very light boat (a kayak , couple in a canoe , etc.) ... same scenario of course it's understood the towed vessel get's snagged abruptly to a stop , the line is attached directly boat to boat (let's just say it's a direct knot attach) , so those things are constants I beleive ... but what happens if the towing vessel's weight is altered ??
Maybe the speed could be altered some too , perhaps 2 mph instead of 3 ... or perhaps 4 , or all of them resulting in different values .
Altering tow vessels weight and speed become variables I believe .
Maybe this 1/4" poly rope is being used by a couple guys in a Jon boat , weight aprox. 1000-1200 lb.(good samaritans) , maybe it's a Tanker and the Dingy gets wrapped on a channel bouy or Moby Dick comes up and swallows it , lol ...
OP don't need to fear in the circumstances he explained , I think he's pretty much always know that .
I realize these things are not in context w/the OP's situation , and the Tanker / Moby Dick thing is rediculous . Mostly just curious about the 1/4" poly rope and what "minimum" weight towing vessel it would take to reach a 310-387 lb. stress point range .
1st thing that comes to my mind is a 310 lb. tow vessel should have no problem breaking the rope in the snag/shock loading situation ??
not out there
Similar concept was used by Mythbusters for their crash scenarios. Basically, they used sacrificial rope segment to make sure that the towing vehicles did not get damaged when the tow-ins smashed against obstacles.
I didn’t think of that at first.
I’m pretty sure that movable contents of the boat is a factor. I was trying to think of a good reason I’ve never felt the slightest jolt when towing another boat, and that may be part of it. I’m still confident I could tow a boat every day for the rest of my life and never feel a huge shock or break the connection to one of the boats (because the more I think of it, and whitewater rescue instances I’ve been part of, the more I realize that thwarts or carry handles will break long before the rope does).
I understand what you are saying though, even if I don’t think it will happen due to a rope between to tiny boats. Once in my guide-boat, I drifted very slowly into a bridge pier while trying to explain to a newbie how to do eddy turns. I bet the boat was only going 1.5 mph, but it was a direct hit on the stem and it fractured the hull at the point of impact. I’ve had lots of other impacts that were much more severe which caused no damage at all, but they were in a manner that allowed the boat to shift ever so slightly at that moment of impact, which must have made all the difference.
I vote for trilliumlake’s recommendation
Go ahead and go pop for a good tow line system. After all, you’re towing your son, not a log. At least buy some floating line.
Nope
You are correct that F = CdA(1/2)rhou^2
However the A in this equation is the frontal area. I have never seen a 6’ X 2’ kayak, perhaps 0.6 ft x 0.2 ft.
Further, so far, the stretch in the line has been neglected. One needs to integrate the tension of the line over the distance which the line stretches, then equate that to the kinetic energy of the running start. Then solve for force. That is the only way to do it.
Guideboatguy is right.
I use 1/4" floating NRS line.
chuckle
OP- has your question been answered? LOL
reading is a lost skill
– Last Updated: Jun-18-11 8:10 AM EST –
I believe I used words "worst case scenario" a couple of times.
Oh, since you are such an expert, might run a quick search on WW pins and look for pictures. Then we can discuss how unrealistic my number were.
Reading?
The OP was regarding towing a child.
The tangent went off to the maximum force in the line.
The sub-tangent involved the drag force.
I missed (and still miss) the connection between the shock loading of a tow line and a WW boat pinned in a strainer.
I am certainly not an expert in anything, but I have a fundamental understanding of dynamics. The max acceleration experienced by a rock climber after a fall is a classic problem - very similar to the max shock load in the tow line. Hence, the integral approach to the problem.
No personal offense intended.
The max force in the line is a function of acceleration (d2x/dx2), not the jerk (d3x/dx3) or the drag. Please correct me if I’m wrong.
D
this works
http://www.nrsweb.com/shop/product.asp?pfid=1831&pdeptid=1039
this works even better
http://www.nrsweb.com/shop/product.asp?pfid=1829&pdeptid=961