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Working load vs. breaking strength
From http://www.boatsafe.com/marlinespike/safeload.htm :
“Knowing the maximum safe working load for line can help prevent accidents and tragedies. You should never stress a line anywhere near its breaking strength. As line is spliced, stretched, wears, is subjected to sustained loads, shock loads, loads of many times the recommended working load, subjected to great heat or ultraviolet light for long periods of time it will continually lose some of its strength… Safe working load is generally thought of as no more than 1/5th of a line’s breaking strength.”
Let me try 
– Last Updated: Jul-15-09 11:13 AM EST –
Follow these to check my back of the envelope calculations:
http://hypertextbook.com/physics/mechanics/acceleration/
and
http://www.ajdesigner.com/phpforce/force_equation_m.php
g (the acceleration of the earth's gravity) is roughly 9.8 m/s^2.
The acceleration during the crash that killed princess Diana is estimated at say 85 times g or 9.8*85=833 m/s^2.
Plug this in the formula for force F expressed in terms of mass m and acceleration a for a 60lb kayak:
F=m*a = 60*833 = 22671 newtons.
Find out what this is worth in terms of mass (weight) by using the reverse formula and using acceleration of 1 g (or 9.8 m/s^2):
m=F/a = 22671/9.8 = 2313 kg or roughly 5,000lb
Even if you had a 5000lb rated tie down I very much doubt the tie down points would hold, especially for those of you with tie downs screwed under the hood in a less than 1mm thick sheet metal that can hold may be a couple of hundred pounds if that. So why use a rope stronger than that?
However, this is the force the boat would experience if it were to slam into the dashboard at the speed that Dianna crashed at (fast).
==============
For a more reasonable scenario, assume the tie-downs and straps act like a seat belt. That would decrease the acceleration so the result, skipping the math, for acceleration that is now down to "only" 30g (per that same article above) for a 60lb kayak, we get a force of 8001 newton and thus a mass equivalent of 816kg or roughly 1,700lb. Again, that would be the mass of a 60lb kayak crashing to a dead stop at fast highway speeds and held only by the tie downs stretching a little...
How many of you have 1,700 lb tie downs? How many would worry about their kayaks coming loose in a 90 mile per hour head-on crash?
A much more realistic scenario is to calculate for the main straps or the rack failing and thus the tie down having to hold the rack and kayak in place or near the car. Say the same 60lb kayak starts to shift. Say at 3g acceleration, equivalent a F1 car cornering (your car or SUV would corner at something like 0.6g or 5 times less!). Repeat the above calculations for 3g and get ... 82 kg or rougly 200 lb.
That is well within the OP's rope rating, it just so happens, and has a 3-5 times margin, that should account for say the weight of the rack... Remember, the main tie downs will likely loosen or snap one at a time and the tie downs will be pre-tightened so there will not be a huge dynamic load above the acceleration of the car maneuvering. To me a 500lb load rating seems very reasonable for most situations which the driver would survive...
DISCLAIMER: As with any internet advice - do not take mine as the "gold" standard - make your own mind as I'm not even pretending I have the right data to calclulate and even if I did, an actual event has too many varialbles to say one can predict it!
voice of reason. thank you.
550# is generally the steady pull
strength.
BUT, a snap or sharp tug can multiply that force by ten easily.
Pull a piece of string and you cannot break it.
But release the tension then snap your hands apart and it breaks easily.
That is what theyare talking about.
You can hang from the cord easily but if you jump up and fall even a couple feet, it may easily snap.
Same for a boat on the roof… it flexes! The bow & stern lines may be tight when you tie them down but as you drive, the boat flexes so the cord relaxes and tightens. The wrong flex and it can snap!
I’ve snapped 1" cotton webbing with a cam-strap when I hit a bump on a road.
ah
1) Forgot to factor in 50% strength decrease for knots. So even at a 5 to-1 safety factor, your working load limit is 55 pounds.
2) For reference, NRS describes their straps as “1 inch, high-tenacity, polypropylene webbing rated to 1500 lbs. Buckle rated to greater than 2000 lbs. tensile strength.”
3) If you loop it a few times, things look much better, especially if the turning points are low-friction so that the load is distributed along the entire length.
Well, another forgotten point
Is where the tie downs are tied to the kayaks! How many kayaks have a loop that would withstand 500 lb load? I think if I pull hard enough on any one deck fitting on my kayaks I will probably snap it - it will either break or pop-out. Of course, some canoes meant to be portaged with 500lb load main in fact have strong enough points to attach to, but many sea kayaks I would gather do not (especially the mass-produced entry level boats that are weak to save $$$ or the racing variety that is fragile to save weight).
So I think some of this discussion is mostly academical any way, in that we need to identify the weakest link and go from there -
Ummm…then why did you ask?
– Last Updated: Jul-15-09 12:30 PM EST –
"Guys, the paracord I have has a load strength of 550 lbs! The diameter is about 5mm. I've already tried it on my rack and it ties great. Are you really telling me to get something stronger than 550 lbs for a bow and stern tie down when the canoes are already securely belly fastened to the rack with cam buckle straps. I was just at Home Depot and to get a rope stronger then 550 lbs I would need to get something over an inch in diameter."
Why are you defending what you're asking advice about? Let you in on a little secret: if you run something up the pole on P.net, you are not going to get a bunch of folks telling you how wise and clever your solution is.
for that you need a different kind of website and your credit card...
Good point
That’s why I mentioned the stretch factor in relation to how tight the line is to begin with, noting that bow and stern tie-downs are barely snug, not tight. A thick rope and a thin rope may have the same degree of stretch over their intended load range, but at very low amounts of tension, the thick rope will stretch much less, and thus provide a more stable anchorage at THAT degree of load. You are right that something else is likely to “give” if a huge amount of strain is applied. It would probably take a vehicle collision to make that happen, and at that point you’ve got plenty more to worry about, huh?
Well, I have been convinced.
I am going to convert to mason line for my bow & stern tie-downs. 5mm is overkill.
Jim
That’s funny, I’m going the other way.
It’s stainless aircraft cable for me!
Actually, now that I’m thinking aircraft, the last time I watched a canoe-lashing contest at a float plane fly-in, those maniacs were using the rattiest-looking clothesline I’ve ever seen. What kinds of forces are involved in taking off a Cessna 195 with two canoes tied to the struts, I wonder?
Not sure but .
his fuel range and GPH I bet suck. Do you need bow & stern lines a 125MPH,and 3000 ft?? Lets ask JackL
billinpa
Ratty plane tie-downs
Yep. Been there and seen that too. Bradley Air Service DH Beaver with what appeared to be thick cotton rope used to tie on my Mad River Northwoods.
I guess if the boat stays put during the radial’s run-up, the pilot figures that it will stay on at cruise. And I doubt we ever got over 1000’ above the lakes!
Jim
5mm
All the 5mm cord I have holds enough for a canoe. Stuff from rei etc is better than the home depot stuff. That yellow twisted rope is absolute crap.
-James
http://bit.ly/AdventureCanoe
cutting edge
one thing not mentioned yet is that smaller diameters will “cut” over an edge easier that larger diameter lines over that same edge. When you bend a line, the outside of the bend stretches farther than the inside of the bend, so there is an uneven pull on the strands - tensile strength is measeured in a straight line pull, without any bends (that is also why you can more easily break a branch by bending it over your knee). Then bend in a knot is what causes the reduction in strength - the more severe the bend, the more reduction (which is why some knots reduce strength more than others). So if that paracord is led over a sharp edge, such as the bottom edge of a car bumper, under load stresses, it would cut much faster than a line of the same tensile strength but of a larger diameter. Just something else to consider.
“Real” 550 cord
used in military applications - where this stuff traditionally came from before it was commercially available in so many guises - has multiple strands inside the nylon casing. That’s the stuff that has the actual 550-lb rating. The other stuff is crap.
But I agree - a sudden stratch under load can be far more than the line is rated for, even if that load is comparatively light. That’s why the military used MULTIPLE strands of 550 cord for each parachute and not just one.
If…
If you have a relative/friend that is a farmer, and has hay fields; they usually have some excess bailing twine left over after haying is done. Ask them for some.
Might not hold up for a prolonged period of time if traveling at interstate speed, but should be good for short trips. In a real pinch, use some kite string.
:^)
BOB
– Last Updated: Jul-16-09 1:34 PM EST –
I can't help but get a chuckle over the precision engineering folks are posting. This doesn't have to be rocket science, folks. Overengineer, get some strong rope that takes knots easy and allows for them to be untied easily. Or get some straps. Pretty simple.
We don't have to prepare a thesis in order to make a decision on bow and stern tie downs. If we did I could well understand why someone would just forgo them.
But…
– Last Updated: Jul-16-09 1:44 PM EST –
add inta de eequasoon de effects o' dark energy on JackL's 57' canoo witout bow an' stern tiedowns bein' passed by a fat lady drivin' in de opposite direction in a 1972 Mustang Mach 1 convertabool at 55 mph wit a crosswind of 27.7 knots...
Me'reckon's it be high time fer a rhyme fro' CWDH on dis subject!
FE
LOL!
But I’m intriggued by the theory of “passing someone in the opposite direction”, regardless of the Mustang’s lady mass and velocity - Sounds … relative to me!