How do they determine the strength of fishing line?
A 4 lb. test can hold alot more than four pounds before breaking.
There are no uniform standards for labeling. The International Game Fish Association independently test line used for world record catches. The length of line submitted is held in dry condition at room temp. The line is then soaked in fresh water for two hours. The line is then stretched apart at the rate of 12 inches per minute on a machine that registers the break point of the line. The average breaking strength of 5 tests is the official line test.
Anything that’s labeled “X lb. test” can hold a lot more than X lbs; there’s usually a safety factor of 4 to 10. This applies to everything from lumber to submarines.
How do you know this? You can catch a fish heavier than 4 lb. on 4 lb. test line but the fish doesn’t weigh anything in the water, its weight in air being balanced by its buoyancy.
This young lad finds that the 5 biggest selling brands of 10lb test support from 6 to 17lbs. Only 3 support 10 or more.
Tie off one end of the line to a fixed object, pull up on the other end until it breaks. My kid tried this yesterday. To break it, he had to pull alot harder than if he were lifting a 4 lb. weight.
I hope you won’t hate me if I don’t find this exactly iron-clad evidence.
'Cuz I’m a mechanical engineer 
The way I’ve seen the fishing line test described is as follows:
IGFA approved- will support NO MORE than the specified weight.
Regular old consumer line- will support AT LEAST the specified weight.
To expand on my previous post: It’s a principle of physics (that I learned way back in Machine Design I) that when you suddenly put a weight on something, it causes a momentary* force equal to twice the weight of the object. So if you want to hang 400 lbs from a fishing line, it better have an 800-lb tensile strength. And If you want the 400 lb weight to bounce and fly around, the line could experience forces as much as 4000 lbs.
*The momentary force lasts only a fraction of a second.
I tried lifting a gallon of milk (8 lbs.) with four pound test. It snapped before I got the jug off the counter. I guess my kid didn’t use alot more force than 4 lbs. :rolleyes:
Once we drink it half way, I’ll try again.
I have measured the diameter of nylon fishing line and computed the unit stress when loaded with the nominal breaking strength. As I recall, the result is that the nominal breaking load is quite close to the nominal ultimate strength of nylong.
The relevant physical principle would appear to be F = ma. The actual force carried in a fishing line would thus depend on how you accelerated the test weight from a standstill, which could itself depend on the “stretchiness” of the line. Pegging the peak force at twice the weight seems greatly oversimplified.
Well, that’s me, the Great Simplifier!
When fishing, the other consideration that has not been mentioned is the drag on your reel. You can catch fish much heavier than the weight of your line if you set the drag appropriately as you try to land your catch.
What does the apparent weight of the fish have to do with it? It’s the force being exerted by the fish which is an amalgam of a lot of factors – the strength of the fish, its mass, whether you’re trolling from a boat or stationary, the speed that you’re reeling in, and so on.
I would assume in many cases the fish is lifted out of the water by the line (not always, I know, but it’s a routine enough activity that it should be accounted for). Thus, the line should at least be as strong as the expected weight of the fish you wish to catch. Granted, this is a minimum; factors such as drag and strength of the fish should also be considered.
The back-of-the-envelope explanation is that when a sudden force is applied to a stationary object via pulling a string or rope, the force is transmitted from the hand, down the rope, and to the object via a transverse wave. Like any wave, this force wave will be (partially) reflected back up the rope toward the puller.
These two waves overlap right at the portion of the rope nearest the object resisting the pull, so that portion of the rope experiences (in tension) something more than just the original force applied; at a maximum it could see double that force.
This portion is therefore under the greatest stress during the initial jerk of pulling, and so if a uniform rope breaks under this jerk, it is most likely to break near the object. As a test, tie a string or thread to a solid object, let it hang loose, then jerk hard on the free end (please, no jokes :-)); repeat until the string breaks; it will always break nearest the tied end.
This may be due to the well-known fact that essentially all knots weaken the line in which they are tied.
If you have an identical knot in both ends (at the weight and at your hand) then the situation is symmetrical and (despite any “force waves” present) the string won’t know at which end to break. In practice, the knots won’t be identical - one will weaken the string slightly more, and thus be the point at which the break will tend to occur.
Perhaps I haven’t explained the situation very well. The knot is irrelevant; any method used to secure the string/rope to the object will produce the same result, though perhaps the weakening caused by a knot makes it a poor choice in the experiment. Furthermore the situation is not symmetrical because the inertia of the large object being moved is only at one end of the string. In short, the tension in the string is not uniform for the short, transient time it is jerked.
The demonstration here will perhaps better explain. It clearly shows that tension is transmitted though I’ll admit the use of a term like “force wave” is problematic.
As I understand it, we’re talking about a single string connecting two masses - your hand on one end and the weight on the other. So it looks symmetrical to me, and the only thing that will make the string’s tension non-uniform is the mass of the string itself - which can certainly oscillate a bit, but looks to be small enough that it can safely be ignored.
That demo is a bit different - it includes a hand, two masses, and three strings.