Breaking a rope

[DanHoward]

How much ST is required to break a rope? If a person wants to break the 3/8" rope on p.B288 what ST would be needed? This rope can hold 300 lbs but this would be for a static load. A Dynamic load would be half this. So if the person could jerk the rope it would be easier to break than if he applied a slow even force.

If BL is utilised, assuming that 2 hands are employed and there is no time limit, I get ST 19 if he can jerk the rope. Extra Effort would probably be the default assumption too which would lower the required ST.

[Nymdok]

For 1 second 2 handed BL I show ST 20.

For a full 4 seconds to break the static strength at 8xBL is ST 14.

Nymdok

[SuedodeuS]

300 lbs is how much it can hold without risking breakage, isn't it? That means the rope could hold more than that, it's just running the risk that it would snap. I suspect this means a decent amount more than 150 lbs of sudden force need be applied to reliably snap it.

[DanHoward]

True there is a saftey margin of about 100%. Looking at other websites the figure in GURPS is about half the maximum strain a 3/8" hemp rope can handle if it is new and undamaged and the load is static, not dynamic. I handle the safety margin with a percentile dice. If the rope is rated for 300 lbs and the load is 450 lbs (50% overloaded) then roll percentile. 50% or less and the rope breaks (the rope automatically breaks if loaded with 600 lbs or more). If the load is dynamic i.e. the rope is jerked, then the rope can only handle half the load anyway so I'd just use the initial figure as a guide - in this case 300 lbs. So one would need ST 14 to break 300 lb rope. 14 x 14 / 5 x 8

[Figleaf23]

Not that I doubt it, but just curious how you know dynamic is half of staticÉ

[DungeonCrawler]

Originally Posted by DanHoward

True there is a saftey margin of about 100%. Looking at other websites the figure in GURPS is about half the maximum strain a 3/8" hemp rope can handle if it is new and undamaged and the load is static, not dynamic. I handle the safety margin with a percentile dice. If the rope is rated for 300 lbs and the load is 450 lbs (50% overloaded) then roll percentile. 50% or less and the rope breaks (the rope automatically breaks if loaded with 600 lbs or more). If the load is dynamic i.e. the rope is jerked, then the rope can only handle half the load anyway so I'd just use the initial figure as a guide - in this case 300 lbs. So one would need ST 14 to break 300 lb rope. 14 x 14 / 5 x 8

The sources I found list 3/8 in manila rope in excellent condition as breaking at over 1200 lbs. But, they note that knots or a drop over an edge (anything that introduces shear to the rope) can cut that to 2/3s or 1/2. True splices are a 5%-25% lessening in strength and I wasn't real clear on why such a broad range...probably how the rope is made, what it's made from, and the skill of the splicer. However, any wear or extra stress or just plain caution might reduce that further.

And it drops again when dealing with a dynamic load and other real life use issues. Exact percentages are uncertain, but that would certainly explain the differences I found between breaking points and safe loads.

Safe loads seemed to hover around 300 lbs for a 3/8 in manila rope.

I'm not sure exactly how that affects the strength required to break the rope, since the breaker might be stuck just pulling on a rope or be able to flex it or put it over an edge...like a chair or wall, not something actually sharp. But it would seem to require truly enormous strength to break a 3/8 in manila rope in good condition by simple strength. And much easier if one can twist it, knot it, put a kink in it, and/or lunge against it. Naturally, if one can use body weight and leverage, it gets easier yet.

For breaking points:
http://www.hmsrichmond.org/ropework.htm

http://74.125.95.132/search?q=cache:...nk&cd=13&gl=us

For safe workloads (which are much lower than breaking points for obvious safety reasons...and which seem to include a significant "worst case" element.)

http://books.google.com/books?id=Mjs...sult#PPA154,M1

Originally Posted by Figleaf23

Not that I doubt it, but just curious how you know dynamic is half of staticÉ

That seems to be the standard safety assumption for dealing with rope.

[starslayer]

Ropes radically loose strength as dirt and stuff get into them, because even regular dirt will wear at the rope from the inside as it is tightened/loosened like thousands of tiny cutting blades, so a rope that can safely take 1600lbs when dry, clean, un-frayed and fresh, might only get those 300 lbs of static load when its dirty, has UV damage from being left in the sun, fray from use over time, and aged threads that have expanded and contracted from wet/dry cycles.

For breaking the rope via raw ST I would figure out the ST of the rope based on its ability to lift (I'd take that 150lbs for dynamic load and calculate that for the basic lift of the rope), and then let anyone attempting to break that rope by might along engage in a contest of ST with the rope, and have the margin of success applied as damage to the rope- however I would also have the rope loose ST as it looses HP from failed contests (unless its mountain climbing grade nylon, that apparently gets harder as a fails, but also looses flexibility as it fuses together).

Doing this will require extra effort for basically anyone to break a rope any thicker then 1/16" nylon unless they have superhuman strength, but that seems accurate; most of the time when someone breaks a rope it's not via raw strength, but instead via a wearing action from rubbing it against something (which I guess would make a contest of the escape skill vs the HT of the rope with the same results as above with each attempt taking a full 60 seconds and consuming 1 fatigue), or by attacking the knot itself (contest of escape skill vs knot tying skill- success frees knot, failure does nothing, costs 1 FP, and makes the next attempt be done at -1 without a break).

[gilbertocarlos]

ST-penalty of the rope vs HT rope.

So, a rope with 300lbs, would probably get -7(for being able to sustain ST17) vs the HT of the rope.

If someone with ST19 tries it, it would be 12 vs 11(HT of the rope).

[Refplace]

Seems to easy to me.
Breaking 3/8 hemp should be a lot harder and take a serious strongman a real effort.

[Anthony]

Breaking a rope through a sharp tug does not require less force than breaking it through pulling; it's just easier to achieve the high force with a sharp tug. The total energy to break a rope is equal to the integral of its stress-strain curve; there's a fair amount of variance, but a reasonable estimate is (length * nominal strength / 4) ft-lb; that's assuming a 20-25% breaking strain and that the listed strength includes a 100% safety margin (and represents the energy that will reliably snap the rope). If the attachment point (either one) can flex or slide, add the work required to get the attachment point to its maximum strain. Thus, snapping a 6'/300 lb rope requires 450 ft-lb, which an adult human can do by falling one yard unless he can catch some of the force with his body (and in general, that 300 lb test rope will break if a human using it to climb with falls 1/2 to 1/3 of its length; it may do quite a lot of damage to the human in the process).