Barges and Bowlines

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This Fair Leads is the result of a letter from a friend, about the practices of the "rinky-dink towing company" that he works for. In order to preserve his job, I'll call him by a different name - how about Belford? - and will likewise change some other references.

The story begins with a reel of nylon rope, suitable for towing large barges. We're talking serious material here, 8" or so in diameter. Somehow, though the rope was new, the towing company got it for free; maybe somebody knew somebody who knew somebody who happened to be there with a fork lift when the stuff fell off the back of the truck. As Belford relates, "... Soon after they put it to use the splice on the barge end pulled out [italics mine]. They consulted with the people from whom they acquired the hawser and were told to put a bowline in it. Which they have been using ever since.

"Upon seeing the setup I cringed but apparently they are operating on professional advice. My idea (short of another splice which seems difficult if not impossible) was to put a half hitch below the thimble and slap on 3 or 4 stout well done siezings on the tail. Effectively doing what Nico press fittings do for wire. The skipper didn't think much of the idea given that the bowline seems to work fine. I would be interested in your thoughts on the matter..."

First, it was alarming to hear that the splices "pulled out". This should be just about impossible, and is always indicative of less-than-optimal procedures. When splices do pull out, it is usually in new rope, as the fibers are still slick enough to slide, at least under low loads. Under high loads,the fibers compress on each other, creating the "finger puzzle" phenomenom that makes braided splices work. Any good splice is also stitched very thoroughly at the throat, to keep things from sliding under low loads. It is possible that these splices weren't stitched, and that they pulled out in come-and-go load situations, when the rope was new.

The only other reason -- besides damage to the rope -- that I can think of for a splice to pull out would be insufficient bury of core and/or cover. Perhaps someone read the directions wrong, or decided all on their own that the splice would be a lot easier to turn in if the tails were shorter, or perhaps they were using directions for a different construction of rope, one that required less bury. It happens. With too-short tail(s), there would never be enough friction generated inside the splice to prevent slippage at some level of loading. Bad.

Which brings up a related point: What kind of outfit would employ someone who would turn in a splice like that? Probably the same kind of outfit that would buy crummy rope (see "Factor of Safety", below).

Moving on to the matter of the Bowlines, if the splices had been done correctly, they would have had a break strength approaching the rope's ultimate strength. That is, a good splice generates friction without inducing stress risers that would weaken the rope, much. Bowlines, along with every other non-splice knot, also work by generating friction, but unlike splices, Bowlines (and every other non-splice knot) distort the rope in major ways, resulting in significantly lower break strengths. In Nylon, Dacron, and most other materials, a Bowline will weaken a rope by about 40% Most loop knots and bends will weaken rope by about the same amount. Most hitches weaken the rope by only about 20%, as would the hitch-and seizing combination you propose. Seizings alone, properly done, could get to 100%, but any seizing option must be used with care aboard towboats, as the twine is a mite vulnerable to chafe. The half-hitch-and-seizings proposal would be tough to work up against the thimble, and would likely allow the thimble to get sideways, thus chafing the very rope it was meant to protect. I also assume that these Bowlines are not made to thimbles, but taken to a bollard, cleat, or such aboard the barge. Your arrangement would be stronger in that instance, but only if fetched up against the belay, and then it would be impractical to apply or to cast off.

There are two other factors to consider: Factor of Safety; and Load. The Factor of Safety is the reserve strength of the rope, a multiple above whatever load we calculate will come onto the rope. If we figure, for example, that the maximum load that we could reasonably ever expect is 10,000 pounds, and we have a spliced rope with a break strength of 100,000 pounds, then the Factor of Safety (FS) is 10:1. If we reduce the strength of the rope by 40% with a Bowline, the strength drops to 60,000 pounds, and the FS drops to 6:1. The question is, what is an appropriately reassuring FS? The industrial standard for most applications is 5:1, and in our example, even with Bowlines we have better than that.

Now, some people might wonder why we even need 5:1.Why make things 5 times stronger than our artful calculations have shown a need for? The answer is, of course, that calcualations don't and can't take everything into account. They don't consider, for instance, that someone might have been sold a piece of rope that wasn't as strong as advertised. They also don't consider that the rope might become damaged in use, and put under significant load before someone notices the damage. Calculations are also not real good at predicting operator error, weird load fluctuations, particularly in acceleration, or the consequences of having someone with a room-temperature IQ put a splice or knot into the rope. Regarding the latter, there isn't a FS large enough to compensate, but you can improve your odds. So 5:1 is a minimum FS in a field like yours, which is far from the nice clean, predictable world of cruising yacht rigging, where we can usually get away with less than 3:1.For towboats,10:1 would not be unreasonable, and you might want to couple that with a rigorous program of rope retirement after so many cycles, or after even trivial-seeming damage or deterioration.

And as a step further back, we should consider what the consequences of failure are. If we posit some massive FS, all we are doing is adding another layer to the calculations; it's still an intellectual exercise, one in which we are trying to predict loads relative to rope strength. Even if we are successful at predicting those loads, our SF might vary, just based on the consequences of failure. To put things on a personal level, picture yourself on the deck of a tugboat that is towing a large barge. Through some unfortunate combination of low quality, bad rigging, chafe, UV, unexpected load, etc., the FS goes to zero, and the towline breaks. I've assumed you're talking about Nylon here, and if that's the case, you now have some portion of that hawser heading for you at about 600 ft/sec. Yup, two football fields in one second. You would not be able to dodge this. How high do you want the SF to be?

Of course, safety = expense, as the heavier a rope is, the more expensive it is, so industry tends to do a dance between safety and cost-effectiveness. The worst case scenario is one in which rope is only retired when it breaks. Which brings us back to your rinky-dink outfit. What we hope is that your rope is ridiculously strong for the loads, a real possibility since it was free. And that, finally, leads to what I hope that someone considered before tying those Bowlines: What are the loads? We know you are towing stuff, and we know that this work involves all kinds of loads, in terms of both magnitude and nature. That is, a barge under tow at low speeds, on a calm day, can be towed with sash cord, but the same barge, with wind accelerating it, maybe turning, maybe in a chop, can pull the tug sideways and backwards, with some shock loads thrown in for good measure. As a result, towboaters have tables available to them, specifying loads for a given tug and tow configuration; you have these aboard, right? If not, you're flying blind. The rope might be absurdly strong, absurdly weak, or just right; you've got to start with the load.