- 15.8-foot fall, plus rope stretch (typically another 2.8 feet).
- Elongation. To avoid bungee ropes that would bounce a climber dangerously by stretching too far, the rope may not stretch more than 10% under a 176-pound weight. It would also be difficult to climb or descend a stretchy rope. There is no stretch limit under impact forces.
Test procedure is detailed in Standards Downloads.
This is an incredibly severe test of energy absorption. The test wiegh reaches about 32 feet/second. The falling mass gains about (15.8' + 2.8') x 176# = 3,274 ft-pounds of energy, and the rope must absorb this within about 2.8 feet of stretch. 3,274 ft-pounds = 2.8' x 1950# x 0.6. If the rope were a perfect spring, the 0.6 "factor" would be 0.5, but the rope is non-linear and this factor gives a better fit. Basically, a climbing rope must absorb about 400 ft-pounds of energy per foot, because a climber gains 176 ft-pounds of energy for every foot he falls, he can fall twice the length of the rope in a worst-case fall, and a safety factor is needed. This is a true worst case fall, it is VERY unusual in actual practice, and it is impossible to match the forces unless the climber is quite large. In practice, no UIAA approved climbing rope failures have been documented since the 1960s that did not involve cutting over an edge or chemical damage to the rope.
For jacklines, the math is related, but every boat size will give different answers. We will be using more "gentle" assumptions more reflective of actual conditions possible during a wave strike.
- 40-foot boat with 30-foot jacklines (start 2' from the bow and end 8' forward of the stern).
- "Fall" in this case is a man moving 20ft/sec across the deck, ignoring the force of the wave and falling. That is about as fast as any breaker could be expected to throw a crewman.We will then double this result, to allow for the force of the wave and the weight of the sailor sliding down a wet and sharply heeled deck.
- ISAF approved jacklines must have the same strength as 3/16-inch 316 stainless cable, or about 3,700 pounds. 1-inch nylon climbing webbing meets this requirment when dry, but not when wet, when it will fail at about 3,250 pounds. ORC requires a 6,000-pound rating and we will base or calculations on this higher value. 17% elongation at failure is typical of 1-inch tubular webbing; I am assuming that the 6,000-pound webbing exhibits the same elongation at failure.
- Webbing is weakened 12% when wet. Nylon fiber is weakened about 10%, rope about 20%, and webbing about 12%. We will use 5,200 pounds as breaking strength, since jackline systems are only tested to their limits in wet conditions. However, the elongation to failure increases to about 20%, so the energy absorption to failure is little effected by moisture. This result seems to be supported by the observation that climbing ropes do not fail when wet. US Sailing requires 4,500-pound test webbing or SS wire. 1-inch climbing webbing is 4,000- to 4,200-pound test. 1-inch rigging webbing tests at 6,000 pounds.
- Person with gear weighs 200 pounds.
The applicable ISO specification, for harnesses and tethers on small sailing boats:
We are assuming one man per jack line. Clearly, there can be more, but they should not both fall in the same worst-case manner at the same instant, so perhaps a 160% multiplier would make sense for 2 men. Yes, it is not unlikely that the same wave would sweep 2 men, but they would not hit the end of their tethers at the same moment and they would not both be at the mid-point of the jackline.
Knotting vs. Sewing. Sewn jacklines will be effectively 100% strength. Knotted webbing jacklines will be about 85% strength if waterknots are used. A rope jackline will be about 85% strength if a figure-8 is used. A cleated end will be about 95% strength. This has been considered in the calculations, as described in the drop test procedure above.
(note: ISO and OSR standards specify 1,430 ft-pounds fall energy; I have used a higher value because multihulls typically use longer tethers. However, if we use 1,430 ft-pounds of fall energy, all of the stress results will be 43% lower and the ISAF 3,700-pound webbing and cable will be suficient.)
What If We Clip Dirrectly To A U-Bolt?
The forces become much greater because of the abrupt stop. There have been instances of tethers breaking. Generally there is no body to examine for injuries.
We will adjust our assumption, since the sailor is now in the cockpit and not exposed on the bow. It turns out that that a 6-foot fall and a lesser wave strike are not all that different: A falling body accelerates at 32 ft/sec^2; if you fall for 0.38 seconds you reach 12 ft/second and cover 6 feet in that time. This the same 1,430 ft-pound energy fall ISO and OSR standards specify. It's not hard to imagine that a sailor could be thrown across the cockpit at 12 ft/second by a breaking wave, or even faster. Add the force of the wave, the tether can easily break and the sailor is never found. In each case I read (more than the two I copied below) the sailor was attached to a fixed u-bolt and not a jackline (the jackline would have absorbed energy through sliding and stretch). Without the jackline stretch, the sailor is stopped in about 6 inches and subjected to a peak deceleration of about 24 gavities. In independent testing many harness and tethers fail and all show evidense of trama, since the impact force is 2,500-4,500 pounds. A person cannot survive this without serious spine or rib injury. It seems clear that fixed-point anchors are the hazard.
• "A lesson in harness and tether construction can be learned from Tami Ashcraft, who along with a sailor friend was making a sailboat delivery to Hawaii when they ran into a powerful storm. Ashcraft was knocked unconscious. When she came to more than 24 hours later, she climbed to the cockpit where a single tether dangled over the side. The D-ring had snapped where it was connected to her friend's PFD. He was gone. Ashcraft was convinced a round ring might have held, like those on Mustang vests." Ashcroft, in my opinion, was wrong in her conclusions.
• 1998 Sydney-Hobart Race Accident. Glyn Charles was never found, after his tether parted. But the force of dragging his body through the water could not have generated that kind of force, only a sudden impact. The inquest stated that he was attached to a "fixed point" and thus would not have benefited from jack line shock absorption. The lanyard may have also been defective, though I can't locate the information. http://www.parliament.nsw.gov.au/Prod/parlment/hansart.nsf/V3Key/LA20010307027.
I don't think the tethers failed due to bad construction; I think they were simply stressed past 4500 pounds and broke. Making them strong wouln't help, as they would simply break the sailor in half. We need to make them softer.
Well, at least that's one possible answer, something very simple that would only need be deployed in extreme conditions. ISAF now requires an over-strain indicating flag instead--a half measure which only warns that you nearly died, instead of doing something about by absorbing shock. This is much how climbers originally use Screamers; other applications came later.