I've always used locking carabiners on the jack line end; non-locking biners can lever off a fixed point or tight rope in any number of ways, and the jackline end gets dragged all over all sorts of edges and fittings. Ice climbers, wearing heavy winter gloves, take advantage of this very mechanism to clip gear in and out without fumbling with the gate. On our cat, we generally leave the tethers attached to the jackline anyway, so a screw-lock biner is safe, dependable, and snag-free. We use climbing biners rather than "marine" biners; their clunky stainless designs bang on the gel coat and their weight makes them more snag-prone, and to me, snags present a safety hazard. We keep them greased.
This style carabiner was invented for use in via ferrata systems, where climbers follow "iron ways" through the mountains. Frequent clipping and unclipping require a dependable locking carabiner suitable for one-handed use. Notice also that it is used with a 2-legged lanyard system, not unlike some sailing tethers, but with a shock absorbing section mandated by UIAA standards, because of the potential for high impact loading. I don't know.... It seems to me the marine deck harness, tether and jackline manufacturers are always following climbing innovation, a few steps behind.
Notes on lubricating biners:
- Waterproof grease is best on screw gates. very dependable.
- A good corrosion blocking spray should be applied to all pivots, and critically, to the spring inside the base of the gate. Be liberal there.
- Cor-Block and Bo-Shield did very well in Practical Sailor wire corrosion testing in a salt environment chamber/accelerated aging test. Many others, like WD-40, are basically useless.
I have several other posts on jacklines, climbing gear for sailors, and the stresses involved. As a life-long sailor, rock climber, and engineer, it's a favorite topic.
What about energy absorbing tethers, as used rock climbing and construction? This study is the best I've found on survivable tether impact loads, and energy absorption.
IF the subject is well secured in a body harness it concludes g-forces over 10 Gs have significant injury potential and forces over 20 Gs are generally fatal. Since 20 Gs ~ 3500 pounds, it seems clear that the sailor will be dead before the harness breaks (cutting over sharp edges excluded) and probably seriously injured at 1,000 pounds, since he is only wearing a chest harness. Much depends on geometry, but the general conclusion is unarguable.
I found the discussion of variable arresting force depending on a persons weight (page 14-15) interesting. To me, this also suggests a sailing tether should have a lower arresting force, perhaps 600 pounds. A lesser total energy absorption capacity is needed, perhaps only 40% of the OSHA figure, so the required extension will not be excessive, perhaps 2 feet, based upon a comparison of industry figures and potential wave impact/falls on a boat (they are looking at a 12-foot fall, we are considering the wave force plus boat free board).
These are not just numbers on a table to me. I've taken thousands of roped falls rock climbing--most top-roping, but other sorts as well--and have a pretty well tuned gut feel for the issue. I have a strong feeling also, that non-climbers have no respect for how dangerous a fall on a chest harness, without shock absorption, really is.





















