Knots & Hitches
Where the rope gives up
Rope weak points & knot efficiency
Every knot, bend and sheave takes strength out of the line. Here is roughly how much, and what else quietly destroys a climbing or rigging rope.
Knot strength reduction
A knot fails the rope by bending fibres around each other, so the strands on the inside of the curve carry far more load than the rest. Published figures vary by rope construction and how well the knot is dressed — use them to compare choices, not to calculate a safe load.
| Knot | Approx. strength loss | Remaining strength | Notes |
|---|---|---|---|
| Bowline | 30–35% | ~65–70% | Easy to untie after loading; can shake loose — finish it (Yosemite or stopper). |
| Running bowline | 30–35% | ~65–70% | Same loss as the bowline, plus it cinches on the wood. |
| Figure 8 loop / bend | 20–25% | ~75–80% | The strongest common terminal knot; bulky but easy to inspect. |
| Timber hitch | 30–35% | ~65–70% | Holds only under steady pull in one direction; back it up for rigging. |
| Clove hitch | ~40% | ~60% | Slips under variable load; never a standalone life-support tie-in. |
| Cow hitch with a better half (toggle) | ~30% | ~70% | Girth hitch plus a half hitch and toggle — releasable under load, popular for rigging. |
| Anchor / buntline style terminations | 20–30% | ~70–80% | Depends heavily on dressing and setting. |
| Spliced eye (professional) | 5–10% | ~90–95% | Why factory-spliced slings beat knotted ones. |
Ranges are typical published test results across rope types. Dressing and setting the knot properly matters as much as the knot choice.
D:d bend ratio
D:d is the diameter of the thing the rope bends around (D) divided by the rope diameter (d). The tighter the bend, the more strength you lose.
- Aim for 4:1 or better on sheaves, blocks and any hardware the rope runs over under load.
- A 1/2 in rope wants a sheave or bollard at least 2 in across; 4:1 keeps loss in the rough range of 10–20%.
- At 1:1 — rope bent around something its own diameter, like a thin carabiner spine or a hard steel edge — you can lose half the rope's strength.
- Running a rope directly over a branch union or a steel shackle pin is both a bad bend ratio and an abrasion problem. Use a block, a low-friction ring or a friction saver.
- Sling angles work the same way: the tighter the choke over hardware, the higher the real load in the legs.
Rope over rope, rope over edge, rope over a pin — three ways to halve a line.
Heat, friction and glazing
- Friction devices turn load into heat. Polyester and nylon fibres soften well below any visible burn — around 300–350 °F (150–175 °C).
- Glazing — shiny, stiff, fused-looking fibres — means the rope has already been heat damaged. Retire it.
- Long controlled runs on a Port-a-Wrap or bollard are where heat builds: add wraps, share the load across more surface, and let the device cool.
- Never let a rope run across another rope under load. That contact point melts fast and is a classic cut-through failure.
- Descending long lines fast cooks hitch cords first — they are the cheapest part of the system and the first to fail.
Chemical and environmental damage
- Acids, bleach, battery acid and many solvents attack nylon and polyester with no outward sign until the rope breaks.
- Two-stroke fuel, bar oil and hydraulic fluid do not necessarily destroy fibre, but they hide abrasion and hold grit — wash and inspect.
- UV exposure degrades the outer fibres steadily; a rope left on the truck deck all season is older than its use log says.
- Grit worked into the braid grinds fibres from the inside. Wash dirty ropes in cool water, no detergent stronger than mild soap, and air dry out of sunlight.
- Any suspected chemical contact is an automatic retirement. You cannot inspect your way out of it.
Shock load and cycle fatigue
- A shock load is force from a falling or suddenly stopped mass — a short drop can multiply a piece's static weight several times over at the block.
- Ropes have a finite number of high-load cycles. Bend-over-sheave cycling under load is what ages a rigging line, not calendar time.
- Log each severe load event. A rope that has taken a hard catch is no longer the rope on the tag.
- Signs to retire on: flat or mushy spots, core showing, inconsistent diameter, pulled strands, glazing, stiffness.
- Lower with a let-run, not a dead catch, and the whole system — rope, block, sling, union — lasts longer.
Two identical ropes, one used for smooth lowering and one for dead-catching pieces, are not the same age after a season.
Cautions
- Efficiency percentages are approximate test results, not a design standard — never calculate right to the edge of a rope's rated strength.
- Inspect life-support ropes before every climb and retire on any doubt. A rope costs less than a rescue.
- Manufacturer instructions and retirement criteria always override a general chart.