Technora Elastic Ropes 

Sandows (elastic ropes, bungee ropes, shock ropes) are ubiquitous on boats. However, they can be treacherous. A sandow that breaks and/or lashes back is a painful experience.  The problem is that sandows have low rupture loads, e.g., 10 to 60 kg.

The sleeve always breaks first, at elongations between 75% and 200%.  The sleeve is made of polypropylene, polyester, polyamide, cotton and/or polyethylene, i.e., low resistance fibers. The rubber core is not a problem: it holds up to 700% elongation. 

Solution: Technora sleeve and rubber coreTechnora(r) is a brand of Aramid  like Kevlar(r), Nomex(r). Aramid is a heat-resistant, high modulus fiber, with low elongation and high rupture load.   A Technora elastic rope is 3 to 11 times more resistant than a common sandow. 

Technora elastic rope is as resistant as a non-elastic rope made of Kevlar and/or Dyneema. (indeed the equivalent rope is thinner, because it contains no rubber core). Compare:  

2.7mm Technora elastic rope breaks at  95-120 kg, 2mm polyester sandow: 8-10 kg
4mm Technora elastic rope breaks ~170 kg, 4mm polyester sandow:  45- 60 kg.

Technora-rubber rope 2.7mm. Rupture load 95 to 120 kg.  Developed specially for the battens project

Hanging from a 2.7 mm Technora elastic rope

Double sleeveFor specific applications, the Technora is protected by an additional sleeve, polyester-Technora for UV-exposure, polyethylene-Technora for immersion in sea water.

Left: polyethylene-Technora 8mm sandow. Center: polyester Technora 6mm. Right: plain Technora 6mm.

With higher diameters (4mm to 8mm) Technora elastic rope stands loads between 160 and 280 kg. Its resistance is further multiplied by forming loops and/or torsades.

Left: 4 loops of 6mm Technora-polyester elastic rope.  Right: torsade of 6mm Technora polyester elastic rope. 
Both have rupture load above 2.000kg. 

Static vs. dynamic rupture load. In real conditions, a rope has to absorb impacts. The more elastic a rope is, the better it absorbs the energy of impact.  There are 3 types of ropes depending of their elasticity: static, semi-static, dynamic. 

For instance, with a mass M  tied to a rope and falling from a height H, the impact transmits an energy MgH to the rope. A static rope (e.g., dyneema) must absorb this energy almost instantaneously when the rope reaches 100% of its nominal length. The (static) rupture load Fs must be much higher than M to stand the impact.

A dynamic rope with 10% elongation (e.g.,  polyester) absorbs the energy progressively, when the rope stretches between 100% and 110% of its nominal length. For a given (static) rupture load, a dynamic rope has a dynamic rupture load much higher than a static rope.

Technora  elastic is 'hyper dynamic'. Because its length  increases much more than 10%  it has a very high dynamic rupture load. Look at the following photos.

  + 163 kg concrete block
  + 2m static rope (halyard 12mm, prestretched polyester)
  + 2.20m free fall


result: no problem, see photo below. Compare with a 10mm stainless steel snaphook after the same test.

After a 160kg concrete block falling from 2.20m including 2m of  static rope. 
Left: loops of 6mm Technora polyester sandow. Right: 10mm AISI 316 snaphook.

Fixations. Technora elastic rope requires special fixations.

The terminals for common sandows are too weak  for loads above 100 kg.  Metal hooks, metal pods to crimp or screw, etc. just slip and/or break.

Be careful with knots: under traction, sandow rope thinners and slips out. For instance  bowline knot, cleat hitch, clove hitch, constrictor... all these knots tend to slip.

Sewing or punching holes weakens the elastic rope. In general, it is difficult to insert something in an elastic rope, because the rubber core fills completely the sleeve. For example, a sewing machine tends to jam because of the rubber core.

Knots that hold (and do not rest too much resistance) multiple loops and additional stopping knot on the loose threads; double eight knots, triple fisherman's knot (grapevine), 

Knots that work on Technora elastic rope. Left: double eight with stopper knots. 
Right: fisherman's knot with at least 3 loops and stopper knots.Center: detail of Fisherman knot.

On ring: anchor hitch with at least 3 loops, stopping knot on loose thread (no need of additional half-turns).
Anchor hitch for Technora elastic rope, at least 3 loops and stopper knot. 

On chain: chain splicing, loops on a chain link, always with stopping knot on loose thread.
Left: Technora polyethylene elastic rope, chain splicing. Right: Technora polyester elastic rope, chainlink loops.

Universal fixation splicing on a dyneema braided rope (hollow braid).  Insert elastic rope in dyneema braid (e.g., 30cm) and make a knot at 1/3 of the length.  

The splicing preserves 85% of the sandow resistance and dyneema braid is easy to secure (knot, rigid eye, spliced loop...). 

Splicing Technora elastic rope - dyneema braided rope. Use a splicing needle. Simple knot works, but height knot and nine knot are even stronger.

Terrminals. Technora elastic rope tends to fray. Heat-cut (that would melt the extremity) does not work with aramid fibers: you have to use good scissors.

Solution : crimped anodized aluminium tube (standard diameters 6mm, 8mm and 10mm with 1mm thick wall work for  4mm 6mm and 8mm elastic ropes, respectively)
Aluminium tube, crimped.

Other solutions: marine adhesive tape (e.g., adhesive fabric for sails), thermoretractable tube.
Left: adhesive fabric. Right: thermoretractable. Enlarge images for details.

Combining Technora elastic and non-elastic ropes. For punctual uses, you can braid Technora elastic rope and non-elastic rope and/or stuff a braided non-elastic rope with Technora elastic rope.

Problem. it takes an unreasonable time to produce significant lengths.

Shock absorber for mooring. Braid 3 threads of Technora elastic rope in a braided mooring line. Absorbs shocks but the elasticity and elongation are limited and difficult to control precisely.

Braided Technora elastic rope and mooring rope (on photo: polyester rope/elastic).

Anti-tangling rope. Stuff a Technora elastic rope in a braided dyneema rope, stretch the elastic and knot the extremities. Problem: the dyneema braid is loose in its short configuration, which makes it vulnerable to scratches and cuts.
Stuffed dyneema braided rope. A length of Technora elastic rope is inserted in the braid (use splicing needle), pre-stretched and knotted at extremities. Enlarge leftmost photo for details.

Boating applications.  Technora elastic rope is suitable for all the applications of common sandows.  Technora elastic rope is stronger, lasts longer, therefore it is safer

More important, the resistance of Technora elastic rope opens new applications, indoor, outdoor (with polyester sleeve and/or any other form of protection) and immersed (with polyethylene sleeve).

For instance, tensors for lifelines and jack lines (2.000 kg rupture loads), shock absorbers for individual safety devices, tensors to remove the slack of rigging, small shrouds, etc.
Shroud of wind generator rotating mast. 2 loops of Technora-polyester elastic, 
stainless steel rings & snaphooks and dyneema ropes.


Design 4 Sea is a registered trademark.  Disclaimer, Terms of use, Credits and Copyrights