HMPE, UHMWPE and Dyneema®: What's in Your Rope
How a plastic powder becomes the strongest rope on a boat, what it does well, where it falls short, and why we only sell it from makers we can name.
Three names, one family
People use these words as if they mean the same thing. They don't. Each sits inside the next.
- HMPE: the fibre type, high-modulus polyethylene. Any maker can produce it.
- UHMWPE: the polymer it is spun from, ultra-high-molecular-weight polyethylene.
- Dyneema®: a registered trademark for one maker's HMPE fibre. All Dyneema is HMPE. Not all HMPE is Dyneema.
How the fibre is made
The process is called gel spinning. It starts with the same chemical as a carrier bag and does something very different with it.
- Ethylene becomes polyethylene. The result is UHMWPE powder, with chains around 100,000 carbon atoms long. Ordinary plastic has about 10,000.
- The powder is dissolved and heated in an extruder until it forms a gel.
- The gel is pushed through a spinneret, a plate with hundreds of tiny holes, to form filaments.
- The filaments are drawn. Stretching lines the molecules up along the length of each filament.
- The yarn is wound onto spools ready for braiding.
Why it is so strong: in ordinary plastic the short chains are tangled and slip past each other. In gel-spun fibre the long chains are all aligned, so every chain shares the load.
From yarn to rope
There are four steps. Two of them decide how the rope behaves on the water.
- Braid the core. The yarn is braided into the load-bearing core.
- Heat set. See below.
- Coat. See below.
- Add the cover. A braided jacket protects the core and gives the rope its feel.
Heat setting
New HMPE rope has slack in its braid. Under its first loads it tightens and stretches. Heat setting does that tightening in the factory instead of on your boat. The rope is held under tension at a high temperature, below the point where the fibre would melt, for a set time.
- What it does: removes most of the bedding in and early stretch, and reduces early creep. It does not remove creep.
- What it costs: the rope can end up slightly stiffer.
- Makers do not publish the temperature or time, so compare ropes on their stated stretch figures.
Coating
A coating is a thin polymer finish: typically polyurethane, it is applied in one of two places.
- On the yarn, before braiding. The finish reaches inside the rope.
- On the finished rope, by dipping, then fixing. It sits on the surface.
A coating improves abrasion resistance, UV resistance, water repellence, mould resistance, colour retention, grip, and fatigue life over sheaves.
Trade-off: a heavier coating makes the rope stiffer, and it wears off with use but can be reapplied with basic tools at home.
What the fibre does well, and where it is limited
Strengths
- Best strength-to-weight of any rope fibre.
- Very low stretch.
- Floats (specific gravity about 0.97).
- Excellent UV and abrasion resistance.
Limits
- Low melting point, so friction heat matters.
- Creep: slow stretch under a constant load.
- Slippery. Splice it, do not knot it.
- Needs bedding in unless it has been heat set.
Fibre grades
The grade tells you what is inside the rope. Ask for it by name.
| Grade | What to know |
|---|---|
| SK75 | The earlier grade. Superseded for most marine rope. |
| SK78 | The standard marine grade. Much better creep than SK75. |
| SK99 | About 20% stronger than SK78. At the same percentage of break load it stretches the same; at a fixed load it stretches less. |
| DM20 | Slightly lower strength than SK78, with virtually zero creep. The choice for permanent standing loads. |
Why a named maker matters
Two ropes can look the same and carry the same claim on a listing. What differs is how much you can find out when something goes wrong.
With a named maker, every link in the chain has a name, an address and a paper trail: fibre maker, rope maker, test and certification, retailer, you. With an unbranded import, you often cannot name any of the links before the last one.
Five questions to ask before you buy
- Who made the fibre, and who made the rope?
- Which fibre grade is in it?
- Is the break load a tested figure, and who tested it?
- Is the manufacturer's name and address on the product or listing?
- Who do I contact, and who is responsible, if it fails?
Dont be fooled by very cheap generic HMPE! Know what you are buying before you commit. Its cheaper to buy once and buy well than replace multiple times or suffer damage as a result of breakage.
Origin is not the issue. Reputable fibre makers run plants in several countries, including the USA, the Netherlands, Japan and China. Traceability is the issue.
How Rope44 buys
We always tell you who made it, and we do not stock cheap unbranded rope. Our HMPE comes from two makers we are authorised to sell: Marlow and Gottifredi Maffioli.
Browse Dyneema rope and halyards, or have yours made up with our splicing services.