Understanding Rope Materials: Nylon, Polyester, Dyneema

Cordage marketing is loud and the underlying engineering is simple. Almost every line you will meet on a boat is one of four fibres, and each one is a bundle of trade-offs. Understand the trade-offs and you can pick a line from the specification sheet without any help.

The four fibres

Nylon — the shock absorber

A polyamide, most often nylon 6 or 6.6. It stretches further than anything else in common use, in the region of fifteen to twenty-five per cent at a working load, and it recovers most of that when the load comes off. It is also tough, with excellent abrasion resistance, and it absorbs energy better than any other fibre here.

The costs are real. It loses around ten per cent of its strength when wet, it degrades under ultraviolet light, and it is denser than water so it sinks. Where it belongs: anchor rodes, dock lines, moorings, anything where a snatch load has to be absorbed.

Polyester — the balanced choice

Polyester, sold as Dacron or Terylene, is the middle of the range and the default for most working lines. It stretches roughly eight to twelve per cent at working load, holds its strength when wet, and is the most ultraviolet-resistant of the common fibres. It is also the most resistant to the slow chemical degradation that shortens nylon's life in a marine environment.

Where it belongs: sheets, halyards on a cruising boat, control lines, general rigging. Where it does not: anything that has to absorb a shock, because it has almost no give.

Dyneema — the strong one

Dyneema is ultra-high-molecular-weight polyethylene, or HMPE. By weight it is around fifteen times stronger than steel, and it stretches only three to five per cent at a working load. It floats. For halyards on a boat where a slack luff is unacceptable, and for any application where weight aloft matters, nothing else comes close.

The catches are significant. HMPE creeps under sustained load, so it slowly lengthens if you leave a permanent tension on it. It has a low melting point, so a line running fast through a clutch can heat and fail. It is slippery, so knots are unreliable in it. And it degrades under ultraviolet light unless it has a cover.

Manila — the traditional one

Natural abaca fibre. It has a good grip, it will not glaze or melt, and it looks right on a traditional boat. It is also the weakest and shortest-lived line in this group: it absorbs water, loses about twenty per cent of its strength wet, and then rots from the inside where you cannot see it.

The comparison that matters

Construction matters as much as fibre

Two lines of the same fibre and the same diameter can behave differently depending on how they are built. A three-strand twisted line is cheap, easy to splice and holds knots well, but it is stiff and it kinks. A double braid has a braided cover over a braided core: the cover takes the abrasion, the core takes the load, and the result is a line that is pleasant to handle and can be spliced to near its full strength.

Twelve-strand constructions are used for HMPE, because the fibre is too slippery to hold in a twisted or braided structure. That is also why a twelve-strand line must be spliced rather than knotted — the structure relies on the splice to lock it.