What rope is made of

Natural fibres were the only choice until the middle of the twentieth century. Today most working rope is synthetic: stronger for its size, rot-proof, and available with exactly the stretch a job needs.

Fibre	Type	Stretch	In water	Sunlight	Holds knots	Notes
Manila	Natural	Low	Absorbs water	Fair	Good	Stiff and rough on hands. Rots if stored wet.
Sisal	Natural	Low	Absorbs water	Fair	Good	Cheaper and weaker than manila. Garden and craft use.
Cotton	Natural	Moderate	Absorbs water	Poor	Very good	Soft and weak. Decorative work, sash cord.
Hemp	Natural	Low	Absorbs water	Fair	Good	Strong for a natural fibre. Rots without care.
Nylon (polyamide)	Synthetic	High	Sinks	Good	Good	Stretches to absorb shock. Somewhat weaker when wet.
Polyester	Synthetic	Low	Sinks	Very good	Good	The all-rounder for boats: low stretch, wears well.
Polypropylene	Synthetic	Moderate	Floats	Poor	Fair	Cheap and light. Melts easily; sunlight degrades it.
HMPE	Synthetic	Very low	Floats	Good	Poor	Very strong for its weight but slippery: knots can slip, so it is usually spliced.
Aramid	Synthetic	Very low	Sinks	Fair	Fair	Very strong and heat resistant. Weakened by repeated sharp bending; usually covered.

General guide only. Ropes of the same fibre differ by maker, construction and coating; check the maker’s data for loads.

Choosing

Shock loads (towing, anchoring, climbing falls): a rope that stretches, usually nylon.
Holding a position (sheets, halyards, guy lines): low stretch, usually polyester.
On or in the water (rescue throw lines, water-ski tow ropes): a rope that floats, usually polypropylene.
Craft and decorative knots: cotton or soft braided polyester, which take a shape and hold it.

Strength figures on a label are for new rope pulled straight. Knots, bends round small objects, wear, sunlight and age all reduce the strength you can actually use, which is why working loads are set well below breaking loads.