Buoyancy is a binary rope property decided entirely by the fiber’s specific gravity — and for some jobs it is the first filter, before strength or price.
Float or sink
| Material | Specific gravity | On water |
|---|---|---|
| Polypropylene (PP) | ~0.91 | Floats |
| Polyethylene (PE) | ~0.95–0.97 | Floats |
| Nylon | ~1.14 | Sinks |
| Polyester | ~1.38 | Sinks |
| Cotton | ~1.5 (absorbs water) | Sinks |
When floating is the point
- Water-ski, tow and rescue throw lines — a sinking line fouls propellers and disappears. The standard answer is PP hollow braided rope: floats, splices in minutes, torque-free. See the rescue rope buying guide for throw-line specifics.
- Buoy and marker lines — floating PP keeps lines visible and accessible on the surface.
- Aquaculture and fishing — PE twisted rope and twine float, resist rot and stay flexible in cold water.
- Large mooring lines — PP 8-strand plaited mooring rope floats, which simplifies handling and retrieval around the vessel.
When sinking is actually better
Anchor rodes and mooring systems often want a sinking line: nylon’s weight and catenary help the anchor set and absorb surge. See the marine rope buying guide for how floating vs sinking interacts with stretch in real mooring decisions.
Practical note for RFQs
If floating matters, say so explicitly — “rope must float” is a clearer requirement than naming a material, because the supplier can also propose composite options (e.g. a floating PP core with a tougher polyester cover). Build the rest of the specification with the RFQ guide.