Propylene rope floats and repels water because of three reasons:
1. Lower surface tension vs. water causes it to be water repellent or hydroscopic.
- polypropylene: 29 dynes / cm2
- fresh water: 72 dynes / cm2
- salt water: 60-65 dynes / cm2
2. Lower specific gravity vs. fresh and salt water means that polypropylene is lighter and will float.
- polypropylene: specific gravity of 0.9
- water: specific gravity of 1
- sea water: specific gravity of > 1
3. The amount of air between the fibers allows air to be trapped between the fibers results in buoyancy. The fiber packing is so dense that the surface energy of the sorbent will not allow water to replace the air that naturally exists between the fibers, thus the sorbent will not become water-logged. The net result is that an oil sorbent will float indefinitely.
The only thing that may cause polypropylene to float lower than the water line is the presence of a contaminant such as a surfactant or emulsifier in the water. Such contaminants might include soap, detergent, alcohol or chemical dispersant. The presence of these surfactant materials would cause the oil to be at least partially soluble in the surrounding water and would change the surface energy enough to allow water to penetrate between the fibers. The polypropylene will not sink (as to the bottom) but it can hold some water in the presence of the surfactants.
That is why it is the rope of choice for water skiing, wake boarding and tubing. Like jssailem said "the headache is just boat fever." Buy a coil of HIGH BREAKING STRENGTH polypropylene tow rope, stow it and go sailing!!