Does anyone have experience tying knots using polypropylene rope? I'm concerned that just any knot might come untied due to the slipperiness of the rope. My application is to create "tethers" to hold aeration fountains in place on a lake. One end connects to the fountain by a carabiner while the other end goes around a cement block at the bottom of the lake (there will be two tethers and two blocks for each fountain). Any help will be appreciated.

No recommendations on special knots, but I'll give you two hints. First, where you run the rope through the concrete block -- cut some old sections of garden hose and run the rope through that first and let the garden hose come into contact with the cement block. If you don't, the cement block will quickly cut through the rope from wave action. Second, when you tie your knots, leave a 1-2 inch tag end and then melt the end of the rope with a lighter leaving a large blob of melted plastic at the end of the rope. This will help to keep it from pulling through the knot and coming untied.

Hoss

Thanks, CVHoss. I will definitely use the hose concept.

there are only a few knots that will hold on polypropylene line or rope as a tow boater the only knot i would use is a Bowline
The bowline has been called the king of knots. It will never slip or jam if properly
made and, thus, is excellent for tying around a person in a rescue. Begin by formatting
an overhand loop in the standing part. Then take the free end up through the eye, around
the standing part and back where it came from.
http://www.troop7.org/Knots/Images/Bowline.gif