This tightly-woven, oval-shaped mat is called the “ocean plait” (ABOK # 2243). (A plait is another word for a braid.) It’s on page 113 of of Geoffrey Budworth’s book “Complete Book of Decorative Knots” (ISBN 9781558217911). I used about 50 feet of rope — about 16 meters — and it took about an hour to tie. Most of the time is spent hauling the length of the rope through, over and over, and then “dressing” the knot at the end to look even and smooth.
The more rope used in the mat, the longer it takes to pull through — but the larger the final mat will be. Multiple turns along the same route will close up the holes and make the mat heavier and stiffer. Notice that all the openings are closed up in this one; it weighs a lot and is pretty rigid.
This is a flattened Turk’s Head knot; I first found it on page 69 of a document from the International Guild of Knot Tyers (IGKT)
(original drawing by Eric Franklin). It looks like a star, and would be really fun using a few different colors of rope.
I tied it by following the very simple instructions on page 112 of Geoffrey Budworth’s book “Complete Book of Decorative Knots” (ISBN 9781558217911). The diagrams are about the same, but Budworth’s are a little easier to read.
The finished piece is almost two feet across, and used about 50 feet (20 meters) of rope. There’s a gap in the middle that actually can’t be closed up because of the geometry. At sea, this mat could be tied around a ship’s mast to provide traction or decoration.