The thump mat was used on sailing boats to protect the deck from the movement of rigging blocks attached to the deck.
To make the large sisal mat (6" o.d.) I started with 12' of 1/4" rope. After tightening there about 5" left over to trim.
I will use the cotton mat as a coaster.
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I started with 8' of 3/16" cotton cord, folded in half
Bring the upper half around to make a loop . . .
. . .and down across the loop
Bring the same end up over two strands and under one.
Bring the end down over two strands
Bring the left hand strand over to the right
and back to the left going Under, Over, Under, Over, Under, Over to complete the knot
Begin to double the knot by taking that same strand and follow the path of the right hand strand until you reach the end of the cord.
Take the remaining strand and follow the path around in the other direction.
There is a short bit between the two ends that needs to be completed
Begin to take the slack out of the mat by pulling on the strands in either direction. This will take three goes around to take slack out of each of the three plies
When the tightening is complete (there are three plies all around the mat) tie a constrictor knot to lock each end to its adjacent strand. Trim the ends