Q:

How to attach a shackle to vertical square tubes by using a rope?

I want to hang up a hammock.
There are vertical square tubes made of galvanized steel of 10cm x 10cm.
I want to hang up my hammock by using ropes (polyester, ⌀6mm, 9500N) and shackles.
The rope should remain attached to the square tubes. The shackles are for fast and easy mounting and dismounting of the hammock.
I also bought some thimbles.

What is the best knot or method to attach the rope to the square tubes. With:

Preferably highest knot strength.
No slipping down of the rope, since the square tubes are vertical.
The square tubes must not be modified. No drilling, etc.

What is the best knot or method to attach the Rope to the shackle?

Using a thimble?
Using a rope loop instead of a rope ending to increase the holding force?
If yes: How to mount the thimble to the rope loop?

I am generally able to hang up the hammock. What I am asking and interested in is what would be the best knot or method to do so. Taking into account knot strengths and other factors about knots and ropes I have no clue about.

A:

Coming from a climbing background, I would propose the following:

Tie the cord/rope into a loop, for example using the One-sided overhand bend or the double fisherman's knot
Secure it around the pole using a Prusik knot. Note that the english wikipedia entry is only about climbing; the german version also mentions that this friction knot can be used e.g. to drive stakes into the ground (and has some images of that, section "Heim und Garten"). So I would assume that this knot would work for you, maybe with a few more loops to make it "grab" the tube stronger if it slips down when you load it.
Put the shackle into the remaining loop of cord of the Prusik.

I haven't yet tied a Prusik around a square tube, but I would assume that it should hold well enough. The upsides of this method is that the rope would easily be removable (just in case), and that you can move it along the tube as long as it is not loaded.

