This is the Poly-Tube that the steel skeg cable slides in. It is about 8' long which is longer than needed and you will need to cut it to fit your kayak - use a "tube cutter" to make a clean, square cut. You can buy tube cutters on HarborFreight.com for about $10.

If you are replacing this tube you will also need to replace the compression fitting (or at least the brass ring piece of the compression fitting) at the back end of the boat (where the poly-tube connects to the skeg box). You will need two 13mm open end wrenches to tighten this compression fitting (one wrench to hold the brass fitting that is screwed into the skeg box and the second wrench to tighten the nut part of the fitting that is on the poly-tube). When tightened, the brass ring is permanently compressed onto the poly-tube (so the old ring can't be re-used).

Put the front end (the controller end) of the skeg system together (minus the cable) before cutting this poly-tube and cinching the compression fitting at the skeg box end.

For the newer models such as Etain, Gemini, Sirona which have a black plastic control block with four mounting screws at the front end:

To assemble the front end, you will need to temporarily use a separate compression fitting to permanently compress the brass ring on the poly tube about 3/8" from the end of the poly-tube; then remove both pieces of the compression fitting leaving just the brass ring on the tube. The brass ring at the front end of the poly-tube is what the set screw on the control block uses to lock the poly-tube into the controller so it won't slip out of the controller.  The set screw at the aft end of the controller should should just touch the poly tube (don't clamp it hard on the poly-tube or there will be too much friction for the skeg to work) and be just aft of the brass ring. So when assembled you can pull on the poly-tube but it won't come apart because the setscrew and brass ring locks the poly-tube into the controller.

Once the front end of the poly-tube is assembled in the kayak you can run the poly-tube through the bulkheads and back to the skeg box where you will cut it to length and slip the female half of the compression fitting and a new brass ring on the poly-tube before assembling it into the other half of the compression fitting at the top of the skeg box. Before putting the poly-tube into the fitting on the top of the skeg box be sure about 1/4" of the poly-tube is sticking out past the brass ring.