Thanks guys!
The rope is a piece of string, of which I once bought a spool in a modelling shop, from their section for wooden ship modelling. Must have been one of the best modelling investments I have ever done. It's color is just like real rope, it needs no paint and responds to weathering real well, it doesn't fray or peel like cotton string and to top it off, it can even be glued with liquid poly cement!
I have no idea what it is made of, but that one tiny spool will last me a lifetime. I guess if you were to rig a trimaster with it, you'd run out real quick, but since I only use a couple of inches per tank model, I should be okay for a good number of years.