A better approach would be to define your own line type rope and assign it an already defined linetype (like 'border'). [This is quite easy but I'd have to look it up -obviously not so easy as I would have done it already! - someone else may have it on the tip of their tongue]
This would allow you to change all the 'ropes' at some future time just by adding the symbol definition - when you could find time to make a line rope definition to your liking.
This would be currently the best option. In the future there should be a special symbol for ropes. If you are ready to experiment with metapost code and user defined symbols, there is already a preliminary symbol for ropes in elevation maps (imitating freely hanging rope) in therion sources (mpost/thLine.mp -> l_rope_SKBB). martin