I am still pretty much a beginner...but I would probably use a thin bead of royal icing....I find Royal icing holds things very well.
I did a cake similar I just used water to attach the waves and flowers you can see it in my photosGOOD LUCK!
Water should work fine! Just use a paintbrush and put a line of water on the back edge of the rope and attach to the cake. You may have to do a little at a time and hold it in place for a few seconds for it to adhere.
Antonia74's icing will hold a freighter in place, if you make it the right consistency. Good luck!
make a fresh batch of tylose glue.. that will hold it.. or melt your fondant with the water.. let it be pretty thick.. that will hold with no problem..hth
As you can see from all the responses, you can do it several different ways. The way I would do is using water. I would lay damp paper towels down and lay the ropes on that. Then when you pick them up they will be very sticky on the back and will stick right to it.