Generally we use a mild soap and hand wash the ropes in warm or room temp water, and let them air dry in the sun. Remember that these ropes come from the fishing industry so they are out in the sun for long periods at a time. I have seen ropes destroyed from being put in a washing machine, so do this at your own risk. A trick for washing winch ropes is to push them together and get all of the grit out from between the fibers, this will help to extend the lifetime of the rope!
-Alex