What Kind of Dirt?
Probably the first question is what kind of "dirt" it there? Is the dirt principally from dirty air, like diesel from jet aircraft or smog, or is it from mold and mildew? If the dirt includes mold or mildew then don't use detergent because it will actually provide food for more growth. This goes for not only the lines but also for the dodger and sail covers.Non-detergents include Ivory flakes and stuff like Dr. Bronner's Pure-Castile Soap (www.drbronner.com).The other option, especially for dock lines, is to use a pressure washer. I've used a Karcher 1250 psi washer for about eight years and it works really good, especially for the really recalcitrant stuff. If the soil is from hydrocarbons then pure water will probably not work as well.The Northwest is great for mildew growth and normally I take the summer dock lines off and put on the winter lines, and remove the dodger, and sails. Even with these preventative measures I still have to fight mildew growth.If mold and mildew is a problem then I'd be on the lookout for it developing on the inside as well. Last year we bought a high-temperature (and high priced) Lady Bug steam cleaner and this did really well, especially on the liner and gel coat.Good luck in your battle!