So my dock lines are nasty and need cleaning. What is the best method? Use bleach?
Thanks
Rope manufacturers have lots to say on this subject. They are all pretty conservative about what you can do to clean without weakening your rope. For the first few years, they advise a simple soak in water for a few hours, and then rinse because the individual fibers of your line still have coatings the manufacturer used to make the rope more flexible and reduce friction. Soaking does not remove the coatings but does dissolve the salts and chemicals that make ropes stiff. Many of them talk about soaking in a bucket then rinsing with a hose. When that doesn't work anymore, they talk about using moderate amounts of MILD detergents -- Woolite or a gentle laundry detergent at half-strength. Don't be concerned so much about looks. The fact is that mildew and algae don't weaken synthetic ropes -- they just look ugly -- but the salts and chemical increase friction and wear, shortening the rope's life if left in place.
Once clean, sane doses of fabric softener can improve flexibility, leaving ropes easier to handle. However, it doesn't make much difference when a rope is near end of life.
Both strong acids and strong bases are a bad idea all around. A high molar solution of Muriatic acid will melt a nylon rope in a few minutes. Soaking polyester ropes for an hour in weak acid cleaners such as Lime Away or Shower Power reduces a rope's strength by as much as 50%, according to Practical Sailor. Since cleaning green stains from your ropes is for looks, why do it since the stuff that does so inevitably damages the rope?
As to cleaning techniques, a plunger and bucket is good. Hot water at household temperatures is fine and will not damage rope, as it is usually below 135F. I soak my ropes for a day in the bath tub and sometimes add Woolite late in the soak, then either use a plunger and bucket or use the soak and rinse cycle of my washer, with no spin. I then hang them in the foyer from the 2nd floor banister (out over tile) and let them drip dry. If this hand-powered action is too much for you, you can use an automatic washer on its gentlest cycle, but unless you coil the rope, daisy chain it, or put the rope in a mesh bag or a pillow case that you can close, you get a tangled mess; worse, sometimes a section in the middle of double braided line will unlay, making that section useless and forcing you to cut the rope. Also, if you don't constrain the rope's movement in some way as described above, you it can break your washer when it wraps and binds the agitator (ignore this part if you have a modern machine with no agitator).
Never put your rope in a dryer! Heat and tangles will not help your cause.
If you can't hang your, fake the line loosely on a flat surface for drying. Don't leave it in a bite -- I like the Navy's shipboard faking method myself, either figure 8 or long fake -- as they maximize exposure, minimize ground used, and prevent getting your line in a bite. It will be easy to coil when it is dry. See page 7-13 in the link below.
http://www.navybmr.com/study material/NAVEDTRA 14325/14325_ch7.pdf