There are a number of ways to clean sails. Some are more effective than others. Unfortunately, the most thorough methods also cause the most serious cloth breakdown which leads to greater stretch and, thus, a poorly setting sail. All mechanical methods fall into this category. Even large agitating tubs designed just for sails with carefully regulated water temperature will have the same effect on the sail as several weeks of hard use.
Most sailmakers recommend frequent rinsing with fresh water.
The latter is an enzyme detergent that can actually be used to soak away dirt and some stains. Cleaner's benzine used to be used here, but it is evidently rather dangerous and, as a result, very hard to obtain. Paint thinner and gasoline also work although they will leave an oil stain in some cases. My favorite is a proprietary product called GooGone that will not harm the fabric or stain it. It is available in most hardware stores and, of course, from Sailrite.