If you make a repair to your boat's engine or gear box, there's a good chance you'll get grease on the exterior of your boat. If your boat has that nice, glossy gelcoat finish on its exterior, the grease will simply wipe off, because the gelcoat is almost impenetrable. If you've done spot repairs, or painted, the problem becomes a bit more complicated, but isn't insoluble. With a few simple items found around the house or your garage, neither is the grease.
Emulsifying Dish Detergent
Dish detergents cut grease and oil because they contain emulsifiers. An emulsifier is "a surface-active agent (as a soap) promoting the formation and stabilization of an emulsion." In other words, it breaks the grease or oil down into small droplets and allows these droplets of grease or oil to mix with water, then be flushed away. Nearly every liquid dish detergent for your kitchen is an emulsifier. One such liquid detergent was used extensively by the veterinary clean-up crews that responded to Prince William Sound, Alaska, to clean up sea birds trapped in the floating sludge that followed the oil spill that occurred when the M/T Exxon Valdez grounded on Bligh Reef in 1989. Apply the undiluted dish washing liquid to the grease. Wait for the grease to emulsify, then wash it away with more liquid detergent and running water.
Automatic Dishwasher Detergent
Automatic dishwasher detergents work well to remove grease from your fiberglass boat, just as they do to remove grease from pots and pans. Wet the area with the grease on it, then sprinkle about 1/4 cup of the detergent onto a wet rag. Then use your elbow grease to remove the grease from your boat. Do not attempt this project while you're boating; if you lose some of the detergent "over the rail," you may face charges for a violation of the Clean Water Act of 1975, among other things.
Laundry detergent, just like you wash your clothes with, also contains emulsifiers that "break the grease down." While not as strong as automatic dishwasher detergent, or even liquid dish detergent, laundry detergent is a contender among products to remove the grease from your boat. Use the laundry detergent in the same manner you use the automatic dishwasher detergent, and use it ashore to avoid a fine for eco-terrorism.