The Flying Double Sheet Bend is the knot to use when you want to extend the tow line for your dinghy, or tie on another length of line to your anchor rode. It is much less likely to loosen itself up as the line gets worked back and forth.
The Sheet Bend joins two lines together, and is particularly effective with lines of different diameter. This knot is actually a bowline, and is tied using the same method as the Flying Bowline. When the jib sheet parts on the way to the weathermark, this knot will help you tie it back together in a hurry.