In the right conditions, a spinnaker staysail will increase your speed up to three quarters of a knot, without changing your rating one bit. Staysails are free sail area. The spinnaker staysail, also called the Dazy Staysail, is a tall, narrow, high-clewed very light sail used under a spinnaker, symmetrical or asymmetrical.
The sail is intended to have its tack set on the center-line of the foredeck about one quarter to one third of the way back from the headstay to the mast. Its luff length is as great as will fit between that tack position and top of the foretriangle. Its width, or LP, is about 80% or 85% of J. A wider sail would have greater area, but would not be as effective over as great a wind speed range or within as large a range of apparent wind angles.