Flying an undeclared oversized jib as a spinnaker
There are different definitions for headsails and spinnakers. Generally, the mid-girth (the distance between the half-leach to the half-luff) needs to be less than half the foot or LP dimension, ensuring straight or concave leading and trailing edges. A spinnaker needs to have its mid girth at least 75% of the foot dimension, ensuring that at least one of the vertical edges will be very convex, or roached. The sail''s attachment (or not) to the stay is usually not relevant.