Aaaaa...sorry - I missed that it was a Prindle. The F18 masts are roughly around 29'6" so you would likely be OK with an F18 spinnaker by making sure your spinnaker pole height was correct.

The systems use one line as a "belt" that goes around the perimiter of the mast. A second line, placed vertically, goes to an eye strap above the "belt" and down through it where it shortly ties to a turning block. The halyard runs through this turning block. The block swings like a pendulum from the vertical line while the "belt" line keeps it constrained close to the mast allowing the spinnaker hoist point to rotate around the mast but stay close to it.