An innovative pressure system was used at the Yacht Research Unit's Twisted Flow Wind Tunnel (University of Auckland) to test three asymmetrical spinnakers. The sails were designed for the most recent America's Cup Rule (AC33) and tested on a large-scale model. Force measurements were used to determine the sail characteristics, optimum apparent wind angles and resulting heel angles. Pressures were firstly measured on 5 chord-wise sections with 11 pressure taps on each section, which enabled mapping of the pressure on the sail surface. Measurements were performed between apparent wind angles of 40° and 70° and heel angles between 0° and 20°. The pressure measurements are discussed and related to the flow field around the sails. In particular the pressure on the leeward side of the asymmetric spinnaker is correlated to the leading edge separation and reattachment, and to the trailing edge separation. Subsequently 34 pressure taps were used to measure the pressure on a single section of the asymmetric spinnaker. This allowed an investigation of the effect of the sail trim on the resulting pressure distribution. The results verified that the three sails were suited for their intended design purpose. Over-trimming to reduce luff flapping was also investigated. It was found to reduce both drag and rolling moment. Further successive over-trimming showed a reduction in the leeward suction on the spinnaker, with the pressure distribution becoming more uniform as the flow became more separated. © 2009: The Royal Institution of Naval Architects.