This sailing vessel testing, data collection and analysis project examined the real world potential for a novel carbon-fiber wingsail technology to reduce fuel use in potential passenger ferryboat applications. The project involved building a carbon fiber, computer-controlled wingsail that was then mounted on a 14-meter trimaran test vessel with a complete instrumentation package. The vessel was then operated on the San Francisco Bay over a three-month period. The test results were conclusive that, for a test vessel traveling at seven knots though water, up to 25 to 40 percent of the fuel burned can be saved through the use of the wingsail, depending on wind speed, with a corresponding reduction in greenhouse gases, toxic and criteria pollutant emissions, and fuel costs. The estimated fuel efficiency gains will not necessarily translate directly to those at actual ferryboat service speeds (at 17 or more knots) but are encouraging pending further investigations.