This article describes the design and implementation of a navigation and control system for the autonomous sailing vessel Avalon. This boat, designed for participating in the Microtransat, is engineered to autonomously cross the Atlantic Ocean. We present here a special robust mechanical design and the navigation software that will plan an optimal navigation course and efficiently control the boat on this course. The path planner uses an A* algorithm to generate the fastest path to a given destination. It is able to avoid both static and dynamic obstacles. By using a given polar diagram and measured wind data, it takes into account maneuverability constraints of a sailboat. The control system takes care of rudder and sail to follow a given heading while optimizing speed. Additionally, the system is capable of automatically conducting maneuvers such as tack and jibe. At the time of this writing, Avalon and its software systems have been successfully tested more than two weeks in short deployments lasting several hours with winds ranging from zero to 30 kn.