There isn't anything built-in to take the sail's area into account, but you can write a custom plugin for the boat's dynamics. There is an example here that controls a boat's dynamics based on its size and other properties https://github.com/osrf/vrx/blob/master/usv_gazebo_plugins/src/usv_gazebo_dynamics_plugin.cc

The parameters are passed in here https://github.com/osrf/vrx/blob/master/wamv_gazebo/urdf/dynamics/wamv_gazebo_dynamics_plugin.xacro Another set of parameters for a different vehicle https://github.com/osrf/vorc/blob/master/cora_description/urdf/cora_dynamics_plugin.xacro

Another example to look at is the LiftDragPlugin for aerodynamics http://gazebosim.org/tutorials?tut=aerodynamics&cat=physics