It is not about being completely green. It is about trying to reduce carbon foot print as much as you can. Once a sailboat is produced the boat is greener than most vehicle racing sports.

Below is a link to an auto racing series trying to reduce their carbon footprint. By the simple act of reducing their carbon footprint, they are concidered greener.

http://americanlemans.com/index_green.php

PS: the series is a good development ground for new technologies that will trickle down to production cars which in the long term will help reduce carbon footprint more substantially.