I haven't windsurfed in a few years but this added lots of fun
the speed sensor transmits your speed to a watch that you wear on your wrist (which is perfect so you can see it!!)
the batteries are included
there is a video here and more pics also http://picasaweb.google.com/jimellsworthjr/Speedometer#
comes in a nice case, everything is in good condition
great xmass present
cheers,
Jim