If you have a 2012 or earlier Mirage Drive, there is a hole in the front to accept a "pitot tube" speedometer. This is a very cool devise -- it displays relative water speed. Used in conjunction with a GPS, you can detect tides and currents, debris on your fins, and get instant feedback on various technique effects on speed. For many years, Hobie included them with each new boat. Here's where it goes on the spines that have the capability:
It fits like this:
The faster the speed, the higher the little red ball rises along the indexed tube. It looks like this in action:
These older spines are no longer available, so the pitot tubes have recently been discontinued. There are still a few of these speedos left in stock though, so if anyone wants one, don't wait. I just got a spare in case the original gets lost or broken.