It looks so deceptively simple, it's almost easy to overlook. Who would guess this "tube" is a creative adaptation of one of the most fundamental aircraft instruments -- the pitot tube speedometer?
Though not necessarily accurate, it is extremely consistent for your hull and weight, and can be calibrated with a GPS. But unlike GPS, it gives instaneous readings.
Regardless, whether you're working to improve your sailing technique, paddling style or pedaling speed, you can use this tool as a training device. Here are a few examples:
When sailing, used in conjunction with your "telltales", it helps you adjust the sail position that yields the most speed on any given tack. If you're beating into the wind, it helps you recognize if you're pinching (heading up a little too high into the wind).
Paddling, you can use it to refine your stroke pattern, reach and blade angle. As you experiment you get instant feedback.
Pedaling, it's great when traveling -- the more constant your speed, the faster you get to your destination. With a little experience, you can also use it to tell you how the wind is affecting your speed. Whether to push and pull on the pedals or push only -- your speedo will tell you!
Unlike GPS, it ignores tide and current and lets you focus on your actual performance against the water.
Like everything, it should be rinsed and cleaned occasionally so it doesn't get clogged up, especially in salt water. If you're having trouble seeing the marks, you can enhance them with a Sharpie permanent marker.
So don't leave home without it. Once you get used to using it, it will really help sharpen your skills!