Measure the speed of a motor

In order to design the propeller for my 3D printed RC boat (coming soon!) I had to figure out the performance characteristics of some Johnson motors I bought on Ebay so I designed this little device.

The first 3D printed part is the mount where you fit the Johnson or 540 type electric motor and a photodetector. The second 3D printed part is a rotor for the motor that breaks the beam twice per revolution. When connected to an Arduino this makes it easy to measure the relationships between curr