Building a more-efficient water propeller?
The LEGO RC boat by Slewentogzz claims to have a top speed of 3 km/h, it uses a custom propellor built out of Technic parts.

The actual rudder parts appear to be Technic Panels #87080 and #87086. These panels are very common and are available in over 60 sets.

There are two more parts in a similar design (#64683 and #64391, they are one stud longer each so you can make a slightly larger propellor.







Images: Slewentogzz Lego Creations

Video: "Lego RC Boat with custom built propeller (3,0 km/h!)"