You can see some more information about water striders and the way that they move on water in this YouTube video : Richard Hammond's Invisible Worlds - Water Strider
I originally wanted to use a Windup Motor but it didn't have enough power or enough turns when wound up (it has about 8 turns). I ended up using an M-Motor for propulsion, but the battery box is external and uses a tether to control the water strider. There is no steering in this model.
The mechanism that moves the middle and rear legs uses a modified version of Sariel's Octopod mechanism. The middle legs move in an elliptical motion, and have rubber tips to give them grip and help my water strider move forward when it is 'on land'. When my water strider is in the water, the rubber tips are replaced with 6x6 radar dishes that act as paddles or oars.
To get my water strider to float in water I used the hemispheres from the Death Star Planet set. The hole in the top can be blocked using a 1x1 round plate, and this provided enough floatation to stop my water strider from sinking when it isn't moving.
Unfortunately when my water strider is moving, or when the water is a bit choppy, the hemispheres on the rear legs flood and sink, so I had to use some ping pong balls for a little bit more floatation. This was a last minute solution to my problem, and I hope it is ok. I would have used the balls from the Mindstorms sets if I had any of them.
Here is a quick video of my Water Strider in action:
I took the video of the water strider in the fountain at the back of St Patrick's Cathedral in Melbourne, Australia (StreetView link). The fishing line in the video is just so my water strider didn't float away out of reach .
Here are a few more images of my water strider:
These photos and more can be seen in higher resolution in my BrickShelf folder (once they have been moderated/approved).
I have also created building instructions for this model if you would like to build a copy.
Edited by Splat, 10 April 2013 - 02:07 AM.