We're doing a cargo ship challenge in my tech class at school. We made a tank (our boats are about 4 feet long and a foot wide), to test our boats. Well, our boat held 35 lbs. while only sinking 1 inch. Not bad. But our problem is getting the motor shaft (that connects the motor to the prop) to be water tight. How do the r/c boats do this?? Thanks, it would help a lot, since we are going to competition on April 20.