Thruster Design

We did not have sufficient donations to purchase proffesional thrusters, what we did have was free machine time at a very prestigious CNC manufacturing facility. AXIS FAb inc. With laser cutters and 5 axis precision machine at our disposal, we took a crack at a custom design. On our team was Mindy Ren, our Mech Eng student starting at UT Knoxville this fall. We opted to try a design we saw on thrusters used on tug boats. This was essentially a differential drive, propellers turning opposite directions at either end of the tube. This way the water entering the tube would begin to rotate in the same helical pattern as it continued through the thruster cowl, at the exit port it encountered a second propeller that slapped againts the water rotation adding to the thrust instead of just pushing it along. Using a thin shaft cowl also allowed the drive shaft cross section to be rather small. certainly smaller than if the motor and cover was on axis. The propellers chosen came from a dutch RC subamarine outfit are molded brass 2.5 " diameter and we have both right and left hand versions for the counter rotation. The shaft cowls are light weight delrin, and the gear housing is a 1/2" x 3/4" x 3/4" copper pipe"T" from a local hardware store (they are not a sponser so we wont metion their name) . The motor is 90 degrees so it mounts internal to the sub body.

The motor gear is the pinion in our simple differential, we used belvel washers for tension and back filled the cavitys with silicone grease, The shafts have bearings in either end and turn free. The gear ratio is 3 : 1 so the props turn 3000 RPM max when the motor is most efficient at 9000 rpm. No cavitation occured on the propellers at these speeds due mostly to the design of the props, they are 7 blade swept propellers modeled after a russian acula class submarine.

The cowls and assemblys shown have o-ring seals, so any leakage will be along the shaft through 3" of grease past another 2" of grease and finally into the motor cavity which is sealed and pre presurized. Tets so far show no leakage, as the contest is only to 16-30 ft this hopefully wont be a problem,

Preliminary tests show a current draw of 1.5A at 3000 rpm prop speed, 0.25 A at slow 400rpm prop speed, for manuvering. The whole assembly weighs under 2 lbs. its a little noisy but smooth action the real test is in the pool !!!...