I installed driveshaft anchors in the polycarbonate hemispheres which make the two sides of the isolation chamber, mounted the motor, its batteries, and inertial mass on the driveshaft, bolted the hemispheres to the joint ring, and put it in the water. It didn't leak! The displacement was in the ball-park for what we want. The motor, batteries, and inertial mass are close to being balanced on the driveshaft (this is adjustable)! I was really happy with this test.

Test of the isolation chamber! No leaks!

The O-ring came today in the mail. I couldn't wait for daylight to try it out.

I'm itching to get the motor installed in the isolation chamber. Here, I've suspended the batteries and motor (without the lead) on temporary flanges, just to see how it looks. The same polycarbonate hemispheres and motor will be able to go around two joint rings (see below), which is pretty amazing, all by itself. All robotic fish that I have seen have completely integrated motor and fins; you can't just swap out a joint ring and put a different set of fins on the motor.  

We're going to test efficiency with this simplified system. No, its hydrodynamics aren't great, but the tests will all be at slow speed.

One of the cool things about this setup, is that we will be able to switch between a "Body Caudal Fin-" or BCF-type fin (above) and a "Medial Paired Fin-"  or MPF-type fin, to the left. We'll be able to do this USING THE SAME MOTOR SYSTEM! That's really wild. Before now, different fin types all have different motor systems.

Built around a Torque Reaction Engine

Motor rated up to 218 watts, LiPo batteries providing 29.6V and 15Ah of power. Two Arduino Portenas, one for the motor, another for sensors.  

Isolation chamber... stainless steel or polycarbonate?