I teach a manufacturing class in high school and encouraged four of my students to take on the project of designing and building a variable displacement ballast cylinder. Deceptively simple, the device uses a 12volt linear drive, which actuates a piston. Variable displacement causes the cylinder to rise and sink in the water. Current draw during the extension cycle is near 1A (bad news), but (good news) the cycle time is less than three seconds from negatively to positively bouyant. Neutral bouyancy is easy to attain. As I write this, the cylinder is in a water tank, testing the cylinder for water-tightness with the O-ring seals we made in class.
This is a proof of concept design. From there, we will move to the functional design of a system comparable to the WTC...without the need for compressed gas!
Thus far the main limitation is diameter. This system is suitable only for larger diameter hulls. Piston diameter is 4 inches, which is the width of the linear drive. The drive gives us 27 lbs of force. Finding a smaller drive has been an unsuccessful challenge.
Look for a full report as soon as we submit an article to the SCR. The article will also show you how to do the buoyancy calculations...all done by my high school students.