Hi Dean. I'm happy you are almost ready for a float test. Pretty exciting! I haven't checked the psub site lately, got any new pics?
On the "riding the bubble" thing......I know it works, cause Vance used it, but as an operational method of control it seems like it would add complexity to maintaining neutral buoyancy with the need to adjust the bubble at every depth change of just a few feet. I think Vance used that technique when he was traveling/working at a constant depth.
The whole purpose of achieving "neutral buoyancy" is to avoid constant adjustment of the subs weight so travel over the duration of a dive can be accomplished without adding and dumping air at every depth change.
With a front and back tank like the K boats have you would be adjusting for balance as well, so it's TWO bubbles you need to adjust. Could get very busy at the valve panel when driving the sub.
It will use up a lot of air too, if several changes are made during the dive, and when you get ready to surface at the end of the dive the ascent rate will increase as you near the surface so releasing some of the bubble on the way up may be needed so the sub doesn't get into an uncontrolled ascent. It could be a delicate balancing act adding potential for problems when coming up from a deep dive.
As I understand it, lead ballast bricks are added/subtracted to approximate the weight needed to achieve "neutral" as payload changes, while still at the dock. Then the VBT is used to "fine tune" the buoyancy when you first get the sub fully submerged near the surface. Basically "set-it-and-forget-it" so the pilot can focus on driving the sub, and use the vertical thrusters to control depth.
I'll be diving in salt water but launching in fresh water so I'll need enough VBT to adjust for the change in salinity, plus a little extra capacity just for safety.
One of the ideas I really like was shown on that you-tube video. Dragging a chain when you're near the bottom, so the sub is a little buoyant all the time, and the chain suspended above the bottom is the extra weight needed to maintain a given depth. You'd have to be sure the chain doesn't get hung up on something, and have a means of releasing the chain if it got stuck between two rocks or some how entangled, but it could make it easy to maintain a set distance from the bottom if you were searching a large area and wanted to skim along near the bottom while observing what's there through your down looking window.
I've considered an anchor system where I can get the sub into position over a given spot, set some anchors, and the sub will stay put while I work over that spot. Like using a manipulator arm to dig a hole, or perform some function while in a stationary position. Just settling on the bottom may be too close to work and won't allow any movement should it become necessary. The more options we build into our little boats, the more we can do with them.
Frank D.