As I understand this particular section of answer #2, you're stating that neutral buoyancy can be simply controlled by making minor adjustments to the angle of the dive planes: correct?
neutral buoyancy is a knife edge. the weight of the object must EXACTLY weigh the same as the water displaced by the volume of the object. And or subtract so much as a feather of weight and you have shifted the buoyancy to positive or negative.
Granted... that amount is so slight that it would take forever for the sub to slowly drift up towards the surface or sink to crush depth.
Now the Dive planes...
Like holding your hand out the window of a speeding car with your palms flat... angling your palm up and down causes the wind to deflect your whole hand/arm up and down. Like a plane's control surfaces. The Dive planes act in much the same manner. Not like a wing producing lift, but as a control surface deflecting the air (or in this case water) to force a change in motion.
And just like a plane going very slow or still, no wind over the control surface means no control effected by that surface. Same for the Dive planes. Without the forward motion of the sub through the water the Dive planes have no effect.
The amount of force the planes can effect is directly proportional to the speed of the sub through the water.
Because of what I stated above about Neutral Buoyancy, no sub is ever truly in a perfect state of neutrality but always has some small degree of positive or negative. This means the sub has a slight tendency to drift up or down as the case may be. The forward motion of the sub allows the planesmen to hold the sub against this tendency to drift up or down from the desired depth.
Think about when you drive a car which has a steering alignment problem.
Left to it's own the car will veer off to the right or left and leave the road.
But by holding pressure against the turn you are able to keep driving straight.
A sub that gradually keeps increasing it's Positive or negative buoyancy can be held level by the planesmen by ever increasing dive plane deflections. That is until the point where the planes can no longer hold against the increasingly strong +/- buoyancy and the sub starts to rise or sink.
That small region of +/- buoyancy that's close to true neutral, where the planes can hold the sub in control is for all practical purposes considered by the sub to be Neutral buoyancy.
The same holds true in a way for surface ships but only for the left/right steering of the ship via the rudder, not depth control. A ship must have way on (forward motion) and water flowing past the rudder for the rudder to have any effect. This gives rise to the term "Bare steerage-way" meaning the absolute minimum speed a ship can go and still have any steering control by rudder.
A.) Your (original) question concerns the means of depth control, not neutral buoyancy.
My original question had to do with effecting a change in desired depth, not maintaining a single given depth (which is everything I just discussed above)
Not holding the sub at a depth, but of changing the sub's depth from one depth to another.
B.) You stated above this answer that buoyancy is controlled not by the diving planes, but by the amount of water in the tanks - although changing the buoyancy as needed can aid in the rate of change in depth.
Fine control of a desired depth at Neutral buoyancy, and to make large changes in depth are controlled by the planes. The states of Buoyancy, are controlled by the Ballast tanks.
P.S. Along with simplicity would be the features of reliability/ durability since the buoyancy control system is probably more complicated - and therefore more likely to fail i.e., lower MTBF rating - than the dive plane control system.
I don't know exact "failure rates" and maintenance workloads of each system.
However both systems are equally complex. Both having multiple backups and methods of operation.
The Dive planes for example have both Hydraulic, electrical, and manual backup methods of control.
Same with the Ballast tanks. Each has pneumatic Hydraulic and manual methods of flooding, venting, and blowing the tanks. I would not say either one was more complex than the others.
By way of example. On your previous post you asked about surfacing times (slow vs fast) and I mentioned there is a point during surfacing where the sub can become unstable.
MBT's 2 and 6 are subdivided port and starboard and fore and aft. making four tanks out of each. 2a, 2b, 2c, and 2d for example.
2a and 2c are to starboard and 2b and 2d are to port. The tanks are separated by a partition so that the separate port and starboard arrangements can also be used for list control in addition to overall ballast.
When the sub is surfacing it is subjected to surface wave action. The tanks themselves are freeflooding and any list can cause the tanks to fill or empty unevenly causing a greater list and danger to the vessel.
The simple expedient of having a lever in the overhead connected to a rocker arm to the separate vent valves for the two tanks will restrict flow to one while opening flow to the other restoring balance simply by moving the lever.
I joined the Navy to see the world, only to discover the world is 2/3 water!
I was just expecting several Yes/ No answers with a sentence or two providing the reasons for a No answer.
You already previously & adequately addressed (neutral, +, -) buoyancy vs. dive planes and using said planes to "trim out" neutral buoyancy at a constant depth while moving forward.
As for MTBF, I'd forgotten about multiple types of control systems (electrical, hydraulic, mechanical) as well as their backups. I was thinking in simpler terms of just a water tank's electrical pump/ valves vs. mechanical linkage for the dive planes. My bad!
My defn of depth control (not "constant-depth control") addresses not only maintaining a constant depth, but also controlling the rate & accuracy of ascending/ descending to a different depth setting.
Making minor adjustments (of the dive planes) to maintain neutral buoyancy is much simpler than ....
Q11.) Isn't the (neutral, +, -) buoyancy of a simple object submerged in water the same whether said object is moving (due to an external applied force e.g., horizontally) or at rest (no external applied force)?
If so, then to maintain constant depth of a man-made sub moving forward in a liquid, the dive planes would be needed to:
AA.) "trim out" variations in neutral buoyancy due to limitations in accuracy, precision, etc. of the technology (e.g., pumps, gauges, & tank valves) used to control said buoyancy;
BB.) counter-act various external forces (from varying water currents, densities, etc.) applied against the hull of a 3-dimensional, non-uniform object moving through a virtually in-compressible liquid.
Q12.) Did I forget/ screw up anything?
Nonetheless, (aside from Q11 & Q12) I think we've beaten the snot of this dead horse through 5 of its lifetimes! (Though it feels more like 5 of MY lifetimes at the moment.... ;-)