MovieChat Forums > Run Silent Run DeepĀ (1958) Discussion > Quiz for non-bubblehead sub fans

Quiz for non-bubblehead sub fans


I have a question for you to answer if you can.
Actual submariners please dont answer as I know you guys know the answer, this is for the non submariners among us who just love subs and sub movies.

Though actual submariners may answer to correct anything I get wrong as I make no illusions that I am a submariner. (I was in the Navy though)

The question is this:

How does a Submarine change depth? which of these two ways are correct.
does it:
A) Maintain neutral bouyancy and use its dive planes to change the angle of the boat in the water and drive the ship up or down to the new depth.
or:
B) Alter it's bouyancy by using it's ballast tanks to be either heavy (Negative) or light (Positive) to either sink or float the sub to the new depth.


While a sub CAN change depths by either method, only one is the way they do it
there are at least 4 reasons that only one method is the preferred method and these are based in

1) tactical reasons
2) simplicity
3) Physical limitations
4) Safety

Answer which of the two methods is correct and if you can, give the 4 reasons why.





I joined the Navy to see the world, only to discover the world is 2/3 water!

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CG Sailor: Method A is not sure to work, neutral boyancy won't keep the ship under. Method B is more positive and if on the surface, get the boat under a lot faster...Besides with neutral boyancy, they would not be on the surface very long..........

1) As stated above, get under quick..

2) A lot easier to flood the tanks than try to rig out the planes and drive the boat under

3) Limitations of the bow planes being folded or stowed while on the surface, and not awash in the normal cruising boyancy--on the surface.

4) The boat will submerge a lot faster if the tanks are flooded quickly, if they used the planes,the props would come out of the water when the tried to drive it under..

best guess, from an Army CWO and Aviator..

Dale

"If those sweethearts won't face German bullets--They'll face french ones!"

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We are not discussing getting the ship under from the surface, we are discussing changing depth while submerged.

Even if we were discussing diving from a surfaced condition you would only be partially right and for all the wrong reasons.

A surfaced sub submerges by opening the main ballast tank vents and allowing seawater to enter the freeflooding flood ports in the bottom of the tanks. Flooding the main ballast tanks reduce a subs bouyancy to neutral (not negative) in a properly compensated boat. The dive planes then drive the vessel forward and down with the forward motion provided by the ships screw (propellor). The forward motion is necessary for the planes to work. To put it in Pilot speak for you, it is the same as your aircraft control surfaces becoming ineffective when you are in a stalled condition. Without airflow (waterflow) over the surface, it does not work.

Your arguments (points 2 and 3) are wrong. You are assuming that it is troublesome to rig out the planes, a hinderance. Actually they are designed to do just that and do so under hydraulic power or with electric or hand cranked backup. Rigging out the planes is PART OF the diving process.

If the sub needs to get under faster, Say with a destroyer on it's attack run or a plane diving out of the sun... The negative tank would be flooded in addition to the main ballast tanks. This would give the sub about 7 tons of negative bouyancy. This water must be pumped back out before the sub could level off and return the vessel to a state of neutral bouyancy.

Now to the Original premis of changing depth while submerged.

Method A) Altering the dive planes and driving the boat up or down in depth is used, NOT Method B) Altering it's Bouyancy with the ballast tanks.

1) tactical reasons:
Pumping and flooding the tanks is a noisy process, something you don't want to do with a destroyer hounding you.

2) simplicity
Making minor adjustments to maintain neutral bouyancy is much simpler than making radical changes to alter the bouyancy from neutral to positive or negative and back again. The more you screw with it the more out of balance you are likely to get.

3) Physical limitations
The only way to empty the tanks again once flooded is with compressed air. This is in limited supply and would be used up rather quickly if altering the bouyancy all the time was the method everytime you wanted to change depths.

4) Safety
See reason 3 above... Imagine flooding negative then as you go deeper, you realize you dont have enough compressed air to regain neutral or positive bouyancy.... CRUSH!
Remember, it is volume alone that determines bouyancy. the deeper you go the greater volume of compressed air is needed to displace a like amount of water at a greater pressure depth.

There are 3 groups of ballast tanks aboard Sub. The Main Ballast tanks. The Variable Ballast tanks, and the Special Ballast tank Group.

The main Ballast tanks are flooded to take the sub from Positive to neutral bouyancy and REMAIN flooded during the whole submerged run. They are not designed to withstand full submergence sea pressure so they MUST remain fully flooded. They are used mearly for submerging and surfacing only. Not in depth changing.

The Variable ballast tanks are a series of tanks distributed throughout the vessel to maintaine compensation and balance. The weight of a vessel changes over time but the VOLUME of the tamks remains the same. As fuel, Food, water, and weapons are used up, more and more water must be flooded into the variable tanks to maintain the bouyancy at neutral. They are not used to make large changes to the bouyancy as woud be required in depthchanging, just to maintain neutrality.

The Special Ballast tank group Consists of the Negative tank, the Safety tank, and the Bow Bouyancy tank.

Under special circumstances these tanks are used to change depth as per method B but again, this is under special circumstances, not as a matter of course.

Safety is always kept flooded under normal circumstances and the weight of water in the tank is factored in as part of the overall weight of the sub when it comes to compensation and balance. In an emergency like during a flooding casualty or in an emergency surface, the Safety is blown dry using the 3000lb Compressed air system. this gives the Sub an immediate positive bouyancy even with the main ballast tanks flooded.

Negative is always kept dry (opposite of Safety). It's purpose is to be flooded and give the sub additional Negative weight when an emergency requires a faster dive.

Bow bouyancy is freeflooding. it is blown dry to assist the Safety tank in an emergency surface as well as to give the sub a positive up-angle on the bow for surfacing.


Again. If you used your compressed air and your tanks to change your bouyancy everytime you wanted to change depth, you would rapidly run out of the compressed air necessary to do so leaving you stranded below the surface with no way to reach it. Subs change their depths by swimming the sub up and down through the water column while maintaining a neutral state of bouyancy.





I joined the Navy to see the world, only to discover the world is 2/3 water!

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CG Sailor: I misunderstood the question, but you are absolutely right!!

I have one minor point, if a sub has neutral bouyancy, doesn't it vary with depth? In other words, if the sub goes deeper with the planes and screws, doesn't the water depth affect that bouyancy ??

Sorry, I was an aviator not a submariner, so I do not get all the details on this subject..

Dale

"If those sweethearts won't face German bullets--They'll face french ones!"

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Yes it does.

As the sub goes deeper, the hull is squeezed and compressed just a bit. Not much, a half inch is so but over the whole of the sub, that is a large difference in volume. The sub no longer displaces the same volume of water. It displaces less water. The sub still weighs the same and since the state of bouyancy is the ratio between the weight of the object (Sub) and the weight of the displaced water volume, the sub's bouyancy drifts ever more negative the deeper it goes.

Also on another note: A sub never is in a true state of neutral bouyancy, That is never really achieved outside of a laboratory. It will always be slightly negative or positive. It is too fine a point to achieve exactly. However it is close enough that the forward motion and the dive planes can counteract any slight upward or downward drift. A sub sitting still will drift up or down depending on how close and to what side (positive or negative) it is to the neutral bouyancy.

I joined the Navy to see the world, only to discover the world is 2/3 water!

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CG Sailor: thanks for that answer...I almost joined the Sub Fleet after my duty in Nam, but the Navy cut the program to steal all the Army Warrant Officers and use them in the Fleet, They also recruited me for Naval Aviation, they wanted people who could fly Helicopters and train them for fixed wing as well, they cut that too! I wound up in the CT Army Guard as a flight instructor, and got to transition all the Naval Aviators into Army Flying....how ironic!

Dale

"If those sweethearts won't face German bullets--They'll face french ones!"

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2) simplicity
Making minor adjustments (of the dive planes) to maintain neutral buoyancy is much simpler than ....


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?

A.) Your (original) question concerns the means of depth control, not neutral buoyancy.

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.

Thus, to fix the errors (IMHO) identified in (A) and (B), I suggest replacing the phrase "to maintain neutral buoyancy" with "of the dive planes:" correct?

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.

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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?

Correct.

True 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!

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WOW!

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.

2) simplicity
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.... ;-)

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I'll say A and then 4.

I've never served on a sub, but I did do Nuclear power training on a former ballistic missile sub. But I don't think that disqualifies me since I know nothing about how a sub submerges, I just make the screw turn. All my sea experience is on surface ship "targets".

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