Possible to "stall" a sub?

[Puster Bill]

OK, a question for those with actual, for-real experience in submarines: Is it possible to 'stall' a submarine. In other words, at low speeds and high angles of attack, can the submarine lose dynamic lift in a fashion similar to an airplane?

Logic tells me this should be so, and I have noticed the behavior in SHIII, but does anyone have any actual experience with this?

I've noticed that while running at slow speeds in deep depths, if I try to surface without increasing speed, the planesmen will increase the angle of their planes in order to comply, but if I'm going slow enough, I'll actually slow down and start to sink, yet they were able to hold depth previously with more moderate diving plane angles.

[GoldenRivet]

i dont claim to be an expert.

but:

as far as im aware submarines change depth by altering buoyancy by flooding tanks with sea water to dive, and using onboard high pressure air to force water out of the tanks to surface.

dive planes merely assist the submarine by expediting the process of surfacing or submerging by allowing the crew to set a dive angle or ascent angle for diving or surfacing.

if for example, you were at ahead full and ordered a sub to surface, the dive planes would be set to an angle which would force the bow upward, however, what is acutally causing the sub to ascend from the depths is the increasing buoyancy by use of high pressure air to empty ballast tanks.

the dive planes alone are not what is causing the sub to surface... unlike an airplane, which uses elevators to set an angle of attack to increase lift which causes a climb to occur.

dive planes add stability, and, if the submarine is moving can modify how quickly or slowly the sub ascends or descends... if your sub is not moving, i mean zero speed... and you set the dive planes to full rise, nothing would happen because you are not dumping ballast water overboard.

so my answer is no... subs dont "stall" in the way that airplanes can because they are not "climbing" or "ascending" on the same principles

EDIT: check this out... http://science.howstuffworks.com/submarine1.htm ...scroll down to the picture of the sub's cross section and click the "SUBMERGE" button... observe the process... then click the "SURFACE" button and observe the process. in theory a sub doesnt even need dive planes.

[GoldenRivet]

Additionally, for diving, you can view this video, and you will observe wat appears to be water jets at :17 through :19, this is the forward main ballast tank vents opening and all of the air with sea water mist escaping as sea water floods the MBT. the view changes to the aft section of the sub... here you can see at :22 through about 1:15 another "water jet" which is air and mist escaping from the aft ballast tank vents as sea water floods the tanks. the fuller the tanks get, the deeper the sub goes. dive planes which are not shown in this video have little bearing on the actual process of diving or surfacing, except that is to speed up the process.


For surfacing if you look at this you can see a sub surfacing, where in the video you can see streams of water pouring out of the tank vents as high pressure air is still forcing water out of the tanks.

[Erich dem Roten]

GoldenRivet summed it up nicely. Submarines don't rely on forward or backward motion to propel themselves upward or downward, they rely on the displacement of air and water.

A theoretical situation where a submarine might "stall" is if there is heavy flooding and the sub is making an ascent at a steep angle (as would be likely in an emergency). As flooding increases, it displaces the air inside, and the submarine relies more and more on its own propulsion to carry it to the surface. If the flooding reaches a point where propulsion can no longer drive the submarine "forward"...well...start writing letters to wives.

[Uncle Goose]

Dive-planes (at least those in WW2 fashion) are an important tool to keep the sub level in the water. While you could of course do that too with trim tanks it's not a wise thing to do as pumps make a lot more noise than the hydraulic movement of the dive-planes. If you set the dive-planes to rise together with speed of the screw the hydrodynamic capabilities of the dive-planes will give you a lift but the buoyancy caused by the water in the ballast-tanks will drag the sub back down. So yes, it's possible to stall a sub.

[Kielhauler1961]

I think this might help (taken from Wikipedia article on Submarines).

Submersion and trimming.

All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing displacement of the water. To control their weight, submarines have ballast tanks, which can be filled with outside water or pressurized air.

For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks or MBTs, which are filled with water to submerge, or filled with air to surface. Under submerged conditions, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure. The amount of water in depth control tanks can be controlled either to reflect changes in outside conditions or change depth. Depth control tanks can be located either near the submarine's centre of gravity, or separated along the submarine body to prevent affecting trim.

When submerged, the water pressure on a submarine's hull can reach 4 MPs for steel submarines and up to 10 MPa for titanium submarines like Komsomolets, while interior pressure remains unchanged. This difference results in hull compression, which decreases displacement. Water density also increases, as the salinity and pressure are higher, but this does not compensate for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either fall or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.

Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between these, changing weight distribution, creating a moment pointing the sub up or down. A similar system is sometimes used to maintain stability.

The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are closer to the centre of gravity, and are used to control depth with less effect on the trim.

When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, but it may inflict serious damage on submarine systems.

Hope this answers you question.
KH.

[GoldenRivet]

A stall in the context of the original post is defined as follows:

An aerodynamic (or in this case hydrodynamic) loss of "lift" caused be exceeding an airfoil's (or hydrofoils') cricitcal angle of attack.

a lot of people screw this up. it has nothing to do with weight or speed.

the situation uncle goose describes is a matter of the dive planes (lift) not being able to overcome the negative buoyance of the submerged u-boat (weight)... what he described is NOT a "stall".

uncle goos is partially correct in his statement; that dive planes are responsible for setting a dive angle, and fine tuning the small scale and immediate trim of the u-boat

... but in no way do they make large adjustments to depth... high pressure air or an influx of sea water cause depth changes to occur NOT dive planes.

the situation Puster Bill is describing the way i understand it is as follows:

>>set a steep ascent to surface at high speed... while on the way up, drop the speed down to zero... will the submarine's dive planes "stall" and subsequently cause the sub to drop or sink?

No... they wont. their effectiveness will be negated by the drop in speed, and the submarine will not rise as quickly.

however, as high pressure air is sent to the MBT, the submarine will rise - irregardless of the dive planes.

[Phaedrus]

If the definition of stall is a loss of control of the vessel because of a disruption in the flow of water over it's control surfaces, I would say submarines can indeed stall.

Submarines do rely on forward momentum to some degree to maintain depth and heading.

A good way of looking at it is to say that the ship uses the ballast and trim tanks to determine where it is at the present moment, while the planes and rudder control where the boat will be in the soon-to-be-present future.

The flooding and blowing of the ballast tanks controls depth, but the majority of underway submerged flooding and blowing has to do with maintaining the boat's trim.
Once the boat achieves a neutral buoyancy, trim needs to be adjusted based on water density and other factors to allow the boat to be controllable, and remain on a (relatively) level plane.
The fact that there is air left in a partially flooded ballast tank means that the submarine is destablized by the 'free surface' effect of water (a good example of this is the difficulty in trying to carry water in a wide flat pan)... much of trim adjustment is to find the stabilizing balance.


The dive planes and rudder determine the ship's future position by moving to provide resistance as water hits (or rather, flows over) the surface of the planes.

The minimum amount of momentum needed to be able to maneuver the boat via the helm is known as steerageway.

While it is techincally the true that the dive planes do not make large adjustments to the depth of the boat, they do make an ascent or descent controllable, ensuring that the boat glides smoothly to the desired depth - rather than plummetting to unsafe depths or rocketing out of the water like a cork rising from under water.

Though the formal definition of a stall is obviously aeronautical in nature (disruption in the flow of air over an airfoil, resulting in a loss of lift; thereby inducing a loss of control and altitude until sufficient lift is once again established), a similar state could be said to be induced in a boat during a dive (a negative buoyancy situation) - where no combination of high degree of up-angle on the dive planes, and forward momentum would be sufficient to propel the boat upward because of lack of buoyancy.

... or perhaps during an unwanted ascent, where the boat was trimmed with a slightly positive buoyancy, but a combination of forward motion and bow planes down led to a minute and continuous dive rather than an ascent. If the boat reduced revolutions to a speed below what was necessary to overcome the very slight buoyancy and dive, it would rise without control unless the boat adjusted trim, or increased speed.


I think that a lack of steerageway (though in this case, referring to both the helm and the planes, rather than just the helm in the typical nautical sense of 'steerageway'), would be the submersible equivalent of a stall.

[Letum]

Even a surface ship can stall if you imagine the bottom of the boat as the bottom of the wing.

Think of a speed boat hitting a big wave fast with the flat of the bottom of the hull and it's bow in the air.

BANG! The hull has a horrible angle of attack and the boat stalls losing half it's speed. (And all it's crew!)

[bobchase]

Those textbook answers about using diving tanks to change depth are not the way a sub really works for most of the mission. Yes, the flooding of the main tanks is done to submerge the boat. Once submerged, the boat is put into a state of almost neutral buoyancy and kept there. This is called 'trimming the boat' and it would happen as soon as the boat is submerged, long before it clears the continental shelf.

Some, perhaps most, ship handlers prefer a slightly negative buoyancy. The negative buoyancy proponents like it because the sub handles much like an airplane when it answers to changes in the helm. The downside is that if main propulsion is lost, then the sub eventually sinks down to crush depth. As it is against the regs for a Navy reactor to actually fail, unless its intentionally done as a crew training drill, skippers usually go with the better handling option.

Some ship handlers actually like a slightly positive buoyancy. The positive buoyancy folks trade off better handling against the 'if all else fails, we can always float to the surface' option.

A truly balanced buoyancy is universally avoided. It's almost impossible to maintain depth with that trim and the faster you go, the harder it becomes.

Large changes in depth are done with a combination of the planes and the speed through the water. Certainly not with flooding or blowing the tanks like is often described in literature. Those last two options cause so much acoustic noise that the enemy would hear it half an ocean away and come over to kill your noisy butt.

Boats with sail mounted planes use the sail planes for a fast rise or fast dive at speed and they also use them for small, completely level, changes in depth. This type of boat's rear planes are only used to adjust the angle of attack (or lack of angle) during a rise or dive. Once they are at the new depth or even along the way, small and very quiet changes are made to the trim tanks with pumps to account for the change in water volume in the main tanks as well as changes in water density.

The boats with bow and stern planes can also do a perfectly level change of depth too but it is much trickier to do it in them. They can, however, make a big change of depth faster.

I think that the word 'stall' is not applicable to a sub because there are no lifting surfaces. However, loss of forward momentum will cause a stall-like event to occur. But don't go looking for a flat spin like a Mig 25 gets into with a sub. You've got minutes to hours to contemplate your demise.

Bob

[GoldenRivet]

as i have personally done many many hundreds... if not many thousands of stalls in an airplane i would have to say it is darned difficult for me to imagine a submarine going to a temporary state of full negative buoyancy and sinking like a stone for several seconds simply because of the dive planes going full up.

i have to roll my eyes at the idea.

if you go full up on the dive planes the submarine is not going to arbitrarily sink for a few moments based on the planes position alone.


let me put it to you this way... if you can "stall" a u-boat in the same sense as you can "stall an airplane... shouldnt you also be able to loop a u-boat with dive planes alone?

the hard thing to wrap the mind around here... is that subarines and airplanes operate on a completely different set of principles

[gAiNiAc]

The theory on why the Thresher sank includes what might be described as a "stall". After the reactor scram she didn't have enough mometum to get to the surface. Without propulsion she slid backwards until she hit crush depth.

[GoldenRivet]

thats called sinking. yes?

what about blowing ballast? could high pressure air not have been forced into the MBT causing the ship to become positively buoyant and it would rise to the surface?

[gAiNiAc]

Originally Posted by GoldenRivet

thats called sinking. yes?

what about blowing ballast? could high pressure air not have been forced into the MBT causing the ship to become positively buoyant and it would rise to the surface?

Nope. Emergency blow cannister manifolds froze up. Supposedly they tried to glide her up. Not enough momentum, therefore she "stalled" and, yes, sunk stern first. Certainly not an aeronautical stall...but described as a stall nonetheless..........So in the broader sense of the term it can apply to an event.

[gAiNiAc]

Originally Posted by GoldenRivet

it is darned difficult for me to imagine a submarine going to a temporary state of full negative buoyancy and sinking like a stone ......

Another thing that can happen is if you roll the boat to much it'll flood the MBT's via the open vents in the keel and sink the boat.........submarines have not gotten that far from their diving bell beginnings.........