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StgShultz
12-17-2007, 10:09 PM
Can somebody in the know pls explain why this was not done.
If my sub can get to to say 400m before being crushed surely all I would need to do is increase the internal pressure by a few atmospheres. This would give me the extra depth to possibly excape sub-hunters. Obviously I would need to reduce the pressure slowly to avoid the bends and would also need more air reserves to resurface. But it my only other option was being depth charged why did they not try it?
Or am I missing a basic flaw in this idea?

hueywolf123
12-17-2007, 10:25 PM
in reality, yes. Subs rely on bouyancy, positive is when you are floating, but a sub requires neutral bouyancy to stay submerged. If you hit negative, you sink regardless of how much oxygen you have etc etc.
This then varies depending upon the relative density of the external ocean, and your bouyancy increases to negative the deeper you go.
If this was modded into the game, most would find it a full time job, not sinking

Rood-Zwart
12-18-2007, 06:17 AM
Isn't it so that submariners don't get the bends because the atmosphere inside the sub is on very much the same pressure level as on the surface?

geoffwessex
12-18-2007, 07:51 AM
Increasing the pressure inside the boat could really only be achieved by releasing compressed air (a valuable commodity) into the internal atmosphere. You'd need to run the compressor to recharge the high pressure air to put enough back into the 'bottles' to blow ballast for surfacing at some time, and this would take air from the boat - creating the opposite effect, a vacuum.

The men could adjust to higher pressure eventually, though most eardrums would be ripped, but things like guages, electronic valves (as used in the sonar and radio) and lightbulbs would break. You'd also need to vent the compressed air out of the boat while surfacing or, as RoodZwart says, everybody would be killed by the bends. And as Huey says, the boat would be out of control.

Realjambo
12-18-2007, 09:47 AM
That's a good explanation Geoff http://forums.ubi.com/images/smilies/25.gif

klcarroll
12-18-2007, 12:03 PM
More to the point: ....Each "atmosphere" increase (14.7 psi) would theoretically only buy you an additional 33 feet (10 meters) of depth.

To even increase the pressure inside the boat by ONE atmosphere would require a spectacular amount of air!

Using a typical TypeVII boat as an example, you would have to introduce something on the order of 430 standard cubic meters of air to raise the internal pressure by one atmosphere: .....And if memory serves, you don't have enough high pressure air to do even that!

........So, to increase your theoretical diving depth by some 10 meters, you would have to commit ALL of your high pressure air!!!! (Leaving NONE for blowing ballast!)

Not a serviceable idea in practice!

klcarroll

StgShultz
12-18-2007, 12:03 PM
Not satisfied with some of the answers.
Hueywolf123, are you saying that crush pressure aside, that if I keep sinking eventually I will loose positive bouyancy?
Root-Zwart, internal pressure of subs & planes at atmospheric pres allows fast altitude changes for craft & crew - something divers cannot do - or even pilots, imagine equalising your ears in a plane dive.
Geoffwessex, yes subs have limited air reserves but in extreme situations where you are going to die....
The gauges would not implode due to the additional pressure.Yes bulbs would but who cares given situation above. Also so long as the pressure change is slow enough when surfacing no bends. Else divers would not be able to do it.

Foehammer-1
12-18-2007, 12:47 PM
Root-Zwart, internal pressure of subs & planes at atmospheric pres allows fast altitude changes for craft & crew - something divers cannot do - or even pilots, imagine equalising your ears in a plane dive.

I was about to say that. I can't imagine what those poor souls would feel when the pressure suddenly increases 1-2 atmospheres :S

klcarroll
12-18-2007, 12:48 PM
StgShultz;

From an Engineering standpoint, ....the only consideration that really matters is the one that points out that you don't have enough high pressure air on board to even buy you another ten meters by venting that air into the interior of the boat.

klcarroll

Kaleun1961
12-18-2007, 12:49 PM
I'm not an engineer, so I don't know the mathematics involved, but it seems to me that at such depths as you are talking, the amount of internal pressure required to keep the hull from imploding would have to be so horrendously large as to be humanly unbearable. Aside from that, I can't see them devoting that much space and weight to air containment systems. I think it would be a matter of increased weight from air storage makes the boat sink deeper, which needs more air tanks which make the boat sink even deeper, etc.

StgShultz
12-18-2007, 12:59 PM
Given unlimited air I assumed that it was possible to get another 100'. However the additional large exponential volume of air to empty the tanks was what I thought would be the problem.
What I was trying to figure out was a subs ultimate operating depth. Increase internal pressure > crush depth. Therefore its the supply of air thats the problem and always will be. Unless there's a problem with bouyancy?

klcarroll
12-18-2007, 01:00 PM
K61;

You're quite right! One atmosphere of pressure equals approximately 10 meters of depth in sea water.

Doubling the internal pressue of the boat would only get you a theoretical increase in "crush depth" of 10 meters.

Not only would such an increase be hard on the crew; ...but in real life, it would be impossible to accomplish! In a TypeVII boat, something like 430 Standard Cubic Meters of air would be required: ....An amount that FAR exceeds the total amount of High Pressure Air storage on board!

klcarroll

StgShultz
12-18-2007, 01:25 PM
You're quite right! One atmosphere of pressure equals approximately 10 meters of depth in sea water.

Doubling the internal pressue of the boat would only get you a theoretical increase in "crush depth" of 10 meters.

3 atmospheres = 100' extra depth which is quite alot of extra depth. I still don't see why it would be hard on the crew as scuba diving to 100' is not hard (excluding the risk of the bends).
I will conceed that air reserves will be the limiting factor.

klcarroll
12-18-2007, 01:43 PM
StgShultz;

You're correct: ....The crew would survive a two atmosphere increase, if it was done with any sort of care at all: ...Modern "Saturation Divers" deal with increases that are an order of magnatude more severe.

The fact remains that what you suggest was a practical impossibility due to the limited amount of High Pressure Air available.

This is one of the problems with being a Mechanical Engineer: ....You frequently have to tell people that otherwise good ideas won't work due to "Real World" limitations.

klcarroll

hueywolf123
12-18-2007, 01:48 PM
Hueywolf123, are you saying that crush pressure aside, that if I keep sinking eventually I will loose positive bouyancy?
Yep, absolutely. But you only have positive bouyancy while surfaced. When diving, you acheive and maintain a neutral bouyancy, erring at the positive side.
A U-boat will always try to re-float, the deeper you go, the less this will happen until you get to a point where you will no longer be able to reach the surface.
The maximum rated depth was not just a calculated crush depth, but also a depth factored with displacement in mind. As displacement actually gives you bouyancy.

Please be aware, that not all boats lost during this era, would have been lost violently, or by crushing. Some would simply not have been able to make the surface, (but we are talking deep), and ran out of air.

I hope this partly answers your question.

Compressed air - air by nature has a high compressibility factor, just like any other gaseous substance.
To explain better, it would be simple to fill a u-boat sized space with an equivalent volume of water, as liquid cannot be compressed. But to fill the same space with air requires much more energy. To fill with high pressure air - even more so, and taking the compressibiity into account - quite a length of time.
I realise, that blowing high pressure air into the waist tanks displaced the water enabling the boat to float back to the surface, but that is high presssure air 'Displacing' the water by pushing only some out. As the boat rises, the air naturally expands, thus pushing more out and so on

StgShultz
12-18-2007, 02:17 PM
Question fully answered thanks - simple answer -too much extra air would be needed, complex answer -too complex to achieve a workable solution & not enough storable air.

Kaleun1961
12-18-2007, 02:20 PM
Which brings us to a little known fact. As a U-boat ascended and its cubic volume expanded due to the release of pressure on the hull, it was often the practice to allow water into the boat, to slow down the rate of ascent. Without doing this, the boat could actually broach in an uncontrolled manner.

In addition to the factors mentioned by others earlier is the influence of the diving planes. A U-boat could have slight negative buoyancy but maintain depth by the diving planes. They act like little wings; the water running over them creates "lift" just as with an aircraft's wings. Given sufficient speed, a U-boat can maintain a given depth in spite of certain buoyancy factors. GWX 2 models this quite well; I have encountered situations where I am diving deep and seem to slowly creep downwards. The only remedy is to increase speed, unless one blows ballast.

Liddabit
12-18-2007, 03:27 PM
Question kind of on this subject.... How do Modern subs go so deep? Just thicker hulls or did they find a way to deal with the internal atmosphere like this?

StgShultz
12-18-2007, 04:29 PM
and... how do do get shaft glands to seal under extra extreme pressures?

klcarroll
12-18-2007, 04:48 PM
How do Modern subs go so deep? Just thicker hulls or did they find a way to deal with the internal atmosphere like this?

It's done "The Good, Old-Fashioned Way"; ....Thicker pressure hulls made out of the latest high tensile alloys. Interior pressure just doesn't contribute enough to be worth the pain of dealing with it.

RE: Shaft Seals.

They've come a long way from "oakum" packed cavities! Modern shaft seals are multiple "V Seal" setups with drains to releave inter-stage seepage.

klcarroll

Kaleun1961
12-18-2007, 04:59 PM
And I bet that techology is a state secret of sorts.

Liddabit
12-18-2007, 05:39 PM
Originally posted by Kaleun1961:
And I bet that techology is a state secret of sorts.

Secret seems to be such a funny term lately the way things get leaked :P

VikingGrandad
12-18-2007, 05:39 PM
Very interesting discussion. http://forums.ubi.com/images/smilies/clap.gif


Originally posted by klcarroll:
RE: Shaft Seals.

They've come a long way from "oakum" packed cavities! Modern shaft seals are multiple "V Seal" setups with drains to releave inter-stage seepage.

I'm not an engineer by trade, but am very interested in engineering subjects like this. I used to help my father with his designs for pressurised storage tanks (nothing like deep sea pressures!), which always included numerous fittings (valves, access hatches, measurement probes, sensors, etc.) that penetrate the skin of the vessel. Surely the mechanical seals for such fittings on a submarine are the weakest points on the pressure hull? In theory, would the hull be stronger if it had no fittings whatsoever, not even access hatches? (I appreciate it would be completely useless!)

Whilst we're on the subject, in 'Das Boot' we see flange bolts shearing on internal pipework joints, causing the bolt-heads to fly off at lethal speed and the joints to fail. How does external pressure causes such damage to internal fittings? Or is it caused by the shockwaves from depth charge explosions?

hueywolf123
12-18-2007, 06:32 PM
Originally posted by Liddabit:
Question kind of on this subject.... How do Modern subs go so deep? Just thicker hulls or did they find a way to deal with the internal atmosphere like this?
Basically, they displace more water, therefore, they recover their bouyancy much faster. They also have thicker hulls, bigger air compressors etc.
Gland seals, Super Duplex Stainless Steel,Nickel Aluminium Bronze bearings, High tensile Cast 316 spacer rings, R-PTFE Compressed Seal Rings. The whole thing tightened to particular tolerances, checked hourly, but still drip at depth.

hueywolf123
12-18-2007, 06:36 PM
Whilst we're on the subject, in 'Das Boot' we see flange bolts shearing on internal pipework joints, causing the bolt-heads to fly off at lethal speed and the joints to fail. How does external pressure causes such damage to internal fittings? Or is it caused by the shockwaves from depth charge explosions?
Hydraulic Shock, as you can't compress a liquid, the shock waves build momentum through the reduced bore of the pipe. As some of these pipes already circulate water from external, this added jolt is enough to cause damage. The flange bolts are meant to shear as it is easier to replace bolts than it is to replace sections of pipe work