Tores mailbox VIIC and VIIC/41 operation and technical details

[VIC20]

Tore, do you know what diameter the lines of the compressed air blowing installation to the tanks had (or the inlet openings for that in the tanks itself? Or was that somehow connected to the air vents of the tank?)… I need that information to calculate the time necessary for blowing tanks at a given depth.


The pressure of the air in the air flasks was 200-205 atm - how long could you keep the pressure at 25 atm in the blowing installation? The flasks were at different positions how were they depleted, one by one or all together?


It took about 4 1/2 hours to completely refuel the compressed air flasks, is that correct?

[tore]

Mark.
Ref. plate 17. The Hp air system consist of 12 HP flasks each two flasks connected into a bank thus 6 banks. Bank 1 outside pressurehull, both sides of aft ext. torpedocontainer. Bank 2 .outside pressure hull appr. above the main engines. Bank 3 inside pressurehull both side of PO mess. Bank 4 inside pressure hull port side fwd. torp. room. Bank 5 inside pressurehull stb. side forwd torp room. Bank 6 outside pressurehull one flask inside bow buoyancy tank the other stb. side torp container.
The banks are connected to a distribution panel in the controlroom and then to the various blowing distr. panels as can be seen on plate 17.
The blowing pipes to the ballasttanks are going from the blowingpanels to a NR valve and a hullvalve directly into the tank, not via ventingpipes, see photo below. I don`t have the diameter of the blow line but if you look at the photo I should make a guesstimate of 20-25 mm.
I believe each HP flask has a volume of 325 liters and the E- compressor has a capacity of 6.1 liter/min at 205 kg/cm2, the Junker compressor has a capacity of 8.5 liter/ min at 205 kg/cm2, which should make it possible for you to calculate the time for recharging the air. You could keep the pressure at 25 kg/cm2 as long as your HP flasks had a pressure exceeding that.
Tore

[VIC20]

Thanks, maybe one day when I visit U 995 I'll measure it 

[tore]

Mark.
Give her love and kisses from me.
Tore

[SnakeDoc]

Hi Gentlemen,

I have not been here for a some time, so I'll try to answer for some questions in one post.

Interesting stuff Maciek, details like this won't make it into the first version of my game but I will try to implement it into the later main version which should be much more complex.

Nonsense    It's much easier to do a lot of this now, at least the basic stuff.


Where can I find info how quickly the different tanks can be filled and how quickly they can be blown?

We were more concerned about f.i. diving time rather than surfacing time. Crash diving required a very well trained crew.   With normal preparation and a trained crew you could manage 30 seconds which is pretty fast compared to other similar submarines at that time.

My (hopefully not totally idiotic) calculation for the required time to flood the MBT below the command room (when no other tanks are flooded and the boat can still float on the surface) is something like 11.64 seconds
Do you think that could be OK?

When I use the standard leak formula the result is that the 47 cubic metres of this tank could be theoretically flooded within 2.29 seconds (if the tank would be completely open on top instead of having 2 valves for the air to leave) when the "leak" of a comparable size is at a depth of 4 meters.


My calculation takes the leaving air through the 2 vents into account that's why it's much slower. But 10 seconds fits with some numbers I have read yesterday about flooding a MBT on a "Tauchboot" compared to real modern submarines which flood slightly slower.
Shouldn't the MBTs already have completely filled when you've reached pericope depth? I think the positive buoyancy would be too strong to go that deep that fast otherwise. But that's just my guess… 

I have also used some standard formulas for a leak into the tank with opened top. As a result I had about 10 seconds for flooding all three main ballast tanks. Taking into the consideration the diameter of the vent ducts, I think it fits into the practical times of achieving periscope depth.

In the german original version of the manual I've found the sentence:


"Die Regelzellen sind angeschlossen an die Fluteinrichtung in der Zentrale…"


which means:


"The regulating tanks are connected to the flood installation in the command room"


But the english translation says:
"The regulating tanks are connected to the drainage installation in control room"

…in the description of the Torpedo compensating tanks Maciek used "drainage installation" for the word "Lenzleitung" which is correct (and the opposite of a "Fluteinrichtung")

Literally translation of the word Fluteinrichtung is flood installation as you said. I have translated it as drainage installation, because in other parts of manual, this system is called either Flut- und Lenzeinrichtung or (more common) Lenzeinrichtung and I tried to keep consistency. This Flut- und Lenzeinrichtung was used to drain bilges and tanks or to flooding regulating tanks, as Tore explained.

As Tore said, the British English term is rather bilge system or bilge pump rather than (as in American) drain/drainage system. You can easily see this difference while reading British report  and ONI report on HMS Graph (or American reports on type IXC and XXI U-Boats). I used rather American naval terms.

The torpedo compensating tanks… in the german version of the manual they write "Die Torpedozellen nehmen das Wasser für den Gewichtsausgleich der Torpedos und Minen sowie das Umhüllungswasser auf" - Maciek forgot to translate the word "Umhüllungswasser" in the english version: "The torpedo compensating tanks are flooded to compensate for the weight of a launched torpedo or mine." - this word is pretty tricky to translate it's like "surrounding water", I guess they mean the water used when flooding the torpedo tubes?

Right you are, my mistake.

Have you seen my document on torpedo tubes?
http://www.ubootwaffe.pl/en/u-boats/equipment/torpedo-tubes-of-german-u-boats

Tore, do you know what diameter the lines of the compressed air blowing installation to the tanks had (or the inlet openings for that in the tanks itself? Or was that somehow connected to the air vents of the tank?)… I need that information to calculate the time necessary for blowing tanks at a given depth

The inner and outer diameter of blowing lines are 17 and 20 mm respectively.


--
Regards
Maciek

[VIC20]

Have you seen my document on torpedo tubes?
http://www.ubootwaffe.pl/en/u-boats/equipment/torpedo-tubes-of-german-u-boats

…

The inner and outer diameter of blowing lines are 17 and 20 mm respectively.

Yes I stumbled upon it yesterday.


Wow, where do you found this information about the diameter?


I'm trying a new calculation for the tanks now based on a paper from 2011 about computer controlled ballast tanks for non dynamic diving (from reading this paper it seems that they haven't introduced this at least till 2011 on submarines).

[VIC20]

At 25 atm (and 20°C) the mass flow of air into one tank should be 1.41145831051 kg per second 

[Don Prince]

Hello Mr. Tore,


I have a book "Type VII U-Boat" and it stated the electric air compressor was primarily used while submerged.  That statement looks to be uninformed to me!  First it is noisy and would help the enemy to locate the U-Boat, and would drain the battery.  Just where is the electric air compressor going to get its air supply to compress - from the pressure hull?  And who lives in the pressure hull - People who can be affected by no air and high negative pressure.


What do you think?


Regards,
Don_

[SnakeDoc]

Don,

I have a book "Type VII U-Boat" and it stated the electric air compressor was primarily used while submerged.  That statement looks to be uninformed to me!  First it is noisy and would help the enemy to locate the U-Boat, and would drain the battery.  Just where is the electric air compressor going to get its air supply to compress - from the pressure hull?  And who lives in the pressure hull - People who can be affected by no air and high negative pressure.

In some cases E-compressor was used while submerged:

Excess pressure of more than 40 mb is to be pumped off with the electric
compressor, if possible, to prevent carrying away acid when opening the
conning tower hatch.

Tauchvorschrift, paragraph 202

--
Regards
Maciek

[VIC20]

Hello Mr. Tore,


I have a book "Type VII U-Boat" and it stated the electric air compressor was primarily used while submerged.  That statement looks to be uninformed to me!  First it is noisy and would help the enemy to locate the U-Boat, and would drain the battery.  Just where is the electric air compressor going to get its air supply to compress - from the pressure hull?  And who lives in the pressure hull - People who can be affected by no air and high negative pressure.


What do you think?


Regards,
Don_

no idea but my thoughts are:


1. I think the air of the quick diving tank is always released into the pressure hull, so they could compress at least 6.51 cubic meter without reducing the pressure inside the boat below standard atmospheric pressure. Not much but this would be worth 6.5 tons of weight close to the surface.


2. At an altitude of 2000 meters (which is still OK for humans) the atmospheric pressure is 77% of sea level (783.8 hPa) so for the crew itself it would be safe to reduce the pressure to that amount.
The air inside the boat is 391 cubic meter hence 89.93 cubic meters could be compressed which is 449 liters at 200 atm (the total amount of compressed air in the flasks is about 3900 liters at 200 atm)


3. at greater depths the pressure in the boat should increase due to the slightly pressed hull another light amount of air would be available to compress.


I could only imagine that they would do it in case of emergency when laying on the bottom at a great depth with damage to all saddle tanks and in case that the pumps can not overcome the water pressure (no idea how they perform at lets say 200m) - but maybe they could somehow attach the electric compressor directly to the 25 atm blowing system to speed things up?? This device should have more than enough power to do that.


But I guess the stress on the hull would increase with less air in the boat…


…and you would need to put all that air into the boat again before you could open the conning tower hatch


And of course I have no idea and talking nonsense   

[Don Prince]

But I would not think this was the primary electric air compressor mode for operation; submerged?  This mode of operation would never replenish the air in all those tanks.  This had to be done while surfaced (I think)!!!


Regards,
Don_


PS


Does anybody have a photo of both Toggle Switch Switchboards (Port and Starboard) of the older style U-Boats?  I'm having some issues determining how the circuitry works with only half the picture (Port side only in the manual).  There seems to be a difference in switches according to the circuitry in the manual, but I want to be sure and not guess. 

[VIC20]

Maciek there is another mistake in the manual (possibly at a lot of places):


On page 40 you've translated it to "The highest pressure in the installation is 25 atm"
Correct would be "The highest pressure in the installation is 26 atm" (and for the diesel 1.5 atm)
They've wrote 25 atü - atü is "Atmosphere Überdruck" which means "atmosphere overpressure" and is:


X atü = X atm + 1 atm (or more correct at instead of atm)


You will instantly understand why when you look at the 0.5 atü (1.5 at) pressure used to blow out tanks with the diesel. The 0.5 atm from the english manual would be below the atmospheric pressure and it would not be possible to blow out a tank with that, actually it would do the opposite and work like a drinking straw.
The water pressure at a depth of 5 meters is about 0.5 atm but 1 atm of the atmosphere has to be added which is 1.5 atm hence you can blow out the tanks with the diesel up to 5 meters (depth of the bottom of the tank) which perfectly fits to MBT3 which bottom is approx. 80 cm above the keel at a draft of the boat of 4.74m so it is about 4m below the water surface. So when the tank is empty you still have an overpressure in the tank of approx. 0.1 bar.

[VIC20]

But I would not think this was the primary electric air compressor mode for operation; submerged?  This mode of operation would never replenish the air in all those tanks.  This had to be done while surfaced (I think)!!!

Yes sure. But it seems they could use it at least to compress some air while being submerged (if that makes any sense at all is a different thing)

[tore]

 Don and everybody.
The E- compressor was used when submerged and not schnorchling mainly to get rid of the overpressure in the boat which always built up. The overpressure started to build up as soon as the tower hatch was shut as the pneumatic motors for the grinding of the group exhaustvalves in the engine room were operated. Depending upon the leakages you could use a awful lot of air and you could get a substantial overpressure. Inboard venting and small leakages contributed as well. If we had time we were running the E-compressor prior to surfacing to to prevent a rapid pressure fall when the tower hatch was opened. Many gauges were influenced by the higher ( and lower) airpressure  f.i. some of the depthgauges and at last but not least the cooks preparation for the meals were highly influenced as higher pressure increased the boiling temperatures. Yes people on board a submarine got to learn the practise of the thermodynamics and particularly the Boyle-Mariottes Law. 
Tore

[VIC20]

I've read in "the death of the uboats" that at least one commander died when opening the hatch after (I think emergency) surfacing, he was blown out of the boat like a rocket.