Tores mailbox VIIC and VIIC/41 operation and technical details

[tore]

Mark.
True, we always had a man with a firm grip on the COs legs in those cases we had to surface without pressure equalizing. And Mark, running the E compressor at 200m would be a bit risky. The E-compressor was depending upon a seacooling system consisting of inter stagecoolers, water jackets, pumps and valves. we never operated or opened the seavalves at great depths as blowing a gasket or getting a crack at depths like 100 m would be troublesome.
Tore

[VIC20]

But what if you would lay on the ground at 200m and the boat has negative buoyancy (or dead engines) for some reason? Is the situation then die or blow and die?

[tore]

Don.
In desperate situations you try anything, and improvise. When I studied submarine technique a class ahead of me disappeared with the loss of HMS Affray. I remember the old Commander E`s wise words as the tabloid press screamed about building a multimillion submarine escape tank: why don`t use the money to improve the crews skill to run a submarine rather than how to escape or handle impossible situations.
Tore

[SnakeDoc]

Hi Gentlemen,

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)!!!

Well, generally you are right. First of all, in the book "Type VII U-Boat" are some errors. The sentence saying about usage the compressor while submerged has sense only in the context explained above, or can be treated as another author's mistake.

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:

Mark, you are right. I had seen the difference while translating the text, and even had a discussion with Jerry, how
to handle designation of units (in English there is no simple corresponding term for atü unit). And finally, this matter
was forgotten.
I will correct errors you have spotted.

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.

Here is interesting discussion on this topic:
http://uboat.net/forums/read.php?20,62642,62699#msg-62699

The E-compressor was depending upon a seacooling system consisting of inter stagecoolers, water jackets, pumps and valves. we never operated or opened the seavalves at great depths as blowing a gasket or getting a crack at depths like 100 m would be troublesome.

That's interesting notice indeed, Tore. I have checked, that cooling system of E-Verdichter was tested up to 15 atü.

--
Regards
Maciek

[tore]

Maciek.
Testpressures and deep diving.
It was common, anyhow in my peacetime, not to go deep ,say down to 100m, without shutting off for deepdiving (and depthcharging), meaning boardvalves and branchvalves to systems not absolutely necessary. Air compressor was not considered being absolutely nessesary at these depths when we are talking about routine dives. In emergency situations you always evaluate all options to be taken, balancing risks against benefits. Laying stuck at 200m I guess is one of the situations where "the devil eats flies"you try everything.
Tore

[VIC20]

Maciek,


I think something is wrong with your translation of the main drain pump:


"The drain performance by // and -- mode are similar up to depth of 57 meters.  At greater depth the performance by // mode decreases strongly.  It is practical to switch to the - - mode at a depth of 50 meters.  During trials, by connecting the auxiliary trim and drain pump (358 liters/minute) in parallel, drain performance at depths greater than 50 meters is increased by 40%."


Correct would be something like this:


"The performance of // and -- mode will cross on the graph at a depth of 57 meters. At greater depth the performance decreases strongly when operating in // mode. In practice you should switch to the -- mode at a depth of 50 meters. By the hook-up of the auxiliary trim and drain pump (358 liters/minute) in serial to the suction and pressure lines of the main drain pump, the drain performance at depths greater than 50 meters was increased during trials by 40%"


But it's possible that I just don't understand it all all 
Anyway "Praktisch ist ab … überzugehen" is clearly a directive.

[SnakeDoc]

Hi Mark,

"The drain performance by // and -- mode are similar up to depth of 57 meters.  At greater depth the performance by // mode decreases strongly.  It is practical to switch to the - - mode at a depth of 50 meters.  During trials, by connecting the auxiliary trim and drain pump (358 liters/minute) in parallel, drain performance at depths greater than 50 meters is increased by 40%."


Correct would be something like this:


"The performance of // and -- mode will cross on the graph at a depth of 57 meters. At greater depth the performance decreases strongly when operating in // mode. In practice you should switch to the -- mode at a depth of 50 meters. By the hook-up of the auxiliary trim and drain pump (358 liters/minute) in serial to the suction and pressure lines of the main drain pump, the drain performance at depths greater than 50 meters was increased during trials by 40%"


But it's possible that I just don't understand it all all 
Anyway "Praktisch ist ab … überzugehen" is clearly a directive.

Right you are, these pumps should be connected in serial.

--
Thanks, regards
Maciek

[VIC20]

great - helping to find mistakes is fun

[VIC20]

Tore, this simple formula is from scientific paper about submarines - you can estimate the reduction of buoyancy caused by the outside pressure compressing the hull:


B0 = buoyancy due to vehicle displacement at zero depth
B = current buoyancy due to vehicle displacement at a given depth
Z = depth


B = B0*(1 - ((0.015 / 300) * Z))     (I always use more brackets than necessary  )


So at a depth of 200 meters the buoyancy is reduced by approximately 1% (8.65 tons)


What have you done to compensate that? That's almost 18.5% of the volume of MBT3. Was the lift force of dynamic diving still enough? (I haven't calculated the dynamic forces yet)

[SnakeDoc]

Tore, this simple formula is from scientific paper about submarines - you can estimate the reduction of buoyancy caused by the outside pressure compressing the hull:


B0 = buoyancy due to vehicle displacement at zero depth
B = current buoyancy due to vehicle displacement at a given depth
Z = depth


B = B0*(1 - ((0.015 / 300) * Z))     (I always use more brackets than necessary  )


So at a depth of 200 meters the buoyancy is reduced by approximately 1% (8.65 tons)


What have you done to compensate that? That's almost 18.5% of the volume of MBT3. Was the lift force of dynamic diving still enough? (I haven't calculated the dynamic forces yet)

Mark, in Tauchvorschrift is the following paragraph (no 181):

Because the boat loses volume at greater depth, it must be made lighter by timely pumping (for type VII boats approximately 1 ton per 100 meters).  It is appropriate to pump out a greater amount, because the boat will be heavier by the accumulating leak water anyway.

http://uboatarchive.net/DivingRegulations.htm

Compensating buoyancy reduced by 2 tons (at 200 meters) could be easily done by means of regulating tanks (of total capacity 24,6 ton).

--
Regards
Maciek

[Don Prince]

Hello Mr. Tore,


Is there a master switch for the battery compartments in the Galley and the Officers sleep area?  Do you have any photos?


Did the battery compartments have any type of warning system about pressure or chlorine gas sensors? Was there any means of exhausting the chlorine gas while submerged?


Regards,
Don_

[tore]

Don.
I don`t have any photos of the automatic batteryswitches they were placed in special small rooms as indicated on the below sketch. The batteries and battery compartments were checked twice a day taking acidgravety and water topping. Looking at plate 18 you should be able to figure out the battery ventilation system which did not have any possibility to get rid of the chlorinegases while submerged except rising the schnorchel. However we had a lime installation as can be seen on plate 26. As to automatic chlorine warningsystem I can`t remember we had anything like that.
Tore

[Don Prince]

Thank you Mr. Tore for the information regarding my question about the batteries...


Today, I am just about finished working on the circuit schematic for the Rotary Switchboard.  However, I'm just not completely sure about  the Field 1 and 2 hand-wheel in the center.  I believe these wheels are independent and move separately and the normal Low speed for the outer speed hand-wheel is to the left (see attached photo LOW/Niedrig is to the left).  The photos I have from U-995 shows the wheels in the center and to the right.  I guess museum traffic and people turning the Hand=wheels makes for my confusion because I don't believe they are in the shutdown position.


Does anyone have information about the smaller hand-wheel that is used to select Field 1 or Field 2 on the speed controller?  Where is the normal shutdown, not active position?


Regards,
Don_

[tore]

Mark.
Submarine displacement at great depth.
When you use formulas like you found involving displacement figures, depth and hullcompression you have to be aware of that the submerged displacements figures is a bit different to a surface ship. At great depth the compression of the pressure hull (displacement) is the only component which is influenced by the submergepressure. The steel, wood and other solid meterial like the superstructure,  is not significantly influenced by the waterpressure at 200 meter. Thus the diminishing of the displacement is only that of the internal volume of the submarine which you could estimated to 450-500 m3. Your 865 tonnes (1 % 8.65) is based on the totalt weight of the submarine.
Tore

[VIC20]

Thanks Tore, calculating it with 450 tons is still more than twice as much as the historical documents says (2.25 tons per 100m instead of 1 ton). In this case I trust the older data.