Tores mailbox VIIC and VIIC/41 operation and technical details

[tore]

Don.
I believe the green wheel valve is what Maciek says on his drawing. It acts as a shut off valve with watertrap for airbank 4. See my picture below.
Tore

[SnakeDoc]

Hi Don,

1. The photo with all the red arrows pointing to LP valves that lead to blowing the bow buoyancy tank.   Which valve would they use to blow the bow buoyancy tank, the hull valve, or the one down stream? Where are these valves in relation to the forward torpedo tubes?

I have marked the location of the hull valve on the drawing.The "down stream" valve is located on the aft bulkhead of the forward torpedo room (port side).
Which one was used? Hard to say, I suppose, that hull valve would be opened (closed only in emergency) and blowing was done by opening the "down stream" valve.

2. The green hand-wheel is HP air and what is its purpose? 

As Tore answered, this is cut-off valve for the HP air bank. Each HP air bank has similar one.
See this post: http://models.rokket.biz/index.php?topic=921.msg16560#msg16560

3. The photo is of the aft torpedo tube with the hull valve for MBT1 buried so deeply; am I correct?

I would imagine the hull valve for MBT5 is similarly buried in the forward torpedo room section as well. 

That's right.

1 - MBT1 and MBT5 venting valve hand-wheels in the control room

That's right.

2 - The hand lever for venting the stern buoyancy tank in the aft torpedo room (VIIC/41)

That's right.

3 - The hand-wheel for venting the bow buoyancy tank in the control room (VIIC/41)

That's right.

4 - The info just provided about blowing the bow buoyancy tank in the forward torpedo room

That's right.

--
Regards
Maciek

[tore]

Don.
Junker exhaust assumption.
I "redesigned" my idea a bit to match the prevailing photos. To make the system complete I studied the the coolingwater scheme showing the old system which obviously has a seawatercooled hullvalve, a simplified system of the mainengine  hull exhaustvalve. The new shut of valve to the silencer (sea) is outside the pressure hull and could be seawatercooled by the blue pipe on my systemsketch. Otherwise the sketches should be pretty selfexplaining and it only remains to repeat: these sketches are not facts they are assumptions.

[Don Prince]

Hello Mr. Tore,


I have a question about the Bouyancy tanks...


1.  Was the bow buoyancy tank above the waterline?  I remember you saying if the vent were opened that it drained quite nicely...  That would eliminate the need to crawl among the fwd torpedo tubes to get to the hull valve to blow the tank.


2.  The stern buoyancy tank looks to be partially below the water line.  If they want to run the Junkers compressor, did they need to blow the tank to get the stern higher, or just clear the water from the exhaust system?


3.  I don't recall ever seeing a drawing that shows the location of the Junkers muffler and exhaust lines outside the pressure hull.  Do you have anything?


Regards,
Don_

[tore]

Don.
I don`t have a drawing or even a photo of the exhaust silencer and pipes. But I am pretty sure it is situated at about the same level as the mainengine exhaust pipe and silencer, thus well above the surfaced waterline only the outlet probably below the watersurface. The buoyancytanks are marked on your drawing in the skizzenbuch, see my sketch below and the tanks are as you see above the watersurface having the freefloodgates just below the waterline.
 The problem is not the running of the Junker in calm weather, but in rough sea when the submarine is pitching, then the buoyancy tanks are moving under the surface and above the surface. The amount of water in the tanks increases as the bow or stern dips down in the water and decreases as the tanks are above the surface thus you have a pressure pulsation, a blowing shall not be of any use as air escapes as soon as the freeflood gates get out of the water, but the pitching is reduced by the extra buoyancy when the tanks are submerged as long as the vents are shut. Hence the interlock between the sternbuoyancy  ventvalve and the Junker outboard exhaustvalve. Before starting the Junker you could blow through the pipe and the silencer while the outer exhaustvalve  was shut as indicated on my outer exhaustvalve sketch.
Tore

[Don Prince]

Hello Mr. Tore,


Thank you for the drawing and information... 


I believe the line of green colored valves in the control room are for blowing the ballast tanks.  I remember you saying the tanks were not fully blown and they used the diesel exhaust to complete the blowing process. 


How do they know when the tanks are at say 70% empty? Do they allow blowing for so many seconds depending upon which tank?  I could see them blowing the tanks completely empty and the pressure gauge beside the distributor would drop to indicate that tank was empty and they were blowing HP air out the tide valves.

[Don Prince]

Hello Mr. Tore,


Sorry about bothering you again...  Plate 12 Shows 2 lines going to the LP blowing distributor: one is a direct line with valves but at 205 atm, and the other one has valves and a pressure reducing valve to 12 atm.


Why the two very different pressure outputs?   The pressure at around 122 meters would be approximately 12 bar.  Normally they are blowing the ballast tanks at periscope depth.  Would using the 205 kg/sq cm that equal to 201 bar ever be needed.  201 bar would be water pressure at 2.049 meters depth; the pressure hull would be completely crushed.  I know this just an exercise with numbers.  However, could the blowing distributor and its tributaries withstand the high pressure and not blow something?  If they blew the saddle tanks at periscope depth with 205 kg/sq cm / 201 atu would there be any damage?


Regards,
Don_

[tore]

Don.
The air blowing of the tanks is very much based on experience and watching the gauges and to which extent the CO want to surface on air under the prevailing circumstances. As the ballasttanks have different location in the relation to the surface they have different pressures when emptied. The normal surfacing from periscope depth is checked by the depthgauge and the CO checking the testcock of the top hatch to be sure the hatch above the seawaterlevel before he releases the hatchlocks, having the catch still engaged in view of the overpressure. If the CO want to continue airblowing, the saddletanks are watched and you are able to see air is escaping from the Kingstons. Then you continue with the MBT1 and 5, same procedure, and finally 3. As you see the tanks are blown separately in the last stage. The ideal solution would be to stop blowing the highest tank just a bit before emptied and let the air expansion take the rest when blowing the deepest tank (MBT3) but that takes a lot of experience and normally that part of the blowing is done by exhaust. The primetarget is to save precious HP air and that takes experience.
Tore

[tore]

Don.
LP air system.
A good remark, but I don`t believe there are two different pressures . The ordinary supply is the automatic reduction valve, the other connection shut. However the other connection is a back up system which has a reduction valve manually controlled, as the symbol for the supplyvalve indicates a manually controlled pressure reduction valve. You`ll find the same system used amongst several others f. inst HP blowing of the main ballasttanks. Having a manually controlled reduction/stopvalve direct connected to the HP system does not mean you are using the full pressure, it is a reductionvalve manually operated to achieve the appropriate pressure which is shown on the relevant manometer. As a safety there are reliefvalves at the end users, set at the max allowable pressure.
Tore

[Don Prince]

Hello Mr. Tore,


So, if the U-Boat was sitting on the ocean floor at 200 meters, then the 12 atm blowing pressure would be useless.  At 200 meters the external water pressure would be about 21 atm   However, if they used the manual HP air reducing valve and set the blowing pressure to 25 -30 atm, then they could partially blow the MBT3 to get them off the bottom.  Does this sound logical and doable?


Regards,
Don_

[tore]

Don.
When you are blowing the main ballasttanks  you are using the main blowing panel and having direct feeding from the 205 kg/cm2 airbanks. You are using the main blowing valve manually reducing the pressure to what is required up to 25kg/cm2. The main blowing valve has a special sharp valvecone which allows a very accurate pressure  adjustment  shown on the manometer for blowing. The blowing system has reliefvalves for each mainballast tank adjusted to 25 kg/cm2 being the pressure at designed crushing depth( VIIC/41). However the 3.9 m3 air storage  capacity of 205 kg/cm2 is not enough to empty the MBT 1,3 and 5 as you would, according to my calculation, need about 12 m3 of 205 kg/cm2 , at 200 meter you shall need appr 10 m3 of 205 kg/cm2. If you would like to blow only MBT3 incl ducts ( 47,7m3) at 200m you would need according to my calculation 4,65 m3 of 205 kg/cm2.
Thus at these extreme depths it is rather the capacity above the pressure which is the limiting factor. So I guess it is not a good idea to sit on the seabed at 200 meter and try to surface by blowing air.
Tore

[Don Prince]

Hello Mr. Tore,


I went back and checked with the "Diving Requlation Manual" that Maciek translated...


231 - 239 provides some guidance on settling and breaking loose.



"After lying on the bottom for a long time all bilges are to be pumped into the ballast
tank intended for this. The filling of all bunkers and ballast tanks is to be examined,
and the specific weight of sea water is measured for comparison.



If necessary, continue pumping with the auxiliary drain pump, in order to achieve as gentle a
release from the bottom as possible and to avoid the boat shooting up too quickly.


Blowing with compressed air to release from the bottom must be avoided because of the rising
billow of air. However if blowing can not be avoided, then it must take place in the stern,
because as a result, the boat separates more easily from the bottom than by blowing the center
ballast tanks.
 
After the freeing the boat from the bottom the ballast tanks are to be immediately flooded
again. Blowing the ballast tanks must be carried out carefully, so that inadmissible
pressures in the ballast tanks are avoided, should the flood valves lie in the mud.


239. During release from the bottom the boat is to remain bow down (at least 2°), so that
rudders and screws are not damaged. As soon as the boat is bow down and free forward and aft,
the propellers may be engaged."


So again, I was WRONG!!!  All I had to do was (RTFM) Read The Fine Manual...


Also, the blowing manifold is using HP air to blow the ballast tanks.  It all a matter of the difference in volume between the HP air input pipe diameter; the HP air volume can expand within the ballast tank lower pressure environment, and the water is pushed through the bottom open Kingston/tide valves larger exit area into the sea which provides the blowing resistance according to the depth.



Regards,
Don_

[tore]

Don.
As you correctly say  the diving manual deals with being stuck  in the muddy seabottom where you are using momentum to break loose. If we play just with volumes and pressures, Boyle- Mariottes and good old Archimedes laws are the ruling factors.
 If you blow your tanks at large depth say 150 meters, not stuck in the sea bottom and the boat is balanced to neutral buoyancy by the compensating- and trimtanks, and for some reason you want to surface by air, you don`t blow the tanks empty at that depth even if you need MBT 1, 3 and 5 empty to be fully surfaced. You blow carefully with lowest possible pressure and when the boat start to rise stop blowing as you have started a process with gradually less resistance pressure allowing the air in the MBT to expand and pushing the water out through the Kingstons as you rise. If you blow to hard, the tanks empties too quickly and you loose  air through the Kingstons as you rise. In general you would avoid such a surfacing as it is uncontrolled.
Tore

[Don Prince]

Hello Mr. Tore,



The Safety Valve settings were taken from the U-Boat Type VIIC manual published in 15.7. 40,
and do not reflect changes made to the VIIC/41.  The HP air manifold Safety Valve is set at 205 kg/ sq cm which is approximately 201.03 Bar, and the ballast tank blowing manifold air Safety Valve is set at 30 kg/sq cm (29.42 Bar).


Note, I would think the blowing air would be adjusted to 25 Bar/ ATU since all the gauges were in ATUs? That would put the blowing pressure below the safety valve setting.


Do you know is the Blowing distributor for the VIIC/41 had a different Safety Valve with a higher setting to allow blowing compressed air at a deeper depth since the VIIC/41 could go deeper?  Otherwise, If the u-boat were below 280 meters blowing compressed air would not be an option...  That Safety Valve is causing me grief!!!


Regards,
Don_

[tore]

Don.
Its hard to remember the figures, I was an EO for a VIIC/41 U995 and a VIIC  U(926) and cannot remember any different settings of the reliefvalves. The crush depth was really an extreme depth and we were never even near it. The max operational depth for a VIIC/41 was 120 m and a VIIC 100m we stayed within those limits. To blow ballasttanks at 250 or  280 meters would be a desperate emergency and I don`t believe the boat was designed for such an operation. I guess in a situation requiring a blowing at that depth,  you try anything and if the reliefvalve would blow, you adjust the valve. I feel like citing my old submarine teacher and RN commander (E): Why spend time, effort and money to learn how to escape from very rare and impossible situations, better to learn people how to operate the boat within the design limits preventing you to get into such situations. At 280meters you are lost anyhow and I guess it isn`t  worth while to worry about the reliefvalve setting.
Tore