Tores mailbox VIIC and VIIC/41 operation and technical details

[tore]

Don.
The bow and stern buoyancytanks are not considered as ballasttanks, if you use the German words for the tanks it literally says watertight fo`csle and watertight stern. They are situated mainly above the normal surface line and the main purpose for the tanks are to dampen the longitudinal pitching of the vessel. They have as you say vents opened locally at the forward and aft torpedorooms. They are normally not taking part in the surfacing of the submarine.
Tore

[tore]

Don.
In addition to the tanks you are mentioning there are two  luboil system tanks, port and starboard. May be a short description of how the system works can be useful. Each engine has normally a closed luboilsystem where the system luboiltank (f.i. stb for stb engine) is intergrated. The no 1 and 2  luboiltanks are merely storagetanks. The dirtyluboil tank is pretty much like the fueloil collecting tank taking luboildrains and at the time oil purifier was installed, receiving the oil sludged from same. The tank can be emptied by a handpump on the port aft bulkhead in the engine room as well by a suction hose connection to a shore pump. The system oil tanks port and starboard can be emptied by the electrically driven reserve lub oil pump port fwd  in the engine room or by the a hose connection to pump ashore as well as by the luboil hand pump at stb. fwd. bulkhead in the engine room. Filling the systemtanks  from the luboilstorage tanks can be done by the same pump
Tore

[Don Prince]

Hello Mr. Tore,


I was reviewing information on a web site about starting marine diesel engines...


http://www.marinediesels.info/Basics/air_start_simple.htm


Did the GW and MAN engines have the basic components discussed here built onto the engines for air starting?


Regards,
Don_

[tore]

Don.
The airstarting of the GW engines and almost the same on the MAN was a bit different. Referring to plate 29 below : Each engine had a starting air flask topped up from the HP system. Each cylindercover had a starting air valve which in the starting sequence was engaged with starting cams on the camshaft by a pushrod. The valve had two airconnections, one at the top for pneumatically pushing the starting valve pushrod in contact with the camshafts starting cams. The other air connection is the startingair supply to the cylinder. The starting procedure is done by the starting handle on the maneuveringstand and the mainstarting supply valve having a handwheel ( painted red on U 995). The main starting supplyvalve is opened, starting handle is put in start position , air is admitted to the top of the cylinder startingvalve, pushing the valve rod against a springpressure down to the startingcam on the camshaft. One of the cams is always in a startingair lift(open) position thus admitting startingair to the cylinder and the engine starts to run on air, by the starting valves. As soon as the engine achieve satisfactorily revs you move the fuel handle and fuelrack of the fuelpumps,   admitting fuel to the cylinders and the engine starts to run on fuel. Then you put the starting handle to "neutral" the air on the top of the startingvalve is vented and the spring pushes the valverod up and disengage the connection to the camshaft. As plate 29 shows the system of the direct reversible engine, the real execution of the modified nonreversible execution is slightly different, but the idea is the same.

[VIC20]

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?

[tore]

Mark.
I cannot remember seen any such figures, may be Maciek have an idea. 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. I Guess it could be contributed to the relatively large flowarea of the Kingstons plus the permanently submerged hydroplanes.
Tore

[VIC20]

OK, I might find a solution to calculate that the diameter of the vents is written on Uboatarchive,net…


Do you remember how much compressed air could be stored?

[VIC20]

Found it in the manual 

[tore]

Mark.
3.900 liters at 205 bar/cm2. You didn`t blow the tanks by HP air at depths greater than 14 meters normally only for emergency you are blowing tanks at greater depths. Hydroplanes and speed is a better way of changing the depth, better control and saving of air.
Tore

[VIC20]

But during an emergency situation you can only blow the main ballast tank below the command room right? The other MBTs would be destroyed at higher depths if they would be blown right?

[VIC20]

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?

[tore]

Mark.
Air blowing.
Emergency blowing is usually done by all the MBTks as it is the differential pressure which counts. As long as the tanks have free access to the sea and you are blowing keeping the differential pressure well below I believe o,6 bar, MBT 3 bar it is OK. As soon as the boat stop sinking and the boat start slowly to ascend, you stop blowing and the air start to expand in the tanks forcing out the remaining water out creating a fairly uncontrolled acceleration ascent which is not desirable except for emergencies.
The total air capacity of 205 bar is sufficient to to blow all the main ballast tanks and reserve fuel oiltanks totalling 155m3 at 40 meters and for the main ballasttanks only totalling 105 m3 at 65 meters. If you end up at 100 meters only 71 m3 can be blown, just to mention a few examples I found in the U boat info (1939) translated by Maciek.
Tore

[tore]

Mark.
Flooding MBT 3.
47,7 m3 including venting duct to be flooded in 11-12 seconds seems awfully fast, but with the large Kingstons I guess it could be possible. I never have been experiencing such a test. Both normal diving and particularly crashdiving was done in a more dynamic way using hydroplanes and speed in addition to ballasttanks.
Tore

[VIC20]

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… 

[tore]

Mark.
I believe it is OK. Don`t forget it is not only the MBT 3 vent restrictions, but the fairly long airducts through the saddletanks as well. You would like to have the ballasttanks filled as quick as possible and certainly at periscope depth.
Tore