Diving and Depth  
          Diving and trimming a U-boat is exceedingly complicated and unfortunately there are fewer and fewer people around who know how it was actually done.  Each navy designed its submarines differently and used different procedures.  British and American equipment and procedures can only be applied to U-boats in a very general way.   In the Kriegsmarine, equipment and procedures differed greatly between types of boats and even from boat to boat based on the preference of the chief engineer and commander.  In this article we will attempt to explain in a general way how a U-boat dives and introduce the terminology so that the reader of a KTB can understand entries concerned with diving and malfunctions of diving systems.  
     
  Diving  
          To gain a basic understanding of submarine technology it is recommended that the reader visit the websites below which provide and overview of submarine diving equipment and procedures.  
          Johan Heiszwolf has created an excellent page explaining in general terms how a submarine dives.  German U-boats followed the US/UK model described in his comparison of US/UK and Russian operating procedures.  The Fleet Submarine site provides general procedures for diving a US fleet boat.   For detailed information on US Fleet boat equipment and procedures see the Fleet Type Submarine Online at the website of the San Francisco Maritime National Park Association.  
 
    Click the icons to proceed to the associated websites
  John Heiszwolf's page on Submarine Technology
  Fleet Submarine.com page on Submarine diving procedure (covers only procedures for US Fleet boats)
 
The Fleet Type Submarine Online at the San Francisco Maritime National Park Association
 
   
  Submarine Diving Tanks, Valves and Pumps  
          The table below is intended to introduce basic terminology and the functions of diving equipment.  It is greatly oversimplified.  Some ballast tanks doubled as fuel tanks and some could be emptied and filled by several means.  Additionally, tanks valve and pumps were redundant, and could be adapted to another purpose if need be.  
Equipment
Function
Condition when surfaced
Condition when submerged
 
Main Ballast Tanks When filled - changes buoyancy of the boat from very positive on the surface to very slightly positive Completely empty Completely filled
Safety Tank A ballast tank that may be emptied to provide positive buoyancy in the event of water ingress to the pressure hull or the malfunction of other ballast tanks Filled or empty Completely filled (unless there was an emergency)
Quick Diving Tank When filled (in conjunction with the main ballast tanks) - changes buoyancy from very slightly positive to very negative in order to enable the boat to dive quickly as soon as the boat is headed down it is emptied to bring the boat to very slightly positive buoyancy Filled or empty Completely empty (filled and emptied during the dive maneuver)
Regulating/ Compensating or Main Trim Tanks Located near the center of gravity water level and are filled so that the boat is at neutral buoyancy when submerged - because there is one or more to port and starboard they can also be used to adjust list Partially filled Partially filled (no change during dive)
Trim Tanks Located inside the pressure hull far forward and aft - connected by a line - water is pumped back and forth to balance the boat fore and aft Partially filled Partially filled (no change during dive)
Torpedo Tanks Located in each torpedo room  - water is admitted to compensate for the loss of weight from firing a torpedo Completely empty Completely empty (filled when a torpedo is fired and emptied thereafter)
 
Flood Valve/Kingston Valve located at the bottom of a tank which when opened allows water to enter the tank  
Vent Valve at the top of a tank which allows air to escape when water is filling the tank  
Dive Planes Movable control surfaces located for and aft used to dynamically adjust the attitude of the boat and maintain depth once submerged and at near neutral buoyancy
       
     
  Type VIIC Diving Tanks, Valves and Pumps  
          The table below, adapted from the Admiralty Technical Report on U-570, summarizes the tanks, valves and pumps aboard a Type VIIC boat.   
Tank
Between Frames
Capacity
Fitted with
                                                                    (tons sea water)
No. 1 Main Ballast tank
-10 to 0, external, aft of pressure hull above aft torpedo tube
31
Hand-worked vent, direct blow, exhaust blow - no Kingston valve, the bottom is open to the sea
No. 2 Main Ballast and Reserve Fuel Oil tank
17-34,  external, in saddle tank
11.4
Two kingston operating positions working four kingstons, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 port and No. 4 port fuel tanks, one after-end auxiliary vent leading into a duct to a common auxiliary vent for port and starboard tanks, direct blow, exhaust blow, compensating and blow-out connections.  Saddle tank fuel tanks are tested to only 15 lbs./sq. in.
No. 2 Main Ballast and Reserve Fuel Oil tank
17-34, external, in saddle tank
11.4
As for No. 2 port, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 starboard and No. 4 starboard fuel tanks, one after-end auxiliary vent leading into a duct to a common auxiliary vent for port and starboard tanks.
No. 3 Main Ballast. tank, internal
40-49, internal, below control room
47.75*
Four operating positions working six kingstons, direct blows, exhaust blows, two T-wrench operated emergency vent valves to port and starboard at the pressure hull to a duct which leads through the No. 2 Regulating tank at the end of which are two quick-opening lever-operated vents to the sea - tank tested to full diving depth so Kingstons could remain open at depth.
No. 4 Main Ballast and Reserve Fuel Oil tank
45 1/2-62, external, in saddle tank
13.5
Two operating positions working four kingstons, one T-wrench operated emergency vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 port and No. 4 port fuel tanks, direct blow, exhaust blow, compensating and blow-out connections.
No. 4 Main Ballast and Reserve Fuel Oil tank
45 1/2-62, external, in saddle tank
13.5
As for No. 4 port, one emergency T-wrench operated vent valve leading into a duct to a common quick-opening lever-operated vent for No. 2 starboard and No. 4 starboard fuel tanks.
No. 5 Main Ballast tank
80-106, external, forward of pressure hull surrounding forward torpedo tubes
25.3
One hand worked vent, direct blow, exhaust blow - there is no Kingston valve in this tank the bottom is open to the sea
Water tight stern/Stern buoyancy tank
-15 to -10, external, in extreme stern above torpedo tube
4.04
Locally worked vent and blow - no Kingston valve, the bottom is open to the sea
Water tight bow/Bow buoyancy tank
102-103, external, in extreme bow above torpedo tubes
4.04
As for W/T stern tank.
No. 1 Regulating Fuel Oil tank, port 34-38, external, in saddle tank
3.66 (oil)
Not normally used as a ballast tank - connected to fuel filling system, fuel transfer blow in engine room, blow in control room, suction from bilge pump.
No. 1 Regulating Fuel Oil tank, starboard 34-38, external, in saddle tank
3.66 (oil)

As for port tank.

No. 2 Regulating Ballast tank, port
38-44, external, in saddle tank
7.93
Suction to ballast line, no Kingston valve, reduced blow in control room, inboard vent, differential gauges.
No. 2 Regulating Ballast tank starboard
38-44, external, in saddle tank
7.93
As for compensating tank, port.
Quick diving/ Negative Buoyancy tank, port
44-46, external, in saddle tank
2.17
Hand-worked kingston, inboard vent, blow in control room, differential gauges.
Quick diving/ Negative buoyancy tank, starboard
44-46, external, in saddle tank
2.17
As for "Q" tank, port.
No. 1 Torpedo Compensating tank
4 1/2-7 1/2, internal, just forward of after trimming tank in after torpedo compartment
2.35
Suction to main line, connection to tube blows, water meter, sea flood.
No. 2 Torpedo Compensating tank, port
63-69 1/2, internal, below after half of  forward torpedo compartment
5.8
Connected to ballast pump and to tube blows, water meter, sea flood. (ballast pump vice HP air used to avoid noise of air blowing)
No. 3 Torpedo Compensating tank, starboard
63-69 1/2, internal, below after half of  forward torpedo compartment
5.8
As for port torpedo tank No. 2.
After trimming tank
0-4-1/2, internal, below torpedo tube
3.58
Suction to trim pump and blow from trimming blow line with a flow meter to measure the amount of water transferred between trimming tanks
Forward Trimming tank
69 1/2-74, internal, below forward half of torpedo compartment
3.69
As for after trimming tank
*Admiralty Technical Report on U-570 says 25.3 tons but this seems to be an error. ONI report with original Design and Specification Books say 47.75 tons
   
  Displacement  
        The following is a back of the envelope calculation of the effect of flooding ballast tanks on buoyancy*:
Displacement submerged = 883 tons
 
Surface displacement with 2 & 4 MB tanks empty             = 739 tons
Surface displacement with 2 & 4 MB tanks full of fuel       = 784 tons
Subtracting surfaced displacement from submerged displacement gives:
Reserve buoyancy when fuel is carried in MBT 2 and 4 and all ballast tanks = 99   tons
Reserve buoyancy when MBT 2 and 4 are used as ballast tanks                    = 144 tons
     
  Total capacity of MBT 1, 3, 5, = 104.35 tons  
Total capacity of MBT 1, 2P/S, 3, 4P/S, 5 = 153.85 tons
 
Subtracting totals for ballast tanks from surfaced displacement gives:
  Reserve buoyancy when fuel is carried in MBT 2 and 4 and MBT 1, 3 and 5 are filled = -5.35 tons  
  Reserve buoyancy when MBT 2 and 4 are used as ballast tanks and MBT 1, 2, 3, 4 and 5 are filled = -9.85 tons  
     
  Quick diving tanks when filled = 4.34 tons  
  Regulating tanks when full = 15.86 tons  
     
  Both figures above for buoyancy with full ballast tanks are well within the capacity of the Regulating tanks to compensate for so the math works out.  
     
  * All figures from Admiralty Technical Report on U-570  
     
  Diving Procedure  
          The following tables were started from a description in the book Type VII U-boats, by Robert C. Stern, ISBN 1 86019 8554, 1998 edition.  It is hoped to add to this description.  
 
1. An electric bell gives the alarm.
2. The bridge watch prepares the bridge (close voice tube, demount portable machine guns, remove water-tight binoculars from UZO, remove radar detector aerial if necessary, etc.).
3. Simultaneously, vent valves for ballast tanks 5, 2/4, Quick diving tank (if empty) and 3 are opened in that order flooding from bow to stern to overcome the natural stern heaviness of the boat.  If fuel is in ballast tanks 2 and 4, the main vent valve is opened with the inner vent valve remaining closed to equalize pressure in the ducts. 
4. Simultaneously, the Petty Officer of the Watch goes below to his action station at the forward hydroplanes; the port quarter look-out follows to his action station in the conning tower with the Commanding Officer.  The starboard quarter look-out goes below to the after hydroplanes.  The Officer of the Watch is the last to go below.  It is his duty to close the conning tower hatch. He then goes to his action station in the Control Room.
5. Simultaneously, close Supply and Exhaust air ventilation valves and any other openings in the pressure hull.
6. Planesmen position fore plane down and stern plane up.  If it is a crash dive personnel not on watch might be ordered to move to the forward torpedo room to help get the bow under water.  Initial angle of dive 4-7° to avoid propellers "racing".
7. Simultaneously, engine room personnel shut down and unclutch Diesel engines and shut the external air intake and exhaust valves.
8. Simultaneously, electric motor room personnel start electric motors
9. When the desired down angle is achieved (normally 12-15° but can go as high as 30 ° in a crash dive - at more than 40° acid may leak from battery cells) vent valve for ballast tank 1 is opened. 
10. Simultaneously, the planes are brought to neutral position.  Shut off Papenberg depth gauge at 18 meters and precision depth gauge passing 20 meters.
11. Quick diving tank is blown just short of empty to avoid releasing bubbles.  This should result in the boat being at slightly positive buoyancy because the Regulating/Compensating tank is adjusted and maintained to achieve that.
12. Once tanks are flooded, the valves are shut
13. When desired depth is approached forward dive planes were moved up and stern down to level the boat at desired depth.
14. Boat is maintained at depth dynamically by use of the dive planes. 
15. Because teh pressure hull is compressed with increasing depth the boat would grow heavier and the ballast pump or compressed air would be used to keep the boat at slightly positive buoyancy the opposite would occur if the boat moved to a shallower depth and water would be taken in.
 
     
  Surfacing Procedure  
 
1. Compressed air was blown into ballast tanks 5, 4, 3, 2, starting at the bow and working aft.  Tanks were not blown completely but only enough to get the conning tower above water.
2. Simultaneously, forward dive planes to up and stern to down.  Normal alteration of depth at a rate of 1-1/2 ft. per second
3. When the boat broke surface Diesels were started and electric motors were shut down.
4. Complete blowing of the ballast tanks was completed by Diesel engine exhaust, one tank at a time in Type VIIC at a pressure of .6 atmospheres or 8 1/2 lbs. sq. in.
 
     
          The links below provide background information on U-boat diving systems.  
     
 

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Excerpt on Flooding and Venting from ONI, Report on the German Submarine of the U-570 Class Captured by the British in August 1941
Design and Specification Books, Volume M  (Enclosure B to ONI report above)
Admiralty Technical Report on U-570
 
   
  Depth  
          The link below provides some information on U-boat diving depths, time, and angles.  
 

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Excerpt on Diving from the Cumulative Edition - CB 04051(103) The Admiralty's appreciation of the German U-boat force as of June 1944
 
     
  Diving, Regulating, Compensating and Trimming Tanks (Type VII)  
 
Regelbunker 1 backbord Regulating and reserve fuel oil tank 1 port
Regelbunker 1 steuerbord Regulating and reserve fuel oil tank 1 starboard
Regelzelle 2 backbord Regulating Tank 2 port
Regelzelle 2 steuerbord Regulating tank 2 starboard
Tauchbunker 2 backbord Main ballast and reserve fuel oil tank 2 port
Tauchbunker 2 steuerbord Main ballast and reserve fuel oil tank 2 starboard
Tauchbunker 4 backbord Main ballast and reserve fuel oil tank 4 port
Tauchbunker 4 steuerbord Main ballast and reserve fuel oil tank 4 starboard
Tauchzelle 1 Main ballast tank 1
Tauchzelle 3 Main ballast tank 3
Tauchzelle 5 Main ballast tank 5
Torpedozelle 1 Torpedo compensating Tank 1
Torpedozelle 2 and 3 Torpedo compensating tanks 2 and 3
Trimmzelle Back Forward trimming tank
Trimmzelle Heck After trimming tank
Untertriebzelle backbord Negative buoyancy tank port
Untertriebzelle steuerbord Negative buoyancy tank starboard
Wasserdichte Back Bow buoyancy tank
Wasserdichte Heck Stern buoyancy tank
 
     
          The following vocabulary tables are adapted and keyed to the Design and specification book aboard U-570 when that U-boat was captured.  The translation and in some cases the drawings by the David Taylor Model Basin are poor and it is recommended that the reader refer to the German diagrams where possible.    
     
 

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Design and Specification Books, Volume M  (Enclosure B to ONI report on U-570)
 
     
  Plate 16: Flooding, venting, low pressure exhaust gas and emergency blowing systems  
 
 
Key
 
Flutkappe(n)
a
Flood valve(s)/Kingston as for No. 2 and 4 MB and RFO tanks and No. 3 MB tank
Entlüftungsventil
b
Vent valve as for each of the MB tanks
Restentlüftungsventil
b1
Residual air vent valves at the after-end of No. 2 MB and RFO tanks port and starboard (these are schieber/gate valves)
Notverschlussklappe
c
Emergency vent flap valve as for No. 2 and 4 MB and RFO tanks and No. 3 MB tank
Entlüftungsanschluss
d
Emergency vent valve as for No. 2 and No. 4 MB and RFO tanks (these are Schieber/gate valves)
Restenlüftungsanschluss
d1
Residual air vent valve for No. 2 MB and RFO tank port and starboard (one handle operates two valves one for port and one for starboard)
Abgasklappe
e
Exhaust gas outboard (flap) valve  - prevents water from coming in through the muffler and when closed diverts exhaust gas through Main exhaust blow valve for blowing tanks
Hauptausblaseventil
f
Main exhaust blow valve in engine room leads to the Exhaust gas blowing manifold in the control room
Abgasausblasevert
Exhaust gas blowing manifold
Ausblaseventil
g
Main ballast tank blow valves, one for each ballast let air into Main ballast tank blow lines, one for each ballast tank
Ausblaseanschluss
h
Hull valves on Main ballast tank blow lines at each tank to admit exhaust air
Taucheranschluss Backbord bzw Steuerbord
i
Diver's connection, port or starboard
Notausblaseanschluss Backbord bzw Steuerbord
k
Emergency blow valves from divers connection port and starboard
Notausblasehahn
l
Emergency blow valves for each ballast tank
Entwässern
m
Drain valves as for vent duct between No. 2 and No. 4 MB and RFO tanks
 
  Plate 28:  Flooding and venting system for ballast and buoyancy tanks  
 
  Key  
Anzeigevorrichtung   Valve indicator - showed the position of the valve at the operating position
Entlüftungsgestänge   Transmission shafting for vent valves - between crank or handle and valve
Entlüftungskanal   Vent shaft as for No. 3 MB tank between opening in pressure hull and vent at deck level
Entlüftungsrohr or Entlüftungsltg   Vent duct/line as between No. 2 and No. 4 MB and RFO tanks
 
  Plate 17: Blowing and emergency blowing systems for ballast, regulating and diving (negative buoyancy) tanks  
 
  Key  
Bordventil Anblasen a Hull valves in blow line, located at the end of the blow line for No. 1 and 5 MB tank and No. 2 and 4 MB and RFO tanks
Anblasen b Individual tank blow valves, located on the Blowing distributor (Christmas tree) in the control room
Anblasverteiler   Blowing distributor, charged from High pressure air distributor, pressure was reduced to 40atm/587.8psi for blowing lines
Hochdruck Luftvert   High pressure air distributor, received air from the various storage tanks at 205atm/3012.7psi provided air to Blowing distributor and Pressure regulating discharge distributors
Haupt Anblase Rauchzelle b1 Master blowing valve, located at the top of the blowing distributor in the control room, lets air from the High pressure air distributor into the Blowing distributor. 
Ersatz Anblaseventil Tauchzelle b2 Auxiliary blowing valve, opens alternate path between High pressure air distributor and Blowing distributor, back-up to Master blowing valve
Ausdrückventil c Individual Blow valves for regulating tanks, lets air from Pressure discharge distributors into blowing lines for each tank
Ausdrückvert   Pressure regulating discharge distributors, one for No. 1 Regulating and RFO tank and one for No. 2 Regulating tank
Hauptausdrückventil c1 Master blow valves for regulating tanks, lets air from High pressure air distributor into Pressure discharge distributors, one for No. 1 Regulating and RFO tank and one for No. 2 Regulating tank
Anschluss d Stop valve at High pressure air distributor, stops air from going into line to Blowing distributor another stops air from going into line to Pressure regulating discharge distributor
Drückunterschiedmesser   Pressure difference gage one for each of the No. 2 Regulating tanks and the No. 1 Regulating and RFO tanks, measured difference between sea pressure and pressure inside the 4 regulating tanks.
Notausblaseanschluss Backbord bzw Steuerbord d1 Emergency blowing connection, port or starboard, valve located above pressure hull
Taucheranschluss Notluft d2 Divers connection, emergency air, located on deck
Ausdrückanschluss d3 Hull valves in regulating tank blow lines , located at the end of blowing lines at the hull only for No. 2 Regulating tank
Haupt Notausblaseventil e Main emergency blowing valve, like Master blowing valve only for emergency blow, located above pressure hull
Notausblasehahn f Emergency blowing cock, located above pressure hull
Bordventil Notausblasen g Hull valves, emergency blowing (operable from above and below pressure hull)
Entwässern h Drain valves, located at the bottoms of Blowing distributor and both Pressure regulating discharge distributors
Entlüften Untertriebzelle i Hull valves for venting negative buoyancy tanks, air was supplied directly from the High pressure distributor apart from the Blow distributor
 
  Plate 7:  Regulating and compensating system  
 
  Key  
Bordventil von See a Hull valve, sea connection, for fine flooding of the regulating tank, provides water to the Flooding and draining manifold, also one forward and one aft for for Torpedo compensating tanks and torpedo tubes
Fluten und Lenzvert   Flooding and draining manifold
Von See Innen a1 Inboard valve on sea connection
Bordventil Nach See Lenzpumpe b Hull valve, discharge to sea for the drain and auxiliary drain and trim pumps
Nach See Innen Hilfs-, Lenz- und Trimmpumpe b1 Inboard valve discharge to sea
Bordventil Durchblasen c Blowing valve for sea connection, to clear debris from intake
Eintritt Haupt Lenzpumpe d Intake, main drain pump
Lenzeintritt Hilfs-, Lenz- und Trimmpumpe d1 Suction valve, auxiliary drain and trim pump
Austritt Hilfs-, Lenz- und Trimmpumpe e Discharge valve, auxiliary drain and trim pump
Regelaustritt hauptlenzpumpe e1 Regulating discharge valve, main drain pump allows water to enter the Flooding and draining manifold
Anschluss f Manifold to regulating tanks, individual valves let water move from Flooding and draining manifold to No. 1 Regulating and RFO tanks and No. 2 Regulating tanks
Druckanschluss Trimmschalter f1 Pressure connection, trim control, sends water to Trim controller
Saugeanschluss Trimmschalter f2 Suction connection, trim controller
Lenzanschluss Hauptlenzpumpe f3 Suction connection, main drainage pump
Notlenzanschluss f4 Emergency bilge connection
Bordventil Lenzdruckanschluss Tauchzelle 3 Backbord f5 Hull valve, pump pressure connection, No. 3 MB tank
Fluten g Flood valve for Negative buoyancy tanks and Torpedo compensating tanks
Bordventil Fluten Munitionskammer und Munitionsbehalter g1 Hull valve, flooding or circulating water through magazine and ammunition tank
Entlüften h Air vent, allows air to be bled from regulating tanks, Negative tanks and Torpedo compensating tanks to the interior
Druckausgleicher i Pressure equalization, to flood torpedo tubes so that the outer door can be opened
Geräuschdämpfer k Mufflers on vent lines
Schauglas l Gage glass
Mengenzähler   Flow meter one for each of the three Torpedo compensating tanks
 
  Plate 6:  Trimming, compensating and drainage system  
 
  Key  
Bordventil von See a Intake hull valve, for example, for fine control of water in regulating tanks
Von See Innen a1 Intake from the sea, for flooding the regulating tanks
Bordventil nach See Lenzpumpen b Hull valve, discharge to sea for the drain and auxiliary drain and trim pumps
Nach See Innen b1 Discharge to sea, inboard valve
Eintritt Haupt Lenzpumpe c Intake, main drain pump
Lenzeintritt Hilfs-, Lenz- und Trimmpumpe c1 Suction, auxiliary drain and trim pump
Austritt Hilfs-, Lenz - und Trimmpumpe d Discharge, auxiliary drain and trim pump to manifold
Regelaustritt Hauptlenzpumpe d1 Regulating discharge valve, allows water to flow from drain pump into Flooding and drainage manifold
Bordventil Lenzdruckanschluss Tauchzelle 3 e1 Pump pressure connection, No. 3 MB tank
Druckanschluss Trimmschalter e2 Pressure connection, trim control
Saugeanschluss Trimmschalter e3 Suction connection, allows water to flow from auxiliary drain and trim pump to trim control
Lenzanschluss e4 Drain connection
Notlenzanschluss e5 Emergency bilge connection
Lenzen . . .Hinten Dieselmotor A f Main suction valves for draining various bilges throughout the boat
Trimmschalter g Trim controller
Trimmdruckluftschalter g1 Blow and vent control valve for trimming line (water could be moved between forward and aft Trim tanks by the trim pump or by low pressure air)
Trimmventil h Trimming line valve (to isolate after trim line)
Trimmdruckluftregel-ventil i Regulating valve for compressed air trimming tank vent and blow lines
Entlüften Lenzleitungen k Valve to line to evacuate air from bilge suction (main drain) lines
Entwässern Backbord bzw. Steuerbord Küche l Drain, starboard or port galley
Bordventil Durchblasen von See Fluten Regelzelle und Frischwasser m Blowing valve for sea connection, to clear debris from intake
Bordventil Deckwaschen und Brause n Hull valve for deck wash hose and shower head
Feuerlöschung o Fire extinguishing connection
 
     
     

 


 

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