U-boat Archive - Design Studies

CONFIDENTIAL

REPORT 2G-21

S29

FORMER GERMAN SUBMARINE TYPE XXI

WEIGHT, STABILITY AND INTEGRITY

SUMMARY

GENERAL

The vessel is designed primarily for submerged operation. External main ballast tanks and normal fuel oil tanks (but no fuel ballast tanks) are provided, together with bow and stern buoyancy tanks, and regulating tanks. No negative tank is provided, although one is shown and described in plan and text material available. External trim and WRT tanks are provided, and added compensation is provided by internal poppet valve tanks.

The contents of this section will describe the aspects in which this vessel type differs from earlier types.

Weights, Displacement and Stability

Specification weights for the vessel are as follows:

S I	4	Pressure Hull Plating	179.60
Ship-	5	End Bulkheads	9.39
building	6	Pressure Hull Frames	36.06
Work	7	Pressure Hull Bulkheads	16.32
	8	Machinery Beds and Foundations	25.47
	9	Pressure Hull Platforms	40.90
	10	Inside Tanks	9.97
	11	Outer Hull Plating	66.55
	12	Stem, Stern & Struts	9.53
	13	Rudders and Planes	11.33
	14	Outer Hull Frames & Bulkheads	57.94
	15	Superstructure	10.68
	16	Pressure-tight Outside Tanks	61.99
	17	Insulation, Lagging	1.50
	18	Conning Tower	10.56
	19	Ordnance Foundations	(in group 15)
	20	Torpedo Arrangement	3.23
	21	Hawse Pipe	----
	22	Communications Arrangements	0.14
	23	Arrangements for Personnel	0.12
	24	Keel	(in groups 4,11,16)
	25	Lifting Arrangements	----

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REPORT 2G-21

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S I	26	Arrangements for Machinery Systems	9.06
Ship-	27	Brackets & Mounting Plates	2.51
building		Welding	12.00
Work			_____
			573.85

S II	1	Bulkhead Closures, etc.	7.40
Ship-	2	Railings	0.09
fitter	3	Ladders	0.38
Work	4	Portlights	----
	5	Cleats & Chocks	0.01
	6	Flood & Drain Arrangements	11.81
	7	Rudder & Plane Arrangements	6.15
	8	Armament Arrangements	1.02
	9	Anchor Arrangements	0.34
	10	Torpedo Arrangements	11.24
	11	Ship Control Arrangements	0.08
	12	Ballast Tank Arrangements	5.09
	14	Arrangements for Crew	2.30
	15	Safety & Rescue Arrangements	1.41
	17	Miscellaneous Arrangements	3.27
			____
			50.59
			====

S III	1	Shielding, Insulation (Heat)	2.25
Joiner	2	Arrangements for Officers & Chiefs	0.95
&	3	Arrangements for Petty Officers & Men	0.75
Sheet-	4	Miscellaneous	2.10
metal			____
Work			6.05
			====

S IV	1	Painting	6.00
Painting,	2	Cementing	----
etc.			____
			6.00
			====

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REPORT 2G-21

S29

S I				573.85
S II				50.59
S III				6.05
S IV				6.00
				____
Total hull groups				636.49
				====

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REPORT 2G-21

S29

M I	1	Diesel Engines	37.10
	3	Circulating Water System	4.99
	4	Lub. Oil System	2.77
	5	Fuel Oil System	3.91
	6	Diesel Air & Exhaust Gas System	16.00
	7	Shafting	35.89
	8	Propellers	1.52
	9	Main Motors	29.09
	10	Switching System	5.40
	12	Electric Wiring	6.41
	13	Battery	235.85
	14	Battery Venting System	3.02
	15	Machinery Compt., Flats, Ladders, etc.	0.05
			____
			382.00
			====

M II	1	Rudder Gear	1.67
	2	Windlass System	0.11
	3	Compressed Air System	21.67
	5	L.P. (Exhaust Gas) Blow System	3.99
	6	Main Drain Pump	2.14
	7	Aux. Drain & Trim Pump	----
	8	Heating (where steam) & cooking systems	0.44
	9	Fresh Water Still System	0.49
	10	Compt. Ventilating	10.18
	12	Torpedo Heating, Ventilating & Setting	2.81

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M II	13	Periscope Systems	4.92
	14	Ships Lighting	0.57
	15	Searchlights	0.01
	16	Ship Control Systems	0.01
	17	Command & Announcing Systems	1.37
	18	Voice Tube System	0.37
	19	Signal System	5.74
	20	Emergency & Rescue Arrangements	----
	21	Refrigerating	0.91
	22	Hydraulic System	6.40
	23	Aux. Generator	----
	24	Aux. Switching System	1.76
	25	Electric Wiring	6.03
	26	Shop Equipment	0.65
			____
			73.22
			====

MIII	1	Machinery Tools & Parts	----
	2	Electric Tools & Parts	----
	3	Machinery for Ship's Boat	----
			____
			----

M I			382.00
M II			73.22
M III			----
			____
Total machinery weights			455.22
			====

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REPORT 2G-21

S29

Ballast reported by ballast stowage plans amounts to 175 metric tons (see German plan 21S850.132B, Übersicht zur Ballaststauung), but this has, in the case of U-2513 been found insufficient. Actual ballast on U-3008 when docked amounted to 180 metric tons distributed as shown on Portsmouth Plan 44226. A heavy spar concrete was used in lieu of other material for ballast. A check of the vessel against available curves indicate that about 40 tons additional weight is necessary to make a dive when all fuel oil tanks and variables are filled.

Ordnance weights are unknown.

Displacement figures from Navtechmiseu report are as follows:

Surface	1621 cu. meters
Submerged	1819 cu. meters
Submerged volume including free flooding spaces	2100 cu. meters

The center of submerged displacement, according to the Calculated Contents of Tanks and Bunkers (Gerechnete Inhalte der Zellen und Bunker des Auszenschiffes) Plan No. 21S 850.123 E/2, is 35.60 meters forward of frame 0. This is in the control room, about 2/3 of the way aft from the forward bulkhead.

Specified surface and submerged GM is:

Surface minimum	290 mm
Submerged minimum	320 mm

Tank volumes and moments are as follows:

	Volume in cu. maters	Moment in meters	Flooding area in sq. meters	Venting area in sq. meters	Vent Valve Operating Gear	Flood Valve Operating Gear
Stern buoyancy	14.35	- 29.5	1.44	.03	hand	none
NFO 1a	30.01	- 26.34	Compensating		none	none
NFO 2a	66.72	- 21.22	Compensating		none	none
MBT 1	31.08	- 13.15	3.88	.252	single valve and air piston P&S	none
NFO 3a	65.62	-10.46	Compensating		none	none
NFO 4a	17.46	- 4.29	Compensating		none	none
MBT 2	42.30	- 4.29	3.88	.175	single valve and air piston P&S	none
NFO 5a	14.84	+5.07	Compensating		none	none

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REPORT 2G-21

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	Volume in cu. maters	Moment in meters	Flooding area in sq. meters	Venting area in sq. meters	Vent Valve Operating Gear	Flood Valve Operating Gear
MBT 3	39.92	+ 5.04	3.88	.175	Single Valve and air piston P&S	none
NFO 6a	32.64	+ 9.94	Compensating		None	none
MBT 4	50.80	+13.26		.252	Single Valve and air piston P&S	none
NFO 7a	37.08	+20.06	Compensating		None	none
MBT 5	37.24	+29.59	3.88	.175	Single Valve and air piston P&S	none
Bow Buoyancy	12.11	+31.38	1.64	.110	Single Valve and air piston	none
Regulating	25.12	- .94 )	.03 to	(.002	hand	hand
bunker		)	either	(
Regulating	19.24	+1.60)	or	(.002	hand	hand
tank		)	both	(

Each half of each MBT has a separate vent valve. Operating gear for port and starboard halves is in parallel, but it is possible to segregate either side if desired. When vent valves are power operated from the L.P. air system, the gear for MBT 1 and 2 is controlled by one air valve, that for MBT 3 and 4 by a second air valve, that for MBT 5 by a third, while a fourth controls the bow buoyancy vent.

The stern buoyancy tank vent is operated by hand from within the after compartment.

High pressure blow lines are fitted to the main ballast and the two buoyancy tanks.

Low pressure (Exhaust gas) blow lines are fitted to the main ballast tanks.

For trim control, trim tanks, WRT tanks and poppet tanks (torpedo untertriebzelle) are provided.

Forward trim tanks have a total capacity of 7.10 cu. meters and a forward moment of 24.79 meters. After trim tanks have a capacity of 7.20 cu. meters and an after moment of 28.71 meters.

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REPORT 2G-21

S29

The WRT tanks have a total capacity of 27.30 cu. meters and a forward moment of 19.42 meters.

The poppet tanks come in three pairs. Capacity of each tank and forward moment is as follows:

No. 1 and 2, each	.54 cu. meters,	24.89 meters
3 and 4 "	.50 " "	25.53 "
5 and 6 "	.51 " "	26.32 "

The water content of a flooded torpedo tube is not known. The content of each poppet tank approximates the difference between the weight of the torpedo and the weight of the water displaced by the torpedo.

In addition to the foregoing tanks, the vessel is equipped with internal collecting and gravity service tanks, fresh water tanks, battery water tanks, sanitary and lub oil tanks, each of which is discussed under its appropriate system.

The total fuel oil which can be carried with all suitable tanks filled is 295920 liters.

The total lubricating oil which can be carried is 16430 liters.

The total battery water is 1555 liters.

Drinking water is 6030 liters.

Wash water is 582 liters, excluding the WRT tanks, which can also be used to carry wash water. With WRT tanks, the total becomes 14220 liters.

Sanitary tank capacity is 1680 liters.

No safety or negative tank is provided. The space occupied by the negative tank was used as a fuel tank to augment the normal maximum fuel oil capacity. Capacity of the tank is 14700 liters.

According to the Preliminary General Information Book (Vorlaufige U-Bootskinde für U Boote Typ XXI), MBT 1 to 4 inclusive are located low on the sides of the vessel to provide protection from machine guns when the vessel is surfaced, the buoyancy tanks serve as a substitute for the somewhat reduced main ballast tanks and reduce pitching; MBT is arranged to permit venting inboard.

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REPORT 2G-21

S29

Also from the same source, flooding the regulating tanks serves to eliminate buoyancy remaining after ballast tanks are flooded, and the tanks permit compensation for different densities of sea water.

Damage control information is also obtained from the General Information Book. Therein can be found mush the same information as that described in the X-B report. Points covered are:

a) Weight and trim changes resulting from damaged tanks and compartments when the vessel is surfaced.

b) Inability to compensate for any flooded compartment when submerged except the conning tower or the after compartment.

c) Volume of air in banks is sufficient to blow all main ballast tanks at 70 meters depth.

d) Buoyancy and moment obtainable by blowing fuel oil tanks by way of the compensating water lines (a slow process).

e) A list of bulkhead closures.

f) Characteristic curves for surface and deep drain pumps.

g) A list of the contents of the damage control kit, in which 2 pounds of oakum is added to the items listed in the X-B report.

h) A list of timbers and planks stowed in each compartment.

i) How to flood each compartment in order to escape therefrom.

j) A list of emergency lights (hand lanterns and flashlights).

Draft and Trim

Vessel dimensions from Navtechmiseu report No. 312-45 and General Information Book are as follows:

L.O.A.	251' - 10"
Maximum beam	21' - 8"
Depth from bottom of keel to superstructure deck	25' - 4"
Maximum diameter of pressure hull	17' - 5"
Mean draft	20' - 4"
(The calculated drag at this mean draft is 43")

Inclining Experiment

One has been made on U-2513 in this class. Values obtained were as follows:

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REPORT 2G-21

S29

Stabilizing

Two fins are built out horizontally from the hull at the after end of the vessel, enclosing the propeller shafts and continuing outboard to a point out from the centerline of the vessel. Their purpose is to provide adequate stability at high submerged speeds. On U-3008 and U-2513 they do not extend out as far as the ones shown on German plans or other illustrative matter.

Rolling and Pitching

The vessel is designed primarily for submerged service, and sea keeping qualities as a surface vessel are not as good as those of earlier classes. Her long deep shape, the pronounced tumble-home topside and the low surface GM lead to pronounced rolling. She is also reported to be wet.

Workmanship

Workmanship was inferior to that on earlier vessels. Tolerances on section butts which it was commonplace to hold at the American yard which most nearly approximated the German assembly method were found impossible to attain, and the hulls of the two vessels both have places where one section has been drawn in to meet the next section. Further, the character of the tank structure brings about many places where access is restricted, and welding at such places, and elsewhere out of sight, is definitely not good. Undercutting and incomplete welding have both been observed.

Here, too, as on previous classes, the work of one group has been in part vitiated by the work of another group. This phase is discussed under the heading of the systems affected.

Strength of Hull

The hull was designed for a submergence depth of 135 meters (442 ft.) with a safety factor of 2.5 on collapse. Actual pressure tests on a large scale model resulted in collapse of the lower portion of the figure 8 hull at a pressure equivalent of 900 ft., as reported by the Germans.

Further, while the WRT and trim tanks are considered as external pressure tanks, and while the former is exposed to sea pressure when flooding, the bulkhead at the after end of

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REPORT 2G-21

S29

the WRT tank, separating it from the forward battery, appears to be designed for no more than 73 psi pressure, while the bulkhead separating the WRT tanks from the forward trim tanks is only satisfactory for 107 psi.

Tightness Tests

Tests were, as usual, extensive and elaborate, but there is great discrepancy between test information from different sources. It would appear that tests given in the specifications could not be, or at any rate were not, met by design in certain cases, and that tests were later modified. A sample will demonstrate:

Test pressures given for the WRT tanks are as follows:
Hull specifications	284 psi
General Info Book	356 psi
Diving Systems Instruction Book	356 psi changed to 486 psi external, changed to 106 psi

This picture of indeterminability extends through all the available text and plan material. In view of this fact, it is very difficult to evaluate the expected relationships between test and working pressures. The confusion evident in the changes made hinders analysis. It is believed, however, that the general tendency was to test to about 50% over the working pressure, except where a limit was placed by the type of unit employed.

Hogging and Sagging

No specific information is available bearing on this point.

Riveting and Bolting

Comments remain the same as those on the 9C report.

Comment

The vessel is a radical departure from previous practice, and shows it. It is to be noted that here, for the

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REPORT 2G-21

S29

first time, a power-operated bow buoyancy tank vent valve appears. The vent operating gear, however, is a return to the type used on the type 9 vessels, and the air-on-oil operation which appeared on the type X-B valve gear is here confined to engine clutches although it is shown in the sketch book as being applied to the diesel air induction outboard hull valves.

Here, also, the fuel ballast tank no longer exists.

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