What happens in a U-boat when............

I would appreciate comments re this article ...

The following notes are compiled from our (British Naval Intelligence ) previous knowledge of U-Boats as of January 1941, supplemented by information recently received ; they include information which might be of use should an opportunity occur for BOARDING a U-Boat.

On the Surface.—A U-Boat has one officer and three men on the bridge when on patrol and, when attacking at night, probably the Captain, one officer and/or one rating.
The two larger classes can and do attack at high speed in a fairly heavy sea. Their low free-board gives them an advantage over surface craft when turning in rough weather. Heavy seas over the bridge would tend to render their attack less accurate but, against this, their targets would be receiving less protection from escorts and their speed would be reduced. Look-outs would probably be lashed to the bridge with quick-release lifelines. A rating would be stationed in the conning tower to close the lid and prevent green seas entering. An air shaft is probably fitted abaft the conning-tower hatch.

After a Night Attack.—In order to maintain speed, manoeuvrability and range of vision, the Captain will almost certainly remain on the surface after attack. He knows that his speed is comparable to that of slower A/S vessels and that Asdic contact is unlikely. He will re-load his tubes at a safe distance. If he knows a destroyer is at hand he may dive and adopt avoiding tactics, using ships' wakes if available.

Crash-diving. - A U-Boat can alter course when diving, but a straight course to 100 ft. is preferred. If diving to attack and not to escape she would probably go to 50 ft. and then rise to periscope depth.

The Captain or O.O.W. rings the diving bell :‑
Bridge personnel go below, the officer secures the lid by turning central hand-wheel ; he usually remains in the conning tower till reaching the required depth. The Engineer Officer takes charge of the diving, the vent and blow valves for which are in the control room.

Spray caused by air escaping from tanks may be noticed by attacking vessels.
The forward hydroplanes are given dive helm but the after 'planes are kept nearly horizontal in the early stages. E.R. ratings unclutch engines and run motors at about 8 knots. Torpedomen stand by tubes. Emergency stations are probably manned.
Initial angle of dive is about 4°, increased to 7°, when there is no chance of propellers " racing." When ballast-tanks are full, vent valves are closed so that tanks may be ready for blowing'. A master blow-valve is in the control room for blowing all ballast tanks simultaneously.

RECENT INFORMATION

German U-Boats

Sighting of Convoys
It is confirmed that escorted convoys, though almost invariably attacked at night, have usually been first sighted in daylight.

Torpedo Night-sight.—Tuza (Torpedo Unterseeboot Ziel Apparat), is an illuminated telescope used for night attacks. It rests on the top of an illuminated half-globe, which is probably graduated and is fixed to a stand on the bridge.

Explosive Scuttling Charges.—These are usually located in three compartments --Control Room, Bow and Stern. They have fuzes which may burn for 15 minutes and are the responsibility of the Engineer Officer.


Vent and Blow-Valves.—The main venting and blowing position is in the control room, but there are alternative stations forward and aft. In the case of the 500-tonners there is a master blow-valve which can blow all six tanks simul­taneously. There is no master vent valve, but the six valves are grouped together. Neither vent nor blow-valves are tallied.
Drop-keels.—This device, by which an instant gain of some 15 tons in buoyancy may be achieved by working a lever, is apparently not carried in U-Boats.

Inter-communication between U-Boats.----Normally, German U-Boats do not communicate with each other when on patrol. Information concerning our shipping is passed from aircraft, U-Boats and other sources to enemy headquarters which advise any U-Boat likely to make contact. The reception of one U-Boat's signal by another would probably be fortuitous but, rivalry being keen, the latter might well act upon it.

Later 500-ton U-Boats.—The normal 500-tonner has four bow tubes and one stern tube. The improved type, of which " U 101." is among the first, is said to have two stern tubes, completely inside the pressure-hull. The reason for this may well be to save weight ; as they are not exposed to sea pressure, they are probably built of light metal. Fuel stowage is said to be greater than in the earlier type and there are possibly further improvements as crews of other U-Boats are not allowed on board.

Radio Direction-finding.—R.D.F. is probably only carried in a few U-Boats. It is fitted inside the conning-tower.

Hydrophones.—The normal 500-tonners are probably fitted with " Horch­gerat,' consisting of 10 to 12 plates fitted into the hull and connected to a central control panel. This system is directional and could be used for attacking in poor visibility if the U-Boat is at periscope-depth. Another type K.D.B. (" Kristal­drehbasis "), probably fitted in larger and more recent U-Boats is also mentioned. It cannot be raised or lowered. The detection range is said to be 8 to 10 miles when the U-Boat is proceeding at 8 knots.


Italian U-Boats

Information concerning Italian U-Boats has lately been somewhat scrappy. As there are some eight different types, of widely different design, it is not possible to generalise.

Co-operation.—Italian war-orders contain minute instructions for submarines working in close co-operation but, in the Atlantic, it is probable that their tactics conform to those of the German U-Boats, which prefer to patrol singly.
When proceeding on patrol from a Biscay port, however, they have been known to proceed in company.

Personnel.—Although largely composed of officers and men with little training, their crews have shown skill in attack and, when depth-charged, have made courageous efforts to save their vessels before surrendering. When the relative efficiency of Italian and German U-Boats is compared, the inferiority of the former should not be over-estimated.

Material.
The hulls are extremely strong and stand up well to depth‑
charging, remaining tight at depths of considerably over 300 ft. Fittings, however, do not appear to be up to the same standard. Engines, batteries, periscopes and blowing systems have been known to cause trouble. Leaks in the air-line have produced dangerous pressures.

Batteries.—The urgent need for economy in electrical power is constantly being stressed—statical trims when possible, cutting down lighting and cooking, etc.
The Italians started the war with 63 U-Boats, ranging between 650 and 1,500 tons, besides 53 of smaller type. It is possible that the ratio of battery power to tonnage is on the small side:

Hydrophones.—Ranges of 5 miles for merchant ships and 10 miles for warships are mentioned. A range of 25 miles has been claimed in water which was comparatively fresh.

Torpedoes.—Evidence seems to indicate that neither electric torpedoes nor magnetic pistols are used. Maximum range has been given as 4,000 yards. Eighteen-inch torpedoes are sometimes carried, mainly for use against merchant shipping. A number of 21-in. tubes are fitted to take " sub-calibre " collars for firing the smaller torpedoes.
Difficult shots are seldom taken as these, when unsuccessful, cause recriminations on the return from patrol ; they are also unpopular as likely to give the U-Boats' position away to hunting vessels.

Wakes of Ships.—The Italians realise that an Asdic may obtain echoes from a wake. They are advised to bear this in mind when taking avoiding action.

Fresh Air.—Air-purifiers are probably not supplied, but oxygen cylinders are carried.

Recognition Signals.—Smoke by day and flares by night are used for identification purposes with aircraft.

Scuttling Charges.—These are known to be carried by certain U-Boats.

Boarding a Disabled U-Boat

Apart from the capture of the complete U-Boat it is, of course, essential to make every effort to secure confidential books and papers. Floating scraps of paper or note-books have been found of value. Presumably the C.B.s will be thrown overboard in a weighted bag, but it is known that those connected with W.T. are normally kept in .the cabinet ; these might possibly be left behind in the panic.

It is stressed that the salvage of any instrument or electrical device may prove of immense value. Special attention should be paid to " Tuza " and the R.D.F., mentioned above. A rating with a camera is recommended.

It is assumed that the U-Boat is surfacing to save the crew—about 42 men in a 750-tonner, 37 in a 500-tonner, and 25 in a 250-tonner. .They will take a considerable time to emerge in a single file up the ladder and will probably con­gregate near the base of the conning tower. Several " die-hards " will remain till the last moment, among these will probably be the Captain and the' Engineer Officer. It is almost certain that the latter will ensure, before he leaves, that :--

(1) All ballast vents are open ;
(2) Hydroplanes are set to dive ;
(3) Motors are going ahead ; and
(4) Fuzes for scuttling-charges are lit (unless the hull is so badly damaged that they are unnecessary—at least one such case has occurred).

Our knowledge of the lay-out of U-Boats is very incomplete, but the following notes may be of value :‑

(Note.—The inside of the U-Boat will be in darkness so that an electric torch, preferably a watertight one, will be essential.)

The Hydroplane-wheels are in the control-room, with the depth gauges (about 18-in. in diameter and graduated in metres), immediately behind them. If the 'planes are run over to the " RISE " position, they will at least check the rate of diving if there is way on the U-Boat,

Vent Valves (" Ventil-stutzen ") will probably be all on one panel in the control-room. They must be closed to stop tanks flooding and before blowing. As stated above they are not tallied and it would take even an experienced submarine rating a long time to identify them. As they are about the most important valves in the U-Boat, however, it is probable that the majority of her crew will have been drilled in their usage.

The Master-blow valve (" Hauptstutzen ") is equally hard to identify. It would probably be connected to the air-flasks (out of sight), by a tube of about i-id.; diameter and be in an accessible position. As this valve can blow all tanks simul­taneously, if the vents are shut, it might be the means of saving the U-Boat. The " loyal " co-operation of one of her crew is essential.

Main Motor Switches.—In our submarines the motors are controlled by large " chopper " switches, pulled down to stop ; they are in the compartment abaft the engine-room. The arrangement in German U-Boats is not known, but is probably similar.

Every effort should be made to induce one or two " hostages " to re-enter the boat or to remain inside by closing the lid on them. The Engineer Officer (" Leitender Ingenieur," pronounced ' Light-end-dair Engineer "), is the most valuable man, as he controls the diving and is responsible for the scuttling charges. His acquiescence would be extremely unlikely even under threat of death, however, as up to date these Officers have proved quite as stout-hearted as the Captains. The E.O. has an assistant (" Central maat "), who is probably a mechanician, 1st Class (" Maschinenmaat '), this man's help would also be of value.

 

From what I can remember of U-boats, they seem to have gotten a lot right. What they don't seem to have yet is U-boat endurance under-water and maximum patrol distance. There was a section on "Italian U-boats then they seem to have skipped back to German subs but that wasn't clear.