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GENERAL ORGANIZATION OF GERMAN U-BOATS
The whole German U-boat organization is controlled by Vice Admiral Doenitz, responsible only to the German High Command. His strategy is very effective. He maintains a close liaison with the Reconnaissance Aircraft Command.
On completion of a new U-boat, trials will be held either in the Baltic or off Norway. After making the first patrol into the Atlantic, a new U-boat will return to join an operating flotilla on the French Coast. If severe difficulties are encountered, the new U-boat may return to its building yard.
CRASH DIVES OF A U-BOAT
Weather conditions, initial speed and buoyancy, efficiency of the crew, etc., materially effect "performance" of a U-boat when its diving signal is given. It is probable that a small submarine, such as the German type, dives at an angle of about 10° and gains depth rapidly. If the submarine is turning, when it starts its crash dive, or if it starts turning very soon after, it will have its speed of diving reduced.
It might be argued that a U-boat, should immediately use "full dive" on its hydroplanes in order to gain depth quickly. This is not the case. This action would possibly bring the propellers almost to the surface, and even in a slight sea, they would be apt to race idly and lose thrust.
Aircraft sometimes report submarines diving at very steep angles. This is probably due to the false perspective, since in a 12° angle of dive the top of the rudder and the top of the conning tower would just be going under the water simultaneously.
A diving submarine will usually leave the following indications on the surface:
(a) WAKE - Valuable as an indication of the U-boat's initial diving course.
(b) PROPELLER WASH - Considerable disturbance for the 20 seconds before and 5 seconds after submergence of the conning tower.
(c) AIR BUBBLES - Large air bubbles from ballast tanks will rise from vents until submarine is completely submerged and may continue to rise for some seconds, in much smaller amounts, after the submarine is completely under. (Remember that air bubbles rise at the rate of only 1½ feet per second. Therefore, the submarine is not under the air bubbles.)
(d)- OIL - A faint trace may be seen.
(e) EXTRA BALLAST - A U-boat has two extra ballast tanks (called by some "Q" tanks) to enable it to dive more quickly: When at the desired depth the U-boat will blow these tanks dry and will then have normal trim. When these tanks are blown too long., large air bubbles may be noted on the surface. These "Q-tank" bubbles are sometimes reported as evidence of damage.
(f) SWIRL - This is by far the most important sign left by a diving U-boat.
In an aerial view of the whole scene on a calm day, about 10 seconds after a U-boat submerges, the pilot of an airplane should see a patch of disturbed water about three hundred feet long, broad and indistinct at the rear, but narrowing into a distinct oval or pearshaped swirl at the forward end. This swirl is caused by the water pouring into the open bridge on top of the conning.tower at the moment it goes under the water.
The propellers, about 100 feet abaft the conning tower, would continue to show their wash after the bridge has gone under, but these propeller signs would be unlikely to extend beyond the leading edge of the conning tower swirl. The forward gun also makes a disturb ance as it goes under the water, but this would not be visible when the main swirl appears.
It is considered appropriate here to present the case of a U-boat at l00 feet proceeding at 2 knots. If a depth charge were dropped right on the leading air bubble from an air leak, ignoring sinking time of the depth charge, it would explode 220 feet astern. If the bubbles were oil, the miss would be 670 feet astern.
In considering the angle of lead when making an attack on a submarine just recently submerged, attention is called to the graph of a crash dive. See diagram 4.
This diagram indicates that the largest component of the submarine's speed during the dive is forward, not downward. It takes the submarine 1 minute to go to 70 feet, but during this time it has gone along its track 700 feet at an average speed of 7 knots. This is
considered normal for a crash dive. This diagram further indicates that 40 seconds after the top of the submarine dips under, its conning tower would be about 450 feet from the center of the swirl. Add to that, the distance from the bridge of your ship to the depth charge racks, the dead time and the sinking time and see what lead you must have to insure an accurately placed stern dropped pattern.
Note also that a submarine probably will not turn until it is at periscope depth and also that after it does start to turn its stern will continue along its track for some 250 feet or 43 seconds.
In either submarines or surface ships, the question of efficient lookouts is considered so vital that special men are picked for this duty. A day-dreamer, even when blessed with good eyesight, is useless. One needs a man with a receptive eye, which will at once register any small irregularity in a dull expanse of sea. A moment's inattention (blowing his nose or cleaning his binoculars) may cause the loss of his ship or the missing of an opportunity. Lookouts should use high grade binoculars, be given a sector to search and be relieved at frequent intervals. There must be no lights on the bridge to blind the lookouts. A period of adaptation, of approximately fifteen minutes, is highly desirable before going on watch. The wearing of ruby red goggles for this period has also been recommended as satisfactory. Some persons cannot at any time see well in the dark. All officers and lookouts should be tested for night vision. Those with poor eyesight and night vision should not be used.
Reports of survivors of merchantmen indicate that U-boats use three lookouts who constantly scan the horizon using a large type binocular. These binoculars are believed to be of excellent grade with a large field, equally suitable for day or night use.
The height of the lookout's eyes on German U-boats averages 20 feet.
EFFECTIVENESS OF ANTI-SUBMARINE ACTION
The amount of damage sustained by a U-boat may vary somewhat with the particular part of the hull affected. An explosion some 30 feet ahead of the conning tower, if correct for depth, is probably the most productive.
A very near explosion should crush the pressure hull; the U-boat should fill and sink.
A near explosion should cause leaks in the hull; also in glands and connections through the hull. It frequently jams the hydroplanes in the "rise" or "dive" position, thus causing the submarine to break surface or dive to dangerous depths. Fuses might blow and lamps and instruments break. If oil is carried in external tanks, this might leak and show on the surface, giving an indication of the U-boat's course - but no proof of destruction.
More distant explosions might cause lesser damage such as:
(a) Leaks in the hull.
(b) Leaks in glands and connections through the hull.
(c) Jammed hydroplanes and rudders.
(d) Blown fuses.
(e) Broken lamps and instruments.
(f) Oil leaks from internal tanks, giving an indication of the U-boat's course - but no proof of destruction.
(g) Air leaks from air bottles carried in ballast tanks.
(h) Damaged propellers and line shafting.
(i) Cracked battery jars with loss of electric power.
(j) Formation of chlorine gas, necessitating the abandonment of the affected compartment. (Sulphuric acid leaking from cracked battery jars when mixed with small amounts of sea water will form chlorine gas.)
Evidence indicates that, in the majority of "kills" the 17-boat's Pressure hull has not been seriously fractured, but that recurring explosions, even when they are not within the lethal range, have caused such multiple damage, or so affected the crew that they were forced to the surface. Finding themselves in such situations, submarines have been known to attempt escape on the surface.
U-BOAT ATTACK TACTICS
A great majority of U-boat attacks are made on the surface at night. Submerged attacks are not infrequent however.
Details of convoys are passed to U-boats in the area by their control stations ashore, this information having been gained either by reconnaissance aircraft or other submarines. It is now a common occurrence for 5 to 8 submarines to trail a convoy making either "concerted" or "free-for-all" attacks.
It should be noted that it Is hard to lay down any hard and fast rules for a U-boat's tactics by day or night. A successful submarine commander is an individualist. He seizes his opportunities as they arise. However, certain reasonable conclusions may be attempted.
A U-boat commander will try to reach a position well ahead of his target; failing this, he will attack from any position he is able to reach.
When attacking on the surface at night a relatively high surface speed enables the submarine to maneuver for better position and to attack, evade, overtake and repeat the procedure perhaps several times.
A U-boat's heading on firing will probably be within 20° of a normal to the target's track. Having fired, the escaping tactics will depend on the position of the escorts. In the case of an attack on an unescorted vessel, a U-boat may chase the ship on the surface and attack with gunfire. Attacks by gunfire alone usually occur during the daylight hours near dawn or dusk.
THE PACK ATTACK ON CONVOYS
There are strong indications that Axis submarines working against convoys usually operate in groups of from 2 to 8. Each group has its patrol area. When any one of a group contacts a convoy, indications are that it makes the necessary endeavor to assemble its consorts. Recent German tactics are based on withholding attack until this assembly is effected.
The assembly is usually aided by the use of radio. The contacting U-boat usually shadows the convoy sending sighting reports on M/F at regular intervals. Sighting reports are usually followed by transmission on low frequency for radio direction finding by other U-boats to enable them to close the convoy. There is reason to believe ultra high frequencies are being used by U-boats for voice communication, possibly in plain language (German.)
There is one instance on record, in which it appears the assembly required a period of two days. The time will vary, depending on the size of the submarine group - probably the larger the group, the greater their patrol area and, hence, the longer the assembly period. Concerted and repeated attacks may, therefore, be expected once a group has assembled. These attacks may be expected at night or may come in periods of low visibility during the daylight.
A fair assumption is that a submarine cannot trail effectively while submerged. In any event, the submarine must surface if it is to effect the assembly of its group. Furthermore, sight, sound, or radar contact with the convoy must be maintained, even if only intermittently until the "pack" is assembled. To maintain radar contact, it must surface. To maintain sound contact, it must keep within ranges that are less than those for sight or radar contact. To maintain sight contact, it must at least show its periscope.
If the convoy is of medium or high speed, the submarine definitely cannot trail submerged over protracted periods. It must frequently surface or remain on the surface and run on its engines. This, on clear days, forces upon it a minimum distance at which it must remain from the convoy while trailing. This minimum distance must be presumed to be that distance at which it cannot be seen from the nearest escort. This means that on clear days it may remain hull and conning tower down from the nearest escort, raising its periscope-for sight contact. If low visibility sets in, it is left with only radar or hydrophones as its means of maintaining contact, provided it is to remain at this distance. If radar is not used, it must close to visual distance or hydrophone distance, or both.
In rough weather, the submarine must be presumed to be capable of effective trailing. In such weather, its problems increase, as do those of the screen commander's. In rough weather with low visibility, the submarine's attendant disadvantages are less than those of the screen commander's. The minimum distance at which the submarine may now trail is somewhat reduced.
During darkness, the submarine has its best advantage. It can maintain sight, radar and sound contact, and still be unseen by the convoy or any escort or screening vessel. Their only means of picking it up are by radar and sound gear. The submarine will be on the surface, low in the water ready for diving. Submarines on the surface are usually difficult to pick up with sound gear. Thus, the only advantage left to the screen commander lies in properly functioning radar of the screen or other vessels.
U-BOAT ESCAPE TACTICS
From attacks analyzed and plotted to date, it appears that the Axis submarines operating in the Western Atlantic now have a standard procedure after the attacking vessel has made contact. This consists basically of turning away from the attacker so that he will be forced to take echo ranges through the submarine's wake. The variations on this basic maneuver appear to depend on the range at which the contact is obtained. If the original contact is greater than 1,000 yards, the usual procedure is to turn away, creating in the wake, areas of increased turbulence or knuckles, by rudder and speed variations, in the hope that these' knuckles will confuse the sound operator before the actual attack course can be obtained. When contact is obtained at ranges below 1,000 yards, the general procedure is to turn in as tight a circle as possible, timing this circle so that it starts at 700 or 800 yards range from the attacking vessel. Evidently the submarine believes that the wake interference combined with the fact that the attacking ship is at close quarters when this maneuver is detected, imposes a difficult, if not impossible, situation on the attacking ship. In all these cases it is believed that the submarine continues its turn after loss of contact. Allowance should be made for this in the attack course. The success of these maneuvers will diminish with the increased ability of the sound operator to take bearings through wakes and to determine the bearing and motion of the submarine.
It also appears that submarines are, when under attack, not averse to maintaining maximum submerged speed for periods of time up to 10 minutes. This, contrary to what we have previously believed, might be due to the fact that continued searches during which the submarine will be forced to maneuver over long periods of time have been comparatively rare in the Western Atlantic. This conclusion, if true, does not bespeak a very high regard on the part of the Axis submarine service for the anti-submarine tactics and training of our vessels. A determined search and repeated attacks under these circumstances would stand a very good chance of exhausting the submarine's batteries and thus force him to surface even though no material damage had been inflicted.
Submarines will not from choice stay submerged for long hours. It is not likely they will stay submerged either night or day unless forced to do so by the type of work they may be doing. It is to be expected that they may lie on the bottom in shallow water at times, to obtain rest.
Surface ships which are not completely darkened render the Uboat's task much easier. They can be seen much farther away, thereby giving the attacking U-boat more time to maneuver into a better firing position and to fire with more deadly aim.
Surface ships which do not zigzag make perfect set-ups for even inexperienced U-boat commanders. Those zigzagging have a much better chance of being missed.
A lighted shore line may be used by U-boats to advantage. By keeping station to seaward of vessels passing the coast, these vessels may be silhouetted against the shore line.
In a crowded sea lane a U-boat can, by showing running lights, give the appearance of a small craft. In this way, the submarine may mingle with the ships and pick off the heavily laden ones.
U-boats use radio a great deal. They evidently feel that the knowledge gained by the use of radio far exceeds our advantage gained by knowledge of their approximate location.
They usually transmit during hours of darkness.
The frequency bands used are not often changed.
They use radio direction finders to locate ships not observirg radio silence. (Note: Some oscillating receivers emit a sufficiently strong signal to permit resolution by direction finder.
They transmit so that other other submarines can "home" on them. Submarines homing on these transmissions will make an effort to get ahead of the target in order to attack. The attacks usually occur on the surface at night.
Reports state that submarines have listened on our T.E.S. frequencies and have even answered up in good English, trying in that way to confuse orders.