U-boat  "U-249".        249                     Copy No. _89_ C/W's Robertson & Watson.
Dec.6th.1944.                                      111  
Reg No:  Top Secret 136.                          171  
Top Secret.
Current Order No. 2
Procedure when Schnorkelling.        Issued Nov. 1944.
Many undertakings of 40-60 days duration were carried out without surfacing.  The Schnorkel proved very effective and through it operations were again possible in areas heavily patrolled from the air
  a). In General.
?   Principle:  Under all circumstances attempt to recharge battery completely every day.  On account of enemy positions the Schnorkel is not to be shifted.  Continued reduction of the battery capacity can finally cause the boat to be in especially dangerous situation.
  (1). Chose favorable conditions for Schnorkelling, i.e., in general at night, in a swell, poor visibility.  By day only in clear weather with good visibility, the emotion of the sea from force 2 and is the Schnorkel does not smoke.  Best at night, once 2-3 hours at the beginning and end of the night.
  (2) The Schnorkel is not undetectable by radar.  If the Schnorkel is correctly manipulated, that is to say, with the exhaust under water, it can be scarcely or only inaccurately intercepted.  The motion of the sea reduces the possibility of being located.  If the Schnorkel is fully raised, it can be located and attacked, therefore it is wrong to let the Schnorkel protrude too far when there is phosphorescence, so as to avoid a trail of foam.  Then interception through location by radar is possible at a much greater distance and therefore more often than by a trail of foam.  The range of location of the camouflaged Schnorkel is about 10% of the boat when on the surface.  In practice it is scarcely interceptable.
  3). Air attacks on dark nights are generally only possible with the use of artificial light.  If a searchlight is detected shining directly on the boat or a flare above it, sound the Schnorkel alarm, and dive to at least 80 metres (260 feet).  If searchlights and/or flares are detected searching a long way off, or their location is above the Schnorkel dipole, only sound the "quiet-Schnorkel-alarm", that is to say, dive to 20-30 metres (65-95 ft. approx).
  4). When the Schnorkel is in use, periscope manned by the commander and officers of the watch in watches of roughly 1-2 hours.  In boats of type VII C, turn the main periscope continually by hand.  In boat type IXC in general, only the periscope for air observation.  The raising of both periscopes results in oscillations, and moreover increases the danger of being located by radar.  Do not proceed at too high a speed (i.e., with both Diesels on screw (aufSchraube)) this endangers the periscopes.  They swing, bend and become leaky and cloudy; slight oscillations are frequently corrected by very small alterations of periscope height.
  5). Noise caused by Schnorkelling is about the same as when the boat is running on both E-engines at a rate of 200 revolutions per minute.  In areas where there is hydrophone activity do not refrain from using the Schnorkel because of the fear of being heard.  According to experiments, boats have proceeded unperturbed with the Schnorkel in use, with hydrophone bearings of weak signal strength and with "circular saw noises" of medium signal strength.  "Circular saws" on shadowing vessels cut out other noises.
  6). Circular acoustic sweeps for enemy positions every 15-45 mins. - (unreadable) - dive to a depth of 20 metres.  It is possible that


    C/W's Robertson & Watson.
    the shadowing craft makes towards a U-boat which it has intercepted by hydrophone and in order to remain undetected, will stop when the boat stops her Diesels in order to make a hydrophone sweep.  Therefore measures have to be taken to be ready for a circular acoustic sweep as soon as Diesel engines are stopped.
  a). Order to engine room, central control and listening room:  "Ready for circular acoustic sweep".
  b). Switch off charging, charging diesel engine, electric fan and condenser.
  c). Lighten the boat in order to avoid further pumping, once the Diesel engine has been switched off.  Vent the diving tank (exhaust gas through leaky "blow distributor" in the tank.)  Report readiness of rooms to central control.
  d). Engine room telegraph to "Dive".  Cruising Diesel is to be turned off and the E-engine run, utmost silence in the boat, and keep a listening watch.
  e). If charging was previously carried out, at the end of the acoustic sweep vent the starting battery.  As soon as the Diesel is running, vent the Schnorkel battery.
  f). Complete the venting of the diving tank.  After schnorkelling, or after reaching greater depth, cease venting and report to central control.
  Methods of Proceeding.
  a). Schnorkel in sea up to force 6-7 (according to type of boat) steer broadside on, 1-5 degrees, with the ballast forward, with as little flooding as possible, otherwise current and air unnecessarily consumed when pumping prior to the circular acoustic sweep.  Choice of cruising speed depends on the motion of the sea, and the oscillations and angle of the periscope.
  b). Whenever possible ventilate the boat at least once during the day, for 15 mins. with the Schnorkel; this saves potash cartridges.  If when Schnorkelling, exhaust fumes enter the boat, let them circulate the Schnorkel with the quick-action valves shut, suck out the air, then, after switching off the Diesel, equalize the pressure.  Ventilate thoroughly otherwise casualties through carbon-monoxide gasses penetrate into the boat, the best ventilation only with either "in" or "out" ventilators, thus ensuring the maximum supply of fresh air possible.  Switching on both ventilators does not cause such a great increase.
  c). Overhead fuel tank - keep always full, otherwise air in the pipes.  Strictest attention to the maintenance of pressure in the oil pressure cylinder and to drainage between the stuffing box and the Schnorkel mast.  Any leakage of water into the oil pressure plant soon puts the periscope installation out of order.
  Technical execution.
  1. Switching on the Diesel according to special Schnorkel experiments Nos. 9 and 10.  Diesel not too heavily loaded as has frequently been the case in - (unreadable word) -  Thus seriously effecting the safe running and causing grave - (unreadable word) - or the safety of the personnel on account of high pressure and  - (unreadable word) - exhaust pipes (CO gas).
- (unreadable paragraph - several sentences) -


    C/W's Robertson & Watson.
  a). Switching on the Diesel on boats with GW engines:  In engines which cannot be put into reverse, charge the Diesel forward, with supercharger switched on, 380-400 revolutions.  Anchors parallel, batteries in rows, armature or current up to 750 Amp, magnetic current not more than 32 amp.  Engines which can be reversed, can be charged in reverse without the supercharger.  By this method the pressure is lower than in forward charging with the supercharger.  For other Diesels, propelling shaft at least 250-290 revolutions.  No additional charging.  Supercharger continually switched on.  The least number of revolutions subject to the counter-pressure from the exhaust gas, for pressure up to 0.45 atmosphere, 250 revolutions, for pressure above 0.45 atmosphere, 290 revolutions.  To fall below the least number of revolutions endangers the supercharger-coupling.
Page 3 of original.
  b). Switching on the Diesel on boats with MAN - engines/engine in reverse while schnorkelling:  Anchors and batteries as under 1 a).
  c). Exhaust gas pressure in continual operation.  Not above 0.4 to 0.5 atmosphere.  Otherwise leakages in the exhaust pipes may certainly be expected.
  d). When dipping for a considerable period, first switch off the charge and disconnect the driving Diesel.  When exceeding a pressure of 200 mb. switch off the Diesel propulsion engine, and run the electrical engine until a balance of pressure has been established, which corresponds to normal manner of propulsion.  The highest permissible pressure is 200 mb.  Check up continually on pressure metres, since they are frequently jammed.  Avoid important variations in pressure.  As a result of moisture in the boat there is danger for the periscope (misted), electrical installations (short circuit through moistness), food-stores (tins burst at rusty spots).
  e). When the Diesel is switched on for schnorkelling, proceed with exhaust just below surface of the water, in order that the danger of resistance from splashing may be as slight as possible.  Then when the Diesels are running, proceed to Schnorkel depth.
  f). In Schnorkel boats VII C, the starting up of the Krupps Diesel engine with switched on supercharger, is strictly forbidden, even when schnorkelling.  Starting up with switched on supercharger, against orders, has already caused numerous disorders.
  2. Charging.
  a). "Normal charging" (characterized by 2.34 Volt tension).  To avoid premature sulphurization, continue to charge batteries after having reached a tension of 2.34 volts (145 Volts in Type VII, IX and XXIII) at charging stage 1, in such a way, until, with constant tension and a current evenly regulated to 200 amps. of the battery, the charging current has been reached (200 amps. plus network consumer gives distributor current).  That is:- do not break off the charging as previously when charge current has been reached at the third stage, but continue charging, by maintaining an equal tension.
  b). Additional to normal charging.  "Increased normal charging" characterized by 2-4 volts tension):
    The more often carried out the better for the length of life of the battery.  Charging with charging current 1.  Amount of charging up to 2-4 volts, cell voltage.  (After that, charging current as under 2 a).  Reduce to 200 amps. by maintaining equal tension.  N.B.  Do not measure the tension with the main tension measure of the main switchboard, but with the precision instrument, 2-3 cells for each battery.  In no case go above 2.4 volts.


    C/W's Robertson & Watson.
  c). Full charging according to the directions on the battery, with charging current 3.  Cell voltage up to 2-7 volts, according to the precision instrument.  Boats on operations of three weeks' duration, 1 full charging.  Boats on longer operations, 2 full chargings monthly.
  d). The most careful venting of batteries.  After a "Schnorkel alarm", vacuum for rotating fan batter according to directions on the battery.  Before putting a combined charging into operation ventilate the boat for 10 mins.  At the end of, or after an interrupted charging, vent the batteries for half an hour.  After full charging, a 10 mm. pressure for half an hour, 4 hours later, for another half an hour.
  e). Recharging at sea.
  f). Quick charging as previously in Channel areas (2000 amp. battery current).  Use of a charging Diesel and attached charging are most harmful for engine installations.  Resort to this only in emergencies.



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