U-Boat - the hunter & the hunted
When Royal Malaysian Navy took delivery of the first Scorpene submarine, it marked the beginning of a new era for the Navy and the country. While modern submarine can lay submerged for days and months, its predecessor was designed to travel primarily on the surface, and plunge underwater only to escape an attacking destroyer or aircraft, or to carry out a torpedo attack in daylight.
That was the technology prevalent during that period of time.
The early submarine or U-Boat had two separate means of propulsion. The powerful diesel engines drove it forward at speed on the surface, but as soon as it submerged they had to be turned off otherwise the engines will splutter and the crew asphyxiated. In the early part of the Second World War, the moment a boat dived under the surface, the only supply of oxygen was from inside the boat itself. Therefore, the moment it submerged, the engineers had to switch over to two battery-driven electric motors for propulsion.
The batteries weighing several tons could propel the early designed boats at a maximum speed of about eight knots. At more economical speeds, the battery would keep the U-Boat moving for about twenty-four hours or 60 miles submerged after which it will go flat. Thereafter, the boat had to resurface to recharge the batteries with the help of the diesel engines.
The main core of the U-Boat construction was the pressure hull, a long steel cylinder divided into compartments by bulkheads. Attached to the outside of the pressure hull were the ballast tanks, equipped with a water valve below and an air valve on top. When filled with air, they provide sufficient buoyancy to keep the heavy pressure hull, packed tight with equipment, afloat with the conning tower and decks clear of the water.
When both the water valves and the air valves were opened, seawater forced its way into the tanks, expelled the air and reduced the total buoyancy of the boat so that it began to sink. The boat could then be controlled in its upward and downward motion, and could be brought back to the surface, either by pumping out the sea water mechanically, or by closing the air valve in the ballast tanks, and blowing in compressed air to force out the sea water through the water valve.
Adjustments in depth and trim could then be made by driving the boat through the water and adjusting the bow and stern diving rudders to propel the boat upwards and downwards against the resistance of the water.
The ballast tanks, carried on the outside of the boat, could be less sturdily constructed than the pressure hull. While the boat was submerged, the ballast tanks contained sea water, and therefore were not subjected to the vastly unequal pressure of that of the main hull. For the same reason, the fuel tanks were also constructed outside the pressure hull.
The fuel tanks were open to the sea water underneath, while the fuel intake valve to the diesel engines was placed at the top of the tank. As the fuel was used up, sea water was admitted underneath it to keep the tanks full, and since the less dense fuel oil floated on top of the water, no salt water was admitted into the engine. With this system, the expenditure of fuel oil left no air space in the tank, either to provide unwelcome buoyancy when the boat was required to submerge, or to cause the tanks to crumble like burst paper bags when the submarine went deep.
The U-Boat’s strike power lay principally in its torpedoes. The early versions were driven by compressed air which left a conspicuous wake of white bubbles behind the torpedo and gave the enemy ample time to take avoiding action. Later model incorporated battery-driven electric motor which made them almost invisible.
Early torpedoes were percussion ignited, and would explode only on contact with the ship. Many of them were faulty in that it either failed to ignite upon contact or failed to run at the preset depth.
In later models, magnetic ignitions were incorporated and will explode upon entering the magnetic field of the ships at sea, and as such, depth settings become less critical. Even if they explode below a ship’s hull, the strong shock waves almost invariably resulted in the vessel’s back being broken.
As the war progressed, U-Boat became vulnerable to attacks from surface vessels and aircraft.
The battle between submarines and escorting vessels continued relentlessly in the drawing board of naval engineers and scientists. Towards the end of the war when U-Boat became the hunted instead of the hunter, Germany unveiled their new offensive weapon and sophisticated equipment that enable the U-Boat to operate ‘blind’, firing their torpedoes from a depth of 150 feet or so.
The new boat code named XXI could make as good underwater speed as many of the escort vessels. Its effectiveness was well demonstrated by U-2511, when she encountered an anti-submarine group in the North Sea. With a simple deflection of course, she was able with consummate ease to escape her pursuers.
On May 4th, 1945, when Karl Doenitz instructed his U-Boat commanders to cease fire, U-2511 encountered a British cruiser and several destroyers. She delivered a dummy run under water, and came within 500 yards of the cruiser. Later, the cruiser was to reveal that they had no idea how near to destruction the ship had been.
From the beginning until the end of the war, the U-Boats main task were solely for the purpose of severing the Allies main artery, the ocean shipping routes on which their life depended. At times, Germany came close to achieving this aim.
Had the German naval high command relented to Karl Doenitz request for three hundred U-Boats before the war began, Great Britain’s economy might have folded, her morale crumpled and her people and government brought to their knees. Luckily, Doenitz’s vision was not shared by the high command and hence he had only 56 U-Boats when the war started.
Or, had the end of the war been delayed for another year or so, when the new U-Boats came into their own, and the advanced Walter hydrogen peroxide boat came into commission, the pendulum could have swung in favour of the Germans.
That very prospect had the Allies worried. Churchill later wrote “The schnorkel-fitted boats now in service, breathing through a tube while charging their batteries submerged, were but an introduction to the new pattern of U-Boat warfare which Doenitz had planned. Their high submerged speed threatened us with new problems, and would indeed, as Doenitz predicted, have revolutionized U-Boat warfare”.
Revolutionized U-Boat warfare and bleed the Allies main artery.
In all, the Germans lost 784 out of 1,162 U-Boats built. In human terms, the cost was stupendous. More than 40,900 men were recruited into the U-Boat arm, of which more than 28,000 lost their lives and another 5,000 taken prisoner.
Had the end of the war been delayed, Germany would not only rule supreme at sea, but also superior in the air. The jet fighters came into the war way too late to reverse the tide. Germany had by then been surrounded and waiting for the last rites.
Otherwise, I will be writing this in German.
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