In September 1943 the Axis midget craft, 1 and 2 man “human torpedoes” were employed 
against the Allies invading Italy. They failed to penetrate the Asdic Screen, were detected and 
destroyed. Five days later 29 subs were taken when Italy surrendered, A captured Italian es- 
cort was found to be fitted with a German echo ranging equipment which the Axis had begun in- 
stalling. The equipment was primarily a detector and did not provide the many attack and ship 
control features contained in Asdic-Sonar. 
In the fall, says Admiral Doenitz, “The boats were ordered to remain on the surface when 
attacked by aircraft and to cooperate in fighting off the attack. They were then to attack and break 
up the destroyer Screen, with acoustic : torpedoes and, in the third phase of the battle, attack the 
convoy now deprived of its protection.” But due to the disaster suffered by the U-boats, he said, 
“It was, therefore, finally clear that surface warfare for U-boats had come to an end.” "Surface 
exposure to aircraft and radar detection resulted in the U-boat arm losing about 50 per cent of 
its offensive power and being driven from the surface. Continuous submerged operation was 
envisioned in Gross Admiral Doenitz’ top secret order of December 1943 which reads in part, 
“Tt is essential to victory that we make good our Scientific disparity and thereby restore to the 
U-boat its fighting qualities. * Consequently, German scientists who had been drafted to Shoulder 
rifles were recalled from military service to help create the “Naval Scientific Directional Staff”. 
During 1943 only 598 ships had been sunk, while 219 U-boats, 22 Italian and 22 Jap subs 
were Sunk, averaging 22 per month. During the winter of 1943-44 the.average 60 U-boats at 
sea took extreme care to avoid detection and operated mostly submerged - but, detection by 
Asdic-Sonar was becoming increasingly effective, and we began sinking more U-boats than they 
sank ships. By April 1944 the Battle of the Atlantic had been decisively won. Our Scientists 
had been shouldering detection problems as their contribution to the enormous Navy effort. 
Japanese Shipping was now being annihilated. Jap escorts which had been using mostly 
hydrophone detectors now began greater use of crude echo ranging equipment. Our Submarines 
used bathythermographs to hide under thermoclines, and also for regulating ballast water, since 
a Submarine’s buoyancy depends on water density which in turn depends on water temperature. 
Hydrographic charts were provided to show the type of bottom in various operating areas since 
submarines are more likely to be detected over sandy bottoms which reflect sound than over mud 
bottoms which absorb sound. Maps indicated the areas in which intense shrimp noises probably 
would mask the sounds created by submarines. Our submarine echo ranging Sonar had been 
augmented by: (1) a topside listening hydrophone consisting of a 3 foot long 2 inch diameter mag- 
netostriction tube having excellent directional accuracy on sounds up to about 10 miles distance 
in average water conditions; (2) a shipboard listening trainer, attached to the Sonar to provide 
‘practice in estimating speeds of various ships by their propeller rhythm; (3) other Sonar equip- 
ment which facilitated submerged excursions into Japanese held waters including Tokyo Bay. 
On D-Day (June 6, 1944) when the Allies invaded the Normandy beaches, the invasion waves 
were led by Small naval scout craft equipped with “Beach Obstacle Locators” containing an echo 
ranging Asdic whose recorder traced the outlines of the beaches. Obstacles (mines, fencing, 
etc.) were cleared by demolition out of the path of landing craft. The invasion forces were blan- 
keted by Such heavy air coverage and a Sonar Screensotight thatno Ships were lost to U-boats until 
the end of June when the 7 ships they sank had cost them 21 U-boats. 
Up to this time, the U-boat had acted as a submersible torpedo boat, spending much of its 
time surfaced, visible by, eye and Radar, and audible to radio interception and radio direction 
finders. But the U- boats’ tactics were revolutionized in the summer of 1944 when they were 
equipped with the “Schnorchel” extension stack, which expelled diesel exhaust and drew in fresh 
air. The Schnorchel stack was about the same height as the periscope and permitted: (1) opera- 
tion on diesel propulsion while submerged to periscope depth for which batteries were formerly 
used, (2) re-charging batteries at periscope depth which previously required surfacing fully, 
(3) six knot speeds at submerged periscope depth instead of about 3 knot speeds formerly em- 
ployed to conserve batteries. The U-boats took up stalking positions at focal points of convoy 
routes, listening while lying on the bottom, from where they attacked by blindly firing acoustic 
and looping torpedoes into passing convoys. Periodically they came to Schnorchel depth to re- 
plenish their air and re-charge batteries. Admiral Doenitz’ comment on Schnorchel tactics was, 
A-2405 
Silals 
