"Built to withstand great stress and strain by the renowned ship builders of Bremen, Danzig, Hamburg and Kiel, the St-52 grade chromium, nickel-molybdenum steel of the pressure hull had its limitations. Designed with a high degree of tensile strength, up-to 52 kilograms per square millimeter (or 75,000 psi), the pressure hull could not withstand the direct impact of exploding TNT, compounded by high hydrostatic loading, that coerced metal fatigue leading to catastrophic structural failure. Especially vulnerable was the vertical structure of the conning tower located amidships, where communication and sensing devices penetrating the pressure hull, as also where shafts, linkages and exhaust valves pushed into the engine room.

The conical ends of the cylindrical Unterseeboote contour were also susceptible. The three segments of the torpedo tubes, joined together by flanges and bolts, with one segment inside the pressure hull and two outside, unlocking into the ocean, along with their attendant pipes for flooding, draining and venting the tubes, which also separately penetrated the pressure hull, were also highly susceptible.

A tiny fissure, instantly amplified itself into massive structural failure, as the ‘water-ram’ tore the pressure hull longitudinally and horizontally."

Extract from my ebook "TYPE XXI U-BOAT (ELECTROBOOT) PART ONE The Story of the World’s First True Submarine Capable of Operating Primarily Submerged" available on Amazon.com

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