On 20 January 1994 broke the American Star in half. Final certainty about the exact process could probably bring only the opinion of the salvage company in charge of the investigation, which was however not published in detail. Therefore, only a rough overview:
The grounding of the ship had been pushed to a temperature below the waterline rock formation in which it is then wedged in the front center. The tail swam continue without permanent grounding over deeper water, the bow was partly on sand and the mid-ship got stuck on the rocks. This position was fatal:
First, the resulting unequal distribution of weight and buoyancy. The ship "hung" at the bow and stern, and the median keel area was extremely burdened by the selectively increased resting and deflection associated.
For the second through the surf, which is now able to exercise a kind of leverage on the ship.
The surf caused in combination with each other as two very devastating movements of the tail:
A force applied to the longitudinal axis of the fuselage bending (ie, the movement towards the beach and back again), caused by the impact of the waves on the ship's side, and also a change in height of the water level between the crest and valley (ie a strongly fluctuating buoyancy), which the extreme stresses to which the body has been exposed by the incomplete grounding even worse. This effect was also favored by the tide.
The keel break
This multiple exposure gave the American Star, a few hours after beaching finally at the point to where the resulting forces on the largest and the resistance was the lowest.
The result was a broken keel in midship position in the area between the engine- and the aft boiler room. This break had already occurred a few hours after the beaching and was not initially visible as it of course was below the waterline.
With the keel break (the keel is the anchor point of the hull structure of a ship and runs longitudinally along the vessels bottom) the American Star's spine was broken and the stresses in the ship's hull spread to the remaining supporting structures, as well as the ship's side. Because of this extreme stresses to the remaining structural integrity, the rupture started to extend from below the water line upwards and became increasingly visible up to the main deck. The rupture followed no particular structure on these decks, but was guided by the position of the keel break.
Page 3: The hull rupture- Where, How, Why?
This picture was taken just hours after the beaching.
Clearly visible is a deep crack (mouseover for highlight), which corresponds to the position of the later split of the vessel. But this rupture does not spread (as one might suspect) from above, but rather has its origin in a keel break, while the upper decks are visually intact. Very good to see is the hanging of the stern, caused by underwater rocks the American Star hit midship.
-Picture: TVE Canarias/ Vie et Mort de l'America-
The reasons for the keel breaking and the further breaking up of the hull are to be found in the not designed for this extreme situation bearing structure of a ship, and the resulting typical for metal fatigue fractures strongly varying load stresses.
The keel was broken but partly also assisted by the design structure of the American Star. Like almost all pre-war ocean liner, the former America had their boiler rooms in the center of the ship on the lower deck. When America even the entire propulsion system focused due to their special design on the center of the ship. The engine room I was not as usual at that time common in the posterior third behind the ship boiler rooms, but framed in the middle of them. For this, a look at the plans:
In the lower hull area, in the middle of the ship, was so in height extending over several decks and expansive on overall Beam cavity. The ideal vulnerability. In this area there were hardly divided the fewest cross-bracing and interlocking structures, which have the structure from the inside and reinforced the keel "support" can. Even the most robust of the double hull tanks were largely ineffective, as their box-shaped structure was designed to provide protection against an occasional or limited impact area from the outside as a "crumple zone" and not intercept or offset horizontal and vertical tensile loads.
This vulnerability can also be seen on later images of the wreck. The engine room and boiler room, the front are an easy target for the surf (pictured below) by their physical structure.
Neither fracture nor cause the fracture, which also implies an elongation direction from top to bottom, however, are appropriate.
The real breaking point was about 10 meters ahead and touched not the specified elevator shaft and the rotating staircase. These were still intact after the fracture.
The breaking point was instead on the 140th Framework, directly at the beginning of the cocktail lounge (and not behind the staircase with lift) and just behind the chimney shaft and thus directly behind the chimney. Below the relatively perpendicular fracture was exactly on the dividing wall between the machine and the rear boiler room. At this point, the breaking point was also determined keel fracture.
Why did the upper decks and superstructure initially resist the pressure?
After the ship had already succumbed to the rigors of adverse Strandungsposition and surf at the bottom of the hull, holding the upper decks of the stress was still what the ship to the point of complete collapse can affect intact from a distance. This phenomenon is due to a further feature of the American Star: The extremely stable structure of the upper decks and superstructures. During the construction of America in 1939 had been placed on an unconventional concept. The decks had to increase their stability, no expansion joints, but were riveted throughout. To get at this concept, no problems with stress cracks, you put on a thicker deck plating than usual and offset the reduced flexibility on the type of steel used for. Thus, the structures were part of the support system of the ship. Especially the promenade deck amidships had a decisive share in the ship statics.
With the increasing stress to the weight bearing and the growth of crack, but then gave in the upper decks of the force of the surf and gravity after two days. The tail separated from the bow.
The break occurred not so suddenly unsigned, but was only the end of a 2-day process.
Immediately after the breakup of the secured by the Scots and intact prior to departure carefully locked watertight doors, still floating and again motile tail was pressed parallel to the beach to the shore, where it is then drifted on sandy ground. It was at this time about half a deck lower in the water than the bow, which the enormous burden and extreme voltages shows that the American Star was exposed on the rock formation by the selective resting before the break. (Picture above this text to the right section)
The position of the hul rupture at frame 140
In the german NDR documentary "Das Wrack der America," the breaking point was located at the position of an elevator shaft with encircling staircase (frame 150). The justification for this assumption is underpinned with a longitudinal section of the construction plans. At this point (marked blue on the following picture) there would be a "weak point in the ship's design":
The hull in the area of the engine and boiler rooms was an easy prey for the surf. This picture also shows the extreme solidity of the upper decks and superstructure still in place with nearly no structural support from below and on top of that holding the funnel.