Is there a possibility to estimate how the inner waterline was in the last 20 minutes? I guess that at 2 AM when the forecastle submerged the grand staircase was flooding from D-deck to C-deck. But how did it keep track in the next 15 minutes until the dome imploded? Was water maybe still rushing down the staircase and fill the lower levels, which means that when the dome collapsed it was still around C-deck? Or did it keep track like it is shown in the 97' movie and the dome collapses when water is between A-deck and Boatdeck level? If so, from where did all the water come from to fill the staircase in such a short time?
I believe the water inside the ship did not correspond with the waterline outside because survivor Helen Candee said - "When we reached the water I could see two lines of portholes under water, brightly lighted.....Not until I saw the two lines of lighted portholes under the water had I the slightest idea of the truth."
The ship was already heavily weighed down by her boilers and engines which were below the waterline and kept the ship evenly buoyant. When the water entered the forward compartments the weight of water would settle her down further and drag several decks below the waterline before they had a chance to flood including the decks witnessed by Helen Candee, especially as she listed over to port and dipped that side below the waterline.
I suspect the ship actually rolled over to port as she became top heavy and continued to roll over instead of settling down to port. There are numerous survivor accounts which describe the ship reeling and rolling from side to side with desperate attempts from the crew to keep the ship upright for as long as possible which suggests their efforts above the surface were affecting the position of the ship as she became more top heavy. Mrs. Ryerson saw many open windows on C-deck and observed the water rushing into the rooms on the port side. This would weigh down the port side on C-deck while the rooms directly below on D and E deck were still dry (as observed by Mrs. Candee) and this would make the ship much more top heavy and roll her even more to port.
Miss Glynn said - "We watched the Titanic rolling and bobbing like a cork. All her lights were burning, and over the water we caught the strains of 'Nearer, My God, to Thee. Finally Titanic ceased rolling, seemed to hesitate a moment, and plunged her bow into the ocean."
2nd officer Lightoller described the ship rocking from side to side and rapidly going down. He was then sucked against a grating as the water rushed down a shaft leading to the forward boiler rooms and blown to the surface by a terrific blast of escaping air from deep inside the bow which was so powerful that it bent the grating outwards.
Charles Joughin was down on E-deck after 1.30am and he noticed the large Scotland road corridor was practically dry, despite being well below the waterline at that very late stage of the sinking.
I believe the observance of the dry illuminated decks below the waterline, coupled with the violent rocking and rolling of the ship from side to side, and the efforts of the crew to keep the ship evenly steady on the boat deck, and the enormous volume of water that rushed down the forward shafts which burst so much air outwards, are all indications that the bow contained a great deal of unflooded space inside when she broke and her bow took a sudden lurch as it partially detached and became unsteady on the surface, rolling from side to side as it increasingly became top heavy, before flooding completely and going down.
Regarding the breaking of the Grand staircase dome. William Mellors was on the forward boat deck. He said - "We were trying to fix up a collapsible boat when she gave the first signs of going under. There seemed to be a tremble run through the whole of the ship and the next thing we heard were loud reports inside which I think were the watertight doors giving way and before you could say Jack Robinson there seemed to be mountains of water rushing through the doors." This could either indicate that water was rushing violently into the ship, through the doors and down the unflooded staircase, or the water was rising rapidly up the staircase and lifting the whole thing upwards which caused a large volume of water and debris to burst out of the doors. I have been told that the film set had actually lifted up and if the water had rushed in violently it may have broken the staircase from its seating and propelled the whole thing up. Once the sea inside and outside had equalised the wreckage of the staircase would sink back down inside the ship, or smashed through the dome and float to the surface as the ship went down.
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