That's a good question. Sam Halpern makes a good case about why the Titanic could not have 'grounded' on a shelf like projection of the iceberg during the collision in his article about flooding of Boiler Room 4.I also have issues in the grounding theory, if Boiler Room 4 was affected, then how come Boiler Room No.s 5 & 6 weren't affected?
If I have understood his explanation correctly, Sam says that such grounding of the still moving and turning Titanic would have caused far more severe damage to the double bottom of the ship both forward and aft of BR4. The resultant flooding would have caused loss of the Titanic's transverse stability soon after the first hour and the ship would have capsized.
Sam feels that the relatively small damage to the floor of BR4 could have been caused by a small spur in the iceberg that was not involved in the initial collision itself. As the Titanic continued to move forward while turning to port, it momentarily lost contact with the berg after the first 2 feet of forward part of BR5. Seconds later however, that aforementioned spur opened up a small seam in the double bottom under BR4 as it passed. The resultant flooding was slow but steady with the water accumulating under the stokehold plates. By about 01:10 am this continued flooding combined with the 4-degree downward trim of the bow caused the water to 'pile up' at the forward part of BR4 and appear above the stokehold plates as noticed by the crew.