Did the design of Noah's Ark make it almost impossible for it to sink in the tumultuous 150 days of rain?

Well the Ark's proportions contributed to its stability, preventing it from capsizing. You see, the ark was also designed to deal with the forces that could cause it to pitch lengthwise in heavy seas. Extreme pitching-when each wave lifts one end of the vessel and then allows it to plunge downward-would have been very uncomfortable for the people and animals on board.

You see, the pitching also puts heavy stresses on the vessel. The structure must be strong enough to resist the tendency to sag in the middle when large waves lift both ends of the vessel at the same time. Yet, when a large wave lifts the vessel at its midpoint, with nothing to support its ends, the bow and stern may bend downward. God told Noah to use a length-to-depth ratio of 10 to 1. Later shipbuilders would learn only by hard experience that such a ratio can accommodate these stresses.