Likely the keels did some of their job and prevented the hull from traveling into water shallow enough to do real damage. Outside of single errant rock or coral head the rudders would be ripped from the boat before the props and struts would be damaged. Prudent naval architecture is a design that will handle a sudden grounding at hull speed with no threat to water integrity or the ability to sail to safe harbor. Not having a design capable of a sudden stop at hull speed puts the life of the passengers at risk with collisions of submerged containers and whatever else waits just below the surface at night.