"Remember the cruise liner which was disabled off Mexico. She had at least two engines and drive systems, yet was disabled by a fire. So having two engines for those rare instances, is not justified.
There are many ships which go aground because of navigational or other human errors--how do we prevent these?"
Having two engines would, logic says, decrease propulsion inability due to engine failure by half. Fuel issues are not, technically, engine failure. The fire on the Carnival cruise ship caused secondary engine failure, probably due to loss of electronics control, or fuel transport, (hosing) destruction items beyond their "at sea repair" capabilities. They lost power of the primary electricity generator too, not just the propulsion engines. A fire, of any magnitude, in the engine room would disintegrate the electrical control cabling and panels rapidly, producing loss of communication and control of the mechanical plants.
Four hours out of Ft. Lauderdale, we were inbound on a RC cruise ship, when we had a loss of electrical power. When the generator system came back on line, there was a surge, and that took out control to the Starbd engine and bow thrust system. It gets very dark onboard, at 0400 in a center cabin. The 6 hour additional delay, due to slowed port approach and timing for two additional harbor tugs, cost many folks the loss of airport connection. Having my handheld VHF, it was fun to listen to the marine chatter as we connected with the tugs and were escorted into the dock.
As to the human error part. Good Question Dr. Bob. Maybe integrating the control systems, Forward looking sonar, tighter tolerances on the GPS with inputs from both for and aft GPS receivers on anything over 60 feet, more detailed navigation chart systems with more frequent updates, and for, aft and beam clearances programed onto a high def radar. All these to overcome the human on board missing something. Then of course, each system would need to be redundant, from the sensor to the power unit. Possible, yes; practical, no. So we fall back onto the human, doing his best, most of the time, and hoping not to have a catastrophe.
Harvey
SleepyC :moon