Found the right term: "Redundancy"

Didn’t know the right term for the point I was trying to make with this thread:

We don’t need no stinking compass

The first meaning of the word “redundant” in the dictionary is "exceeding what is necessary or normal, superfluous". But the third meaning, as used in engineering, is
serving as a duplicate for preventing failure of an entire system* but there are other forms: For example Dissimilar redundancy

Aboard ship in the engine department having one ship’s service generator (SSG) running with another on standby is a duplicate, the emergency diesel generator (EDG) is a partial redundancy, the batteries for the GMDSS is dissimilar redundancy.

On the deck side, on the bridge there might be two compasses, two radars, two VHFs, threes modes of steering etc. With methods of navigation and collision avoidance there’s visual, radar ranges, bearings, AIS, ARPA and so forth.

Dissimilar redundancy important many things. Current state of the economy it’s fixed income, securities, bonds, cash, real estate.

Learned what I needed to know about investments in the CG. Navigation several dissimilar methods to fix position. Vessel duplicate all the essential systems. Foolish to trust one critical item

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A VHF radio (or two) is not normally considered to be a redundant part of a propulsion system but the local fisherman around here (and elsewhere) sometimes use the radio to get a tow home in the event of a breakdown.

Another consideration would be cost/benefit. As an example a skiff with an outboard. Depending upon the situation it’s common to carry a set of oars. Larger vessels that venture further sometimes are equipped two outboards.