Most yachts have two engines, two reduction gears, and two propellers. The reason most often cited for this is redundancy. While that seems a reasonable point, the truth is that most engine failures can be attributed to fuel problems. Unless you're drawing from separate tanks, filled at separate times from separate sources, fouled fuel can take down both engines in rapid succession. You'll find that many seagoing vessels, including cargo ships and commercial fishing vessels, operate on a single engine without failure for thousands of hours. They depend on fuel-polishing systems, day tanks, and a good supply of spares in lieu of a second engine. Modern diesel engines, supplied with adequate air and clean fuel, are among the most reliable pieces of machinery to be found.