The typical gasoline marine engine will run fine for the first 1,000 hours. It is at this juncture that the engine starts to exhibit small problems. If these small problems aren't addressed, they can turn into major problems which may make the last 500 hours of life difficult to reach.
Interestingly, an automobile engine may run almost twice as long (3,000 hours) as your marine gasoline engine. The reason is that marine engines normally work harder and under worse conditions than automobile engines.
A well-maintained gasoline run under the best conditions may well run for more than the 1,500 hours without major overhaul. However, many that operate under the most atrocious conditions of salt air, damp bilges, intermittent operation and pure neglect will certainly die early.
Diesel engines are built to finer tolerances than are gasoline engines. They will accept much more abuse and often deliver, if well maintained, 8,000 hours of hard work before need a major overhaul. Theoretically a well-maintained diesel may last the life of your boat. Since the average recreational boater logs only about 200 hours per year, the 8,000 hour diesel would last 40 years.
Although diesels can add considerable cost to a boat, they should be seriously considered because of their durability, economy of operation and safety concerns. Diesel fuel has a much higher flash point than gasoline and does not present the same threat of explosion that gasoline fumes carry.
Engines like to run long and steady. The shorter the running time between stops, and the longer the idle time between runs, the fewer the hours they will deliver before needing major repairs.
The adverse conditions under which marine engines operate have a great deal to do with their longevity. What they really need is rarely what they get. Naval architects recommend that engine compartments should be supplied with lots of dry, cool (50 degrees F), clean air. The very minimum fresh air vent area (in square inches) for natural ventilation without blowers is found by dividing engine horsepower by 3.3.
