You must ensure that all pressures and temperatures are maintained within the normal operating ranges. If this is not possible, secure the engine. You must also check all instruments frequently. The NAVSEA technical manual will provide you with detailed information concerning the proper operating pressures and temperatures. When this information is not available, maintain the temperature of the lubricating oil as it leaves the engine between 160 and 200F (180 F is preferred), and maintain the temperature of the fresh water as it leaves the engine at not less than 155F or more than 185F (170F is preferred). Do not allow the temperatures in the seawater cooling system to exceed 130F. Higher temperatures will cause deposits of salt and other solids in the coolers and piping and will aggravate corrosion.

Make frequent checks of the cooling system to detect any leaks. Vent coolers and heat exchangers at least once each watch. Check the level of the fresh water in the expansion tank frequently and add fresh water as necessary. If the freshwater level gets low enough to cause overheating of the engine, NEVER add cold water until after the engine has cooled.

Critical Speeds

The vibrations resulting from operation at destructive critical speeds will cause serious damage to an engine.

All moving parts of machinery have critical speeds. CRITICAL SPEED means there are certain ranges of speed during which excessive vibration in the engine will be created. Every part of the engine has a NATURAL PERIOD OF VIBRATION, or FREQUENCY. When impulses set up a vibration that coincides with the natural frequency of the body, each impulse adds to the magnitude of the previous vibration. Finally, the vibration becomes great enough to damage the engine structure.

Vibration may be set up by linear impulses from reciprocating parts or by torsional impulses from rotating members. The crankshaft is the part that causes torsional vibration, because pressure impulses on the piston cause the crankshaft to twist. When the pressure acting on the piston in each cylinder decreases, the shaft untwists. If pressure impulses, which are timed to the natural period of the shaft, are permitted to continue, the amplitude of vibration will become so great that the shaft can break. If the speed of such an engine is changed, however, the pressure impulses will no longer coincide with the natural period of the shaft and the excessive vibration will stop.

Since each engine has a natural period of vibration (which cannot be changed by the operator), the only control you have is to avoid operating the engine at critical speeds. If critical speeds exist below the normal speed of the engine, you should pass through the critical ranges as quickly as possible when you are changing engine speed. Detailed information concerning critical speed ranges is provided with each installation. Tachometers should be marked to show any critical speed ranges to make it easier for you to keep the engine out of the critical ranges. Remember, tachometers sometimes get out of adjustment. Consequently, you should frequently compare each tachometer with a calibrated mechanical counter.

Fuel

You must maintain an adequate supply of clean fuel. (The importance of clean fuel was discussed in chapter 9.) Check the fuel system frequently for leaks. Clean all fuel strainers at periodic intervals. Replace fuel filter elements whenever necessary. When diesel fuel purifiers are provided, purify all fuel before you transfer it to the service or day tanks. Frequently check the service tanks for water and other settled impurities by sampling through the drain valve at the bottom of the tank. Drain off water and impurities.