Proceedings Of The Marine

FALL 2015

Proceedings magazine is a communication tool for the Coast Guard's Marine Safety & Security Council. Each quarterly magazine focuses on a specific theme of interest to the marine industry.

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88 Proceedings Fall 2015 www.uscg.mil/proceedings A nswers Engineering 1. Note: To prevent alternator voltage droop during motor startup, large AC motors usually use some method of reduced voltage starting to reduce motor starting current. One of the commonly used reduced-voltage starting methods used aboard a ship is the autotransformer starter. A. provide increased voltage for starting Incorrect answer. Autotransformers function to provide decreased voltage for starting, not an increase in voltage. B. provide increased torque for starting Incorrect answer. Although autotransformers function to provide decreased voltage for starting, this results in a decrease in torque for starting, not an increase in torque. C. provide reduced voltage for starting Correct answer. As explained in the Note above, autotransformers provide reduced voltage for the starting of large AC motors. D. provide speed control Incorrect answer. Large AC motors may use any of a number of variable speed drive tech- nologies, none of which include the autotransformer type starter. 2. Note: The steam supply source for an LP evaporator seawater feed heater may be auxiliary exhaust, turbine extraction steam, or "live" steam (steam reduced from boiler pressure). Steam pressure to the evaporator's feed heater is controlled by a regulating valve. An orifce plate downstream of the regulating valve ensures a constant fow of steam to the feed heater. As the supply steam passes through the regulating valve, the steam pressure is reduced, resulting in superheating of the steam. Superheating of the steam is undesirable because the higher steam temperature leads to increased scale formation in the evapora- tor. To prevent this, the superheated steam is "conditioned" via condensate sprayed through a desuperheater nozzle located between the orifce plate and the inlet to the feed heater. A. desuperheated steam Incorrect answer. Prior to the steam being conditioned at the outlet of the control orifce, the steam is in a superheated state, not a desuperheated state. B. superheated steam Correct answer. As explained in the Note above. C. saturated steam Incorrect answer. Prior to the steam being conditioned at the outlet of the control orifce, the steam is in a superheated state, not a saturated state. D. poor quality steam Incorrect answer. Steam quality is a measure of the proportion of saturated vapor in a saturated vapor-liquid mixture. Since prior to the steam being conditioned at the outlet of the control orifce, the steam is in a superheated state, the steam quality cannot be considered poor. 3. Note: Injection lag is the time interval between the beginning of the delivery stroke of the fuel injection pump and the beginning of injection at the injector nozzle. It is affected by the length of the high pressure fuel lines, the compressibility of the fuel, and the ability of the system to keep the high pressure fuel lines full of fuel between injection events. A. improper timing of the intake valves Incorrect answer. Although improper timing of the intake valves may impact the ignition delay period and combustion effciency, it has no infuence on injection lag. B. setting of the pump plunger Incorrect answer. Although the setting of the pump plunger will impact injection timing, it has no infuence on injection lag. C. compressibility of the fuel Correct answer. Even though fuels are virtually incompressible, the small degree of compress- ibility is one of the factors causing injection lag. D. position of the needle valve Incorrect answer. The position of the needle valve is determined by the high pressure fuel line pressure. The needle valve is open during injection when the fuel line pressure is high and is closed at all other times. The position of the needle valve has no infuence on injection lag. 4. Note: When external forces act upon the hull of a ship, causing it to roll about its longitudinal axis, the period of roll is the time required to complete one roll cycle. The roll period is primarily determined by the vertical weight distribution (or the length of KG). The higher the center of gravity, the longer the roll period (or slower the roll) will be. A. length of KB Incorrect answer. The length of KB is determined by the loaded weight of the ship and does not necessarily correlate to the vertical weight distribution. B. vertical weight distribution Correct answer. As explained in the Note above. C. virtual rise in the center of gravity Incorrect answer. Although weight shifting transversely as a ship rolls will cause a virtual rise in the center of gravity, and thus have an impact on the roll period, the vertical weight distribution (length of KG) is the primary factor contributing to the ship's roll period. D. moment to trim 1 inch (MT1) Incorrect answer. The moment to change trim one inch (MT1) is associated with movement of weight fore and aft on the ship which impacts longitudinal stability, whereas the ship's period of roll is associated with transverse stability.

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