|Diesel Electric Propulsion|
As a concept, diesel electric propulsion is not new. In the past these systems were usually diesel engine driven DC generators therein driving DC motors. Their applications were generally limited to vessels that required a degree of close low speed maneuvering. Vessels such as ferries, harbor tugs and various other applications used diesel electric systems for features that were not available in mechanical systems at that time.
As in all technologies electric drive systems have made substantial progress in recent years. The two (2) dominant systems available today are frequency controlled AC motors and SCR controlled DC motors. Frequency controlled AC motor drive systems are generally more cost effective below 500 H.P. and SCR controlled DC motor systems more cost effective at the higher powers. The reason for the latter is the availability of new and rebuilt DC traction motors for railroad applications. Railroads and the offshore drilling industry have favored SCR controlled DC drives, whereas the manufacturing industry has favored frequency controlled AC motors due to the common requirement for lower power applications. Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue. The deep draft cruise ship industry, due to the very high hotel power requirements, is adopting diesel electric propulsion in most of its new builds. The focus of this article is on shallow draft applications. It is not the purpose of this article to judge the technical merits of either system. Both technologies have a proven record of efficiency and reliability.
The choice of a diesel electric system as the power source for a propulsion system of a vessel has nothing to do with hydro-dynamic efficiency. A propulsion system of a vessel i.e. that which is providing thrust to move the vessel is still chosen by the designer based on its merits for the vessel's application. Conventional propellers, CP propellers, azimuthing Z drives, transverse tunnel thrusters and low speed water jet systems can all be driven with equal effectiveness by a diesel electric system. Diesel electric systems become viable when the installed KW for propulsion approaches or is exceeded by the installed KW for other purposes. The convenience of electric power distribution makes it possible to locate the primary power source i.e. diesel generators exclusive of consideration for where the power is to be applied, whether it be propulsion, thrusters, HVAC or cargo handling purposes. A large variation in propulsion power requirements i.e. long periods of low speed or a necessity to shift power from main propulsion to thrusters for dynamic positioning purposes can also justify diesel electric. Modern turbo charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long periods of low speed, low load, or low RPM high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto start, and load shedding features make it possible to more efficiently utilize the installed H.P. of a diesel electric system.
Norman DeJong of DeJong and Lebet, a Jacksonville, Florida, based firm of naval architects, offers the following design problems that can be resolved using diesel electric propulsion.
- When propulsive or station keeping power requirements are a small or relatively small percentage of the total power requirements. Drill rigs, OSV's with special positioning requirements, research vessels with special maneuvering requirements, and gaming vessels where speed is inconsequential such as gaming vessels operating on the river.
- When space and/or propulsion machinery limitations either exclude the use of direct diesels or adversely effect the construction cost resulting from using direct diesels.
- SWATH vessels with hulls and/or struts too small to accommodate diesel engines, access, ventilation, etc.
- Vessels with potential trim problems, such as stern wheelers, where machinery needs to be located forward to avoid trim problems.
- Vessels that require, due to space limitations, more than one machinery space are subject to increased construction cost due to duplication of or increases in systems such as:
- Engine cooling
- space ventilation
- control facilities
- exhausts, etc.
- Vessels that require maximum torque at minimum propeller rpm's.
- Vessels that have a large variation in power consumption.
No system is without its drawbacks. Frequency control and SCR systems produce harmonic distortion that can affect AC circuits. This can cause problems with certain equipment sensitive to harmonic distortion such as computers, navigation equipment and other electronic devices. Jeff McMahon, Vice President of International Electric Company, a Houston based manufacturer of SCR drive systems, points out that this is not a problem if dealt with in the early design stages of the system. The equipment aboard vessels that are sensitive to harmonic distortion are generally at low power requirements. He states that the simplest and most economical method is to apply harmonic filters to the sensitive equipment rather than the power source. There is also the fear on the part of some operators of additional complexity. The fact remains that most all vessels operating today that meet the criteria for a diesel electric system already have this equipment aboard even if on a smaller scale i.e. generator sets, switchboards, and electric motors. The fact that the propulsion power may be supplied by an electric motor instead of a direct driven diesel engine does not make equipment aboard the vessel any less familiar to the operator. The utilization of the diesel engines is transferred from direct propulsion power to generator power. This provides greater flexibility of the use of the installed KW and in some instances reduces the number of diesel engines installed. The ability to generate only the power required to meet the needs of the duty cycle of the vessel utilizing multiple generator sets reduces fuel consumption and maintenance costs. It also provides redundancy in power capacity. The desirable features of diesel electric propulsion do not suit all applications. Diesel electric propulsion is only suitable for displacement applications. High speed vessels that are weight sensitive, and displacement vessels in which the installed horsepower is primarily for propulsion with a constant duty cycle, have no need for a diesel electric system. We feel that diesel electric propulsion systems will play an increasing role in the propulsion of vessels in the future.
Ken Robbins is President of Marine Propulsion, Inc., a Hammond, Louisiana based supplier of power transmission and propulsion machinery.