Energy efficiency
Improving ship operating efficiency By Henry Chen, Ph.D., Chief Naval Architect, Jeppesen Marine
Q
uantifying true ship performance is a very complex, if not impossible task. Ships are designed, built and operated to carry passengers and cargo as efficiently as possible. However, the desired level of efficiency is often difficult to achieve during a ship’s operating life. The exact causes of the efficiency losses that occur may be difficult to pinpoint, as there are numerous contributing factors interacting with each other.
the environment as well as rudder movements and hull and propeller fouling. The condition of the engine, hull and propeller can deteriorate over time, thereby requiring increased power to maintain the same speed. All these factors, plus the effects of changing environmental and loading conditions (draf t and trim) are interrelated, making it difficult to isolate the causes. Figure 1 on page 22 illustrates the elements that affect ship performance.
Monitoring Performance
Benchmarking Efficiency
Traditional methods of monitoring performance largely rely on data entered in the ship’s deck and engine logs by officers on watch. However, these recordings of ship speed, horsepower, propeller revolutions per minute (rev/min), slip and fuel consumption only indicate significant efficiency losses due to hull and propeller fouling when there is a significant increase in propeller slip or daily fuel consumption. Since vessel resistance from bad weather or deeper drafts can also cause increased power and fuel consumption, it can be difficult to isolate the cause of reduced efficiency and to correct it through hull cleaning, propeller polishing or engine tuning. Today, a performance monitoring system has to identify the degradation in ship efficiency and translate this trend or performance metrics into recommendations for improving overall fuel efficiency. Let’s examine some of the issues involved in developing such a system. Efficiency depends on four major factors, i.e., design, deployment, operations and maintenance. A ship is subjected to wind, waves and current while in transit. The engine converts the fuel into propeller torque, which produces thrust to overcome resistances and maintain desired speed. The resistance can come from
Before addressing the question of how to improve ship efficiency, we need to properly define efficiency and establish a reasonable benchmark as the basis for comparison. Efficiency can be loosely defined as “useful work done per energy unit consumed.” In naval architecture, we have: • Engine Efficiency = Delivered HP / Fuel Consumed • Propeller Efficiency = Thrust HP / Delivered HP • Hull Efficiency = Effective HP / Thrust HP • Propulsive Efficiency = Engine x Propeller x Hull During the ship design phase, these efficiencies are estimated and optimized for a specific size, service speed and vessel type in order to configure the size and design of both engine and propeller. The performance of the entire ship system is then confirmed during sea trials and accepted by the shipowner. During actual ship operation and loading, the weather can differ significantly from the calm and controlled conditions available during sea trials. However, the specific fuel consumption at various power outputs should not change when compared to the test results—unless the engine is out of tune or the quality and calorific value of the fuel are in question. As such, specific fuel May 2014 MARINE LOG 21