Controlling Pitch to Save Fuel

The United States Coast Guard R&D Center and Naval Surface Warfare Center have developed an analytical method to determine the most fuel-efficient combination of throttle and pitch settings for any desired speed through the water. Most control systems for controllable pitch propellers have a pre-determined pitch setting associated with each throttle position. This is often a somewhat arbitrary linear pitch ramp-up at lower throttle positions, followed by a constant pitch at all higher throttle positions. This pitch schedule does not generally give the best possible foe/ efficiency. Under the method recommended here, standard calculations for HP and RPM are performed at various speeds and pitches. Lines of constant speed are then plotted on an engine map, along with lines of constant fuel rate from engine bench tests. It is then possible to determine where each line of constant speed achieves the lowest fuel consumption. The corresponding throttle and pitch settings can then be programmed into a software-based control system to implement the most fuel-efficient pitch schedule. Analyses are presented for a diesel-powered US Coast Guard WLB, a USCG High Endurance Cutter operating in gas turbine mode, and for a gas turbine-powered US Navy CG-47 Guided Missile Cruiser. Fuel savings relative to current practice are presented, and the most fuel-efficient operating modes (e.g. trail shaft, split plant, and full plant) for various speed ranges are readily apparent. The method also gives some insight for non-CPP vessels on the economic implications of selecting a particular fixed-pitch propeller. The paper also includes some discussion on the selection of appropriate service margins.