AN INNOVATIVE ENERGY SAVING PROPULSION SYSTEM FOR NAVAL SHIPS

Abstract

ABSTRACTThe propulsion system on most naval combatants is inherently wasteful of fuel since the propellers, shafts, and engines are sized for dash speed, but operate most of the time at low speed cruiser power. Energy consumption is particularly high on twin shaft, gas turbine driven ships which use controllable reversible pitch (CRP) propellers for reversing.On a typical destroyer such as DD 963, the drag of the propulsion appendages is very high, 10–15% of the total ship drag. This drag is present even when operating at low cruise speed where a system with much lower drag could provide the required thrust.This study evaluates several alternate propulsion systems which could be used on a DD 963 type ship to reduce annual fuel consumption. An innovative system, which uses a single DD 963 type propeller system for cruise and two marine jet boosters for added thrust during high speed dash, shows promise of reducing annual fuel consumption by 23 percent. This fuel saving is due to reduced propulsion appendage drag and lower specific fuel consumption during cruise.Annual fuel saving can be further increased to 45% by replacing the two LM‐2500 turbines which drive the propeller with a single turbine equipped with Rankine Cycle Energy Recovery System (RACER) and changing the CRP propeller to a more efficient fixed pitch (FP) propeller with reverse gear for astern operation.In addition to reduced energy consumption, the propulsion systems using marine jet boosters for dash speed show promise of significant benefits in other areas such as reduced weight and improved survivability and reliability. The marine jets, which use state‐of‐the‐art technology and have demonstrated performance, provide the same top speed as the baseline propeller system. Improved low speed maneuvering and crash stop capability is provided by reversers on the marine jets. Improved propulsion system redundancy is provided by the three shaft system. A single marine jet will provide nineteen knots speed with a non‐operating (trail shaft) cruise propeller.While marine jet propulsors are inherently less efficient than propellers, the large drag reduction for eliminating one propeller system more than compensates for the additional fuel burned by the marine jet turbines during high speed operation. The single propeller, twin marine jet propulsion system is particularly attractive when combined with RACER since it permits elimination of one gas turbine without significant effect on top speed.