Optimising technique in matching combined diesel engine or gas turbine (CODOG) propulsion system to hull and propeller of a frigate

Abstract

In operation of a combined diesel engine or gas turb ine (CODOG) propulsion system, optimal matching of prime movers with propellers and ship hull is of great importance. Selection of an ideal propeller pitch that will be apt for different operating conditions of a marine vessel is an arduous task and requires initial assessment with a dedicated mathematical model. In this work, matching CODOG propulsion system to achieve best operation of type F90 frigate was carried out. A non-complex Java computer program (prop-matching) was developed to facilitate the matching process using dedicated simulation models in design and off-design conditions. The goal is to understand the interaction of either diesel engine or gas turbine with propeller and ship hull. The pitch of a controllable pitch propeller (CPP) was varied to the limit of optimal operation to absorb the power in either diesel engine or gas turbine mode in wide range of engine speeds and load. If a pitch other than that for the appropriate load and speed is selected, there would be an increase in fuel consumption, cavitation, vibration induced stresses in addition to stresses caused by engine load and wave motion on the vessel. The determination of the optimal operating points will lead to significant improvement in flexibility, minimum environmental impact and operating cost. Propeller characteristics were determined with hydrodynamics based on updated B-series regression polynomials which were correlated using Boundary Element Methods (BEM) and tuned with semi-empirical corrections. The analysis showed that the pitch ratio of a propeller has a dominating influence in the selection of the optimal points under operations in diesel or gas turbine mode and that the highest propeller efficiency did not occur at the optimal points. The output results for the open water propeller characteristics from this model are in good agreement with results of other authors and commercial Lindo software.