Optimization of a Chemical Tanker and Propeller with CFD

In the 7 th -Framework EU project “STREAMLINE” one work package was entirely devoted to the optimization of state-of-the-art propulsion. The hull form of a chemical tanker, referred to as the ‘Streamline tanker’, together with its propeller was optimized using CFD. The ship speed was 14 knots, Lpp=94m, B=15.4m, the draft was 6m and the block coefficient was 0.786. The Reynolds numbers at model and full scale were 8.9·10⁶ and 8 6.0·10⁸ , respectively. In this paper we describe techniques for optimizing the lines of the aft body using the RANScode PARNASSOS, together with an optimization of the propeller using the BEM code PROCAL. Sections 2 and 3 give a brief description of the method used for hull form variation and the viscous-flow solver.
The optimization of the lines of the ship was done for zero drift angle and free surface effects were not taken into account. All RANS computations computed the flow around one half of the symmetric ship only and symmetry boundary conditions could be imposed on the (undisturbed) water surface. Since scale effects in the wake field can be quite significant, the optimization should be performed at full scale. Modifications aft of midship were allowed only and the displacement should not decrease. Further, the propeller location as well as the ship’s main dimensions were fixed. Constraints to guarantee sufficient room for machinery were that section 2 and all sections more upstream should stay outside a box indicated by the red lines in Figure 1.
The choice of object functions is of crucial importance in hull form optimization projects. Minimization of the resistance is not be the best way to reduce fuel consumption, since a decrease of the resistance is usually accompanied by a relatively strong decrease of the nominal wake fraction [7] which can reduce the propulsive efficiency. Therefore, as a first object function we used an estimate of the required power. The second object function was a function that represents the quality of the full-scale wake. The latter can be of interest when noise and vibration on board ships, or the risk of damage to the propeller and rudder due to (erosive) cavitation has to be minimized. In Section 4 we describe the object functions in more detail. In Section 5 we will describe the results obtained from several systematic variations, together with the scale effects for some candidate hull forms. In Section 6, we discuss the propeller geometry parameterization developed for this optimization, as well as the results that we obtained.