Large Area Propellers

Licentiate thesis, 2013

Even if shipping is the most energy efficient and inexpensive way of transporting large volumes of goods across the world, it still could be significantly improved with respect to energy efficiency. In this thesis, propulsive efficiency is improved by moving the propeller backwards from its original position and increasing its diameter. Computations are carried out using the CFD software SHIPFLOW. In the first phase of the project simulations are done for three different hulls, suitable for the large area propeller concept. Systematic variations of axial propeller positions are done for each hull, using two propellers; one original and one large diameter propeller. For the first hull, an 8000ton Tanker, the larger propeller is determined ad hoc, but for the other two hulls, a Medium Size Tanker and a RoRo Ship, the maximum propeller diameter is set by the ballast condition, in which the propeller tip should be submerged. To investigate the full potential of the gains possible for the 8000ton Tanker an extra-large propeller is also designed. The diameter of this propeller is maximized for full draft conditions with sufficient clearance to the free surface to avoid ventilation. The 8000ton Tanker has the best potential for propulsive improvement from SHIPFLOW simulations. With the first large area propeller the gain relative to the original configuration is about 11 %, as compared to an increase of about 10 % for the Medium Size Tanker and about 8 % for the RoRo ship. With the extra-large propeller on the small tanker the computations indicate a very large gain, about 21%. This number is likely to be too large, and the reasons for this are discussed in the thesis. All numbers given are from computations at model scale. A full scale computation for the 8000ton Tanker indicates a somewhat smaller gain for both propellers, but this is due to the fact that all propellers were designed for model scale. In the second phase of the project means to further improve the configuration are investigated. First, systematic changes of the skeg/boss are carried out and an elongated boss stretching back to the propeller is found to be best, with a power reduction of 1% compared to the original hull with a long shaft to the propeller. Thereafter a study is made of a bracket configuration, where one of the brackets is systematically turned to yield a pre-swirl of the inflow to the propeller. It is found that the power can be reduced compared to the case with brackets aligned with the flow, but the reduction is not large enough to compensate for the additional friction of the brackets. Validations are reported in both phases of the project. Comparing the original propeller/hull configuration with the optimised hull/extra- large area propeller in the second phase, the reduction in propulsive power is 15.0%. The reasons why this gain is smaller than in the first phase are discussed. The measured gain in tests at SSPA tests is 14.2%. There is thus good correspondence between the computed and measured results. Full scale computations indicated a somewhat smaller gain, about 12.5%, which should be compared with 13.5% for the SSPA full scale extrapolation.