PROPELLER DESIGN FOR MINIMUM HULL EXCITATION

High-amplitude fluctuations in hull-surface pressures induced by unsteady propeller cavitation (with consequent hull vibration) are mainly the result of the nature of the wake field, but it must be accepted that the propeller loading distribution associated with maximum propulsive efficiency makes it difficult, if not impossible, to control cavitation under adverse flow conditions. Experience with unusual propeller-loading distributions associated with suppression of noise from warship propellers has led to a consideration of non-optimum, or arbitrary, loading distributions as a more general means of controlling pressure amplitudes, and a research programme has accordingly been carried out in which a design method for an arbitrarily-loaded wake-adapted propeller was developed. The Authors describe this design method, which, because of the uncertainties of such a propeller being successful, was put together from components of existing methods without undue expenditure of effort on the preparation of computer programs. The application of the method to the re-design of a propeller (for a single-screw container-ship) is also described, and an account is given of the associated tests, carried out on models of a 100% tip-unloaded propeller at the National Maritime Institute and consisting of open-water tests, self-propulsion tests with the model hull, and, in the cavitation tunnel, cavitation viewing and pressure measurements behind a partial-hull model. The results of the model tests, and comparisons with results from earlier experiments on models of the as-fitted propeller, are presented and discussed in some detail. It is concluded that, while the design process requires refinement, this research has clearly demonstrated the possibility of controlling hull surface pressure forces by varying the radial distribution of loading on the propeller. The advantages and disadvantages of tip unloading have been clearly defined, and the shipowner is presented with the choice between (i) maximum propulsive efficiency with the risk of severe vibration, and (ii) vibration-free running with reduced efficiency. Reports from the first full-scale application of this research, a retrofit on a large gas-carrier, are good.