Propeller -Hull Interaction 



Professor J. D. van Manen (Netherlands Ship Model Basin) discussed the 

 effects of cavitation on propeller-hull interaction, a subject on which he said very 

 little is known. It is certain, however, that the change in the chord-wise pres- 

 sure distribution on the propeller blade with cavitation will have a marked effect 

 on the propeller-hull interaction factors as we determine them now in our con- 

 ventional towing tanks, on the surface forces, and on the bending moments on the 

 propeller shaft. Dr. van Manen said that one of the very few publications on the 

 subject is from Russia, and was reported at the International Towing Tank Con- 

 ference in London in 1963, which showed that in a towing tank with a ventilated 

 propeller the thrust deduction was reduced by 50%. That would mean for big 

 tankers a reduction in thrust deduction factor of 50% due to the effect of cavita- 

 tion and about 10% change in power. That would be a very important effect in 

 all our correlation thinking, and maybe if we have to test in the near future 

 500,000-ton tankers and have no experience to predict the power to install in the 

 ship to get the speed, it might mean a mistake of about 6,000 horsepower, which 

 is not too nice for the shipyard that has to build that ship. Dr. Todd asked Pro- 

 fessor van Manen if he meant that there could be a 50% change in thrust deduc- 

 tion factor on a large tanker ? Professor van Manen said that that was his esti- 

 mate, but it could be wrong. If the thrust deduction measured without cavitation 

 was 0.2, it would come down to perhaps 0.1. In addition to the effects of cavita- 

 tion on thrust deduction, on correlation, on power prediction, on blade- spindle 

 torque, and thus on the bending moments in the shafts, there is another aspect. 

 K a conventional propeller and a ducted propeller, or a contrarotating propeller, 

 are tested in a towing tank we may come to the conclusion that there is an im- 

 provement with the ducted propeller, but it might be that the effect of cavitation 

 in one propeller type is quite different from that in another propeller type. It 

 can be expected that the effect of cavitation on thrust deduction in a conventional 

 propeller is larger than in a ducted propeller. That means that the reduction in 

 the required shaft horsepower for the ducted propeller would be smaller, due to 

 the effect of cavitation, which we neglect in our present towing tanks. Professor 

 van Manen was of the opinion that there is a very great need for evacuated or 

 reduced-pressure towing tanks and said that the preliminary design for such a 

 facility at the NSMB was ready and the money problem informally already solved. 

 In the design of an evacuated towing tank it is necessary to start with the model 

 propeller, which cannot have a diameter less than 24 cm. This means that for 

 the very big tankers the models will be 12 m or more in length. With such 

 models a very big tank is needed, for instance, 175 m long, 18 m wide, and 8 m 

 in depth. This is the line being considered at NSMB at this moment. Professor 

 van Manen also referred to the small reaction to E. V. Lewis' remarks in his 

 contribution about optimum hull, optimum propeller, and now the optimum hull 

 and propeller combination, as read out by the chairman. In the last two years 

 NSMB had adopted the duct design to nonuniform flow. That is normally done 

 with the conventional propeller, too, but with the ducted propeller there is a 

 chance to adopt the duct to the flow direction. There is an upward flow at the 

 stern, and if the duct is designed in such a way that the flow into the propeller 

 is horizontal, that means a lot to the propeller efficiency, and for a ducted pro- 

 peller it is always favorable to change the stern shape to achieve such flow. 

 With such a flow-directed nozzle, giving horizontal flow at the stern, NSMB suc- 

 ceeded in the case of three tankers in getting the same shaft horsepower reduc- 

 tion as for the Hogner stern, some 8-10%. Another example of the use of these 

 ducted propellers might be with inclined shafts, where the control of the flow 



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