Propeller -Hull Interaction 



scale effects on different kinds of appendages. The treatment of the latter in 

 different tanks differs quite materially, and leads to anomalous results in the 

 predictions of ship powers. 



Lastly, the chairman said it would be interesting to have from panel mem- 

 bers their views on what research should be pursued in the future to resolve 

 some of these problems and to improve the design of the optimum hull- propeller 

 combination. In this connection, he read an extract from a written contribution 

 from Professor E. V. Lewis, of the Webb Institute of Naval Architecture (who 

 was unable to be present at the Symposium), because it summarized the general 

 state of our knowledge and pointed to a definite objective. Professor Lewis 

 wrote: 



I would like to pose the following question for discussion at the panel 

 meeting: Is it possible to coordinate the design of hull and propulsive 

 device in such a way as to obtain a significant advantage in overall 

 propulsive efficiency over a good hull with an optimum propeller? 

 Many experts, including the late Professor Burrill, have thought other- 

 wise. Professor Horn long ago pointed out the fallacy of "wake gain," 

 and aircraft designers have generally striven to put propellers well 

 ahead of wing or fuselage. A little-noticed paper by Professor Troost 

 in 1957 tends to confirm the negative view by adopting the idea of a 

 "substitute propeller." As you know, this involves considering a pro- 

 peller completely clear of the hull as a standard of comparison. 



Professor Troost' s point is simply that what one gains in hull effi- 

 ciency, he generally loses in propeller efficiency. Perhaps it would 

 be worthwhile to make a broad survey of different types of ships and 

 the various relevant factors, such as ship speed, limitations on pro- 

 peller diameter and RPM, and thrust requirements, to see if there are 

 any circumstances under which one could expect to improve hull effi- 

 ciency more than the loss in propeller efficiency. 



Dr. J. P. Breslin (Stevens Institute of Technology) opened the panel meeting 

 with an account of theoretical work he had carried out to find the force on a 

 cylinder caused by both the loading and thickness effects of a propeller operating 

 in a wake, the propeller shaft being parallel to the axis of the cylinder. He 

 showed that the force can be simply obtained from the fields induced by the pro- 

 peller alone, being due to the sum of the pressures induced by all the loading 

 components and by the blade thickness, and he deduced expressions for the total 

 pressures and forces arising on the hull. From these it was concluded that the 

 dominant contribution to the hull force arises from the (m - l)th harmonic of the 

 wake, where m is the number of blades, and it may be expected that the vertical 

 hull force on a ship will be large when the (m - l)th wake harmonic is large. 



Increasing the tip clearance from 20 to 30% of the propeller diameter only 

 reduced the hull force due to the blade loading by 8%, and the larger the hull 

 relative to the propeller the less sensitive the force to clearance. The reduction 

 of the force due to blade thickness was more responsive to clearance. 



Means for achieving reductions in the hull force will be studied by further 

 detailed evaluations of theory. 



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