Ducted and Contrarotating Propellers on Merchant Ships 



2.2. Ducted Propellers ' ' 



Results of self-propulsion tests with ducted propellers on large merchant 

 ships have been presented by van Manen from NSMB, Wageningen (5), Minsaas 

 from SMT, Trondheim (6) and English from NPL, Feltham (7). 



At NSMB, tests have been carried out with three different tanker ship proj- 

 ects, 32,500 TDW, 48,500 TDW, and 90,000 TDW (5) For each project a con- 

 ventional stern arrangement fitted with conventional propellers was compared 

 with a Hogner-type stern arrangement fitted with ducted propellers. The results 

 indicated that, for all the cases, the ducted propeller alternative was superior 

 from the point of view of efficiency. The shaft power reduction was about 5%, 

 3%, and 6%, respectively, for the three projects in loaded conditions at a speed 

 corresponding to trial speed. 



In (8) van Manen has presented a diagram based on the results of self- 

 propulsion tests with about 15 tanker models and results of systematic open- 

 water tests with ducted propellers. This diagram indicates that, for 30 000 SHP 

 and 100 RPM, the power reduction due to the introduction of ducted propellers is 



4 - 7% for 50 000 TDW, 

 7 - 9% for 100 000 TDW, 

 9 - 12% for 150 000 TDW. 



At Skipsmodelltanken in Trondheim, an extensive program of tests with 

 ducted propellers on large tankers is under way. Some preliminary tests (6) 

 with a 100 000 TDW tanker resulted in a power reduction of about 12% at 110 

 RPM, compared with a conventional propeller with the same number of revs. 



Some tests with a 150,000 TDW tanker, fitted with a ducted propeller, car- 

 ried out at NPL, have been presented in (7). Unfortunately, the report does not 

 include any comparison with a ship with conventional propulsion. It is only 

 stated that "the results show that a very high propulsive performance (QPC) has 

 been achieved. This is similar to the findings of van Manen. ..." 



Common to all the reports mentioned above is that no attempt has been 

 made to analyse the results nad present components of the propulsive factors. 



3. CONTRAROTATING PROPELLERS, THEORETICAL BACKGROUND 

 AND EXPERIMENTAL VERIFICATION 



3.1. Design Method 



The results with contrarotating propellers, included in the present report, 

 have all been obtained with propellers designed according to the same method. 

 This method is essentially the same as that of (9) and (10), i.e., a development 

 of Lerbs' method. Below are listed in the main differences between the method 

 used in the present investigation and the original method of Lerbs (9) and the 

 refinements according to Morgan, outlined in (10). 



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