Propeller Design 



1. The seven-bladed screws are worse from an efficiency point of view 

 than the four-bladed screws. Even at equal diameter and a corresponding dif- 

 ference in rpm this is the case. 



2. The ducted propeller is better from an efficiency point of view than the 

 four-bladed screw propeller, although at decreasing rpm this improvement in 

 efficiency decreases and even disappears because of the smaller propeller load 

 due to the larger screw diameter. 



3. However, the most interesting point is a further decrease in rpm from 

 that which is now usual. If the rpm be decreased from 80 to 50, then the screw 

 diameter will increase in this case from 9 m to 12 m. The manufacturers of 

 large screw propellers consider a screw propeller with a diameter of 12 m 

 within their technological capabilities. 



The improvement in efficiency, consequently, reduction in required SHP, 

 amounts to more than 20 per cent. Such savings in SHP force us to consider 

 the consequences for reduction gears and propeller shafts at these extremely 

 low rpm values in order to approach an economical optimum. Attention should 

 also be paid to diesel engines with relatively high rpm combined with reduction 

 gears. In addition to these conclusions it is interesting to note that at the 

 N.S.M.B. recently developed asymmetric nozzles have delivered an average 

 reduction of SHP from 3 to 5% with respect to the results of conventional noz- 

 zles as indicated in Fig. 1. These asymmetric nozzles have been adapted both 

 to the wake distribution and the flow direction at the stern. This asymmetric 

 nozzle has the advantage that the conventional shape of afterbody can be main- 

 tained. The extra initial costs of such an asymmetric nozzle are more than 

 compensated for by the reduction in required SHP. 



Preliminary studies on the reduction of the resistance increase of large 

 tankers as a consequence of course keeping indicate that an improvement may 

 be expected by an enlargement of the nonrotatable rudder surface (deadwood). 

 This can even be done by a reduction of the rotatable rudder surface. Another 

 solution to this problem might be the application of nozzles outfitted with 

 maneuvering devices. 



In Fig. 2, results of identical calculations, as shown in Fig. 1, are indi- 

 cated. In this case the ship is a fast cargo liner with a speed of about 25 knots 

 and a propulsion plant of 30,000 hp. The calculations have been carried out for 

 four- and seven-bladed screw propellers and for contrarotating propellers. In 

 most cases it is usual that the propeller specialist has to design a ship propeller 

 for a required speed, a given rpm and SHP; the propulsion machinery has al- 

 ready been selected and the propeller has yet to be designed. For these design 

 conditions we see from Fig. 2 that contrarotating propellers deliver a 4 - 5% 

 higher efficiency than the comparable four-bladed screw propellers. By a more 

 favorable interaction between ship and propeller this increase in efficiency can 

 be enlarged by about 3% with respect to required SHP. It may be that the en- 

 gine power becomes so large that application of the conventional ship screw is 

 only reliable as a twin-screw arrangement. This leads to an increase of re- 

 quired SHP of about 8%. So in the future contrarotating propellers may lead to 

 reductions in SHP of 15 - 16% compared to the twin-screw arrangement of 



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