Panel Discussion 



5. good behaviour in a seaway; :. .e. . 



6. favorable interaction with the rudder, to improve maneuverability. 



In the selection of a propeller type all these hydrodynamic aspects of a 

 ship propeller play an important role. Besides the dependability, minimum 

 vulnerability and low initial and maintenance costs has to be taken into 

 consideration. i . :<,. 



The conventional ship screw with fixed blades designed for noncavitating 

 condition has been for a period longer than 100 years the most applied type 

 of ship propeller. 



The ducted propeller (screw and nozzle or pumpjet) has shown its ad- 

 vantages for ship types where the propeller load is high or the cavitation 

 danger is serious. Tugs, trawlers, and minesweepers are frequently outfitted 

 with a ducted propeller. Coastal vessels and frigates are other ship types 

 where the ducted propeller shows its favorable characteristics. Results of 

 model tests show that application of ducted propellers to large tankers will be 

 realized in the very-near future. The demand for a cigar- shaped afterbody, 

 if a nozzle were applied to a tanker, is eliminated by the recent introduction 

 of nozzle shapes adapted to both wake distribution and flow direction at the 

 stern. At this moment it can be stated that sufficient theoretical and system- 

 atic experimental data are at hand for any design of a ducted propeller. 



Contrarotating propellers form a type of propulsion that might be a seri- 

 ous competitor of the conventional ship screw on large container ships with 

 such high speeds that the required power cannot be developed by one screw. 

 Gradually, more design information becomes available for this propeller type. 

 The selection of the blade number of fore and aft screws is of particular im- 

 portance for the control of the propeller -induced vibratory forces and also fcv 

 the transmission solution between propulsion plant and propeller. 



The number of controllable-pitch propeller applications has increased 

 very rapidly during the last years. Improvement of the dependability and mini- 

 mum vulnerability and a reduction of the initial and maintenance costs are not 

 the only reasons for this growth in controllable -pitch propeller applications. 

 The solution of mechanical and technological difficulties and the developmert 

 of suitable control systems have pushed the controllable-pitch propeller for- 

 ward to application to frigates and even now to tankers. Shaft horsepowers up 

 to 30,000 have successfully been absorbed by controllable -pitch propellers. 

 The supreme qualities of controllable-pitch propellers with respect to stopping 

 (for tankers) and accelerating (for frigates) promise a continuing growth in the 

 application of this propeller type. The reduction and control of the blade spindle 

 torque by advanced techniques such as ventilation or jet flaps might be a promis- 

 ing field for investigation. 



The paddle wheel, before 1850 the most-applied ship propeller, is still one 

 of these special-purpose propellers, which deserves our special attention for 

 transport in very shallow waterways. In that case, it happens that the screw di- 

 ameters which can be realized are seriously restricted, and the efficiency of 



1573 



