Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERING 



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Figure 25-3. — Navy research hydro- 

 skimmer (SKMR-1). 



Another approach to the problem of cavitation 

 and the resulting reduction in thrust is the use of 

 propeller nozzles and shrouds . Nozzles and 

 shrouds reshape the flow of water to the propel- 

 ler in such a way as to delay cavitation by in- 

 creasing and "containing" the pressure around 

 the propeller. A similar reshaping of the water 

 flow may be obtained by the use of hydraulic jets . 



One "new" propulsion device is actually quite 

 old; although it has not recently had any major 

 application for ship propulsion, it has been used 

 on some torpedoes. This is the contra- rotating 

 propeller , which consists of two screws turning 

 in opposite directions. The advantage of the con- 

 tra- rotating propeller design is that the after 

 propeller is able to utilize some of the energy 

 from the wake of the forward propeller, thus 

 leading to higher efficiencies than are obtain- 

 able with a single screw propeller. The con- 

 tra-rotating propeller also offers possible 

 weight savings and efficiency improvements in 

 the propulsion machinery. 



One variation of the contra- rotating propeller 

 installation is called a tandem propeller instal - 

 lation . In this arrangement, one propeller is in- 

 stalled near the bow and the other near the stern. 

 The propeller blades are mounted on a blade ring, 

 with the blades projecting out through the hull. 

 The pitch of the contra-rotating propeller blades 



can be varied in such a way as to provide com- 

 plete maneuverability, as well as propulsion; 

 hence this device is actually a combination of a 

 propulsion device and a steering device. Although 

 originally proposed for small submersible craft, 

 it is possible that this type of installation may 

 have application for larger ships in the future. 



A number of other devices have been sug- 

 gested which combine the functions of propulsion 

 and steering. Among these are the steering 

 screw, which consists of a propeller mounted on 

 a vertical shaft. The shaft can be rotated through 

 360° in order to propel the vessel in any desired 

 direction. Other devices which combine the func- 

 tions of propulsion and steering to some extent 

 are screw propellers arranged as bowthrusters 

 or as stern thrusters . In each case, the screw 

 propeller is mounted on a horizontal shaft. The 

 propeller is located within a tube which runs ath- 

 wartship through the bow or the stern. The pro- 

 pellers can be reversed to provide thrust in 

 either athwarthship direction. The tubes can be 

 arranged to be closed off when not in use. 



Airscrew propulsion for ships has been under 

 investigation for the past few years. The primary 

 advantage of the airscrew is that it provides very 

 great maneuverabiltiy, particularly at low 

 speeds. In a Navy test of airscrew propulsion on 

 the liberty ship John L. Sullivan (YAG 37), it was 

 found that maneuverability at low speeds was 

 better with the airscrews than with conventional 

 propulsion and steering. When approaching piers 

 or mooring buoys, the ship was able to maneuver 

 without the assistance of a tug because the air- 

 screws provided the capability for applying pro- 

 pulsive force in any direction. 



The disadvantage of airscrew propulsion for 

 ships is that enormous airscrews would be re- 

 quired to propel even a medium- sized ship at any 

 great speed. For certaintypesof craft, however, 

 it is possible that some combination of water 

 screw and airscrew propulsion may be feasible. 



Although none of the combined devices thus 

 far developed have solved all propulsion and 

 steering problems, there is much to recommend 

 the combination approach. Propulsion and steer- 

 ing are very closely related; a truly effective 

 propulsion and maneuvering combination should 

 result in greater simplicity and greater effi- 

 ciency than is obtainable with two separate de- 

 vices. 



Waterjet propulsion for ships was tried out 

 many years ago but abandoned becauseof its re- 

 latively low efficiency. However, the Navy has 

 recently been investigating the possibility of 



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