Principles of naval engineering - Historic Naval Ships Association

Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERINGFigure 25-3. — Navy research hydroskimmer(SKMR-1).3.265Another approach to the problem of cavitationand the resulting reduction in thrust is the use ofpropeller nozzles and shrouds . Nozzles andshrouds reshape the flow of water to the propellerin such a way as to delay cavitation by increasingand "containing" the pressure aroundthe propeller. A similar reshaping of the waterflow may be obtained by the use of hydraulic jets .One "new" propulsion device is actually quiteold; although it has not recently had any majorapplication for ship propulsion, it has been usedon some torpedoes. This is the contra- rotatingpropeller , which consists of two screws turningin opposite directions. The advantage of the contra-rotating propeller design is that the afterpropeller is able to utilize some of the energyfrom the wake of the forward propeller, thusleading to higher efficiencies than are obtainablewith a single screw propeller. The contra-rotatingpropeller also offers possibleweight savings and efficiency improvements inthe propulsion machinery.One variation of the contra- rotating propellerinstallation is called a tandem propeller instal -lation .In this arrangement, one propeller is installednear 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 bladescan be varied in such a way as to provide completemaneuverability, as well as propulsion;hence this device is actually a combination of apropulsion device and a steering device. Althoughoriginally proposed for small submersible craft,it is possible that this type of installation mayhave application for larger ships in the future.A number of other devices have been suggestedwhich combine the functions of propulsionand steering. Among these are the steeringscrew, which consists of a propeller mounted ona vertical shaft. The shaft can be rotated through360° in order to propel the vessel in any desireddirection. Other devices which combine the functionsof propulsion and steering to some extentare screw propellers arranged as bowthrustersor as stern thrusters . In each case, the screwpropeller is mounted on a horizontal shaft. Thepropeller is located within a tube which runs athwartshipthrough the bow or the stern. The propellerscan be reversed to provide thrust ineither athwarthship direction. The tubes can bearranged to be closed off when not in use.Airscrew propulsion for ships has been underinvestigation for the past few years. The primaryadvantage of the airscrew is that it provides verygreat maneuverabiltiy, particularly at lowspeeds. In a Navy test of airscrew propulsion onthe liberty ship John L. Sullivan (YAG 37), it wasfound that maneuverability at low speeds wasbetter with the airscrews than with conventionalpropulsion and steering. When approaching piersor mooring buoys, the ship was able to maneuverwithout the assistance of a tug because the airscrewsprovided the capability for applying propulsiveforce in any direction.The disadvantage of airscrew propulsion forships is that enormous airscrews would be requiredto propel even a medium- sized ship at anygreat speed. For certaintypesof craft, however,it is possible that some combination of waterscrew and airscrew propulsion may be feasible.Although none of the combined devices thusfar developed have solved all propulsion andsteering problems, there is much to recommendthe combination approach. Propulsion and steeringare very closely related; a truly effectivepropulsion and maneuvering combination shouldresult in greater simplicity and greater efficiencythan is obtainable with two separate devices.Waterjet propulsion for ships was tried outmany years ago but abandoned becauseof its relativelylow efficiency. However, the Navy hasrecently been investigating the possibility of633