245MI - Architettura Navale - A.A. 2020/21 Propulsion - Moodle@Units

245MI - Architettura Navale
              A.A. 2020/21
                 Propulsion

  Docente:   Prof. Mitja Morgut
             Università degli Studi di Trieste
             Dipartimento di Ingegneria e Architettura
             Via A. Valerio 10, 34127 Trieste
             Edificio C5 - Stanza C5_2.44
             mail: mmorgut@units.it

Introduction                                                                   waterjet

Classification

        Marine propeller is the most common propulsion device.
        Marine propeller consists of a hub and 2-7 radially oriented blades.
        Marine propellers can be classified in:
               Fixed pitch propellers
               Controllable pitch propellers
               Ducted propellers
               Azimuthing propellers

                                        Architettura Navale, A.A. 2020/21

Introduction                                                                 waterjet

Fixed pitch propeller (FPP)

               blades and hub are rigidly
               connected
               high reliability
               comparatively low price

                                         Architettura Navale, A.A. 2020/21

Introduction                                                                    waterjet

Controllable pitch propeller (CPP)
           Ï blades are not rigidly connected to the hub (blades can rotate)
           Ï using CPP ship speed can be varied by changing blade orientation
           Ï control mechanism is more complex than for FPP
           Ï CPP also requires a larger hub, reducing the efficiency

                                      Architettura Navale, A.A. 2020/21

Introduction                                                                      waterjet

Ducted propeller
           Ï for applications where high thrust is needed at low speed of advance,
               efficiency can be improved by placing the propeller in a duct
           Ï the duct can be fixed, but in general the orientation of the ducted
               propellers con be changed
           Ï azimuthing ducting propellers are commonly employed by active
               dynamic positioning systems

                                     Architettura Navale, A.A. 2020/21

Introduction                                                             waterjet

Podded propulsion system (POD)

           Ï popular in diesel-electric propulsion
           Ï an electric motor is placed in the unit

                                     Architettura Navale, A.A. 2020/21

Introduction                                                        waterjet

Waterjet

The Waterjet has three main components:
   Ï an inlet duct
   Ï a pump
   Ï an outlet or nozzle

                                Architettura Navale, A.A. 2020/21

Introduction                                                                                         waterjet

Waterjet

        Waterjet:
         Ï propulsion system with integral steering capabilities
               Ï stopping and retarding force capabilities are normally achieved with the aid of a
                 reversing bucket

                                             Architettura Navale, A.A. 2020/21

Introduction                                                                                    waterjet

Hull - Waterjet

        In a waterjet propulsion system the hull and waterjet mutuallly interfere with each
        other:
          Ï The naked hull resistance is modified due to a distorsion of the flow over the
             ship’s afterbody which at high speeds may also introduce a change in trim, thus
             influencing the resistance characteristics further.
               Ï The waterjet performance is altered by the distorsions in the hull flow which
                 differs from the normal free stream assumptions of the waterjet theory

        To assist in addressing this problem numerical methods, in particuar CFD
        (Computational Fluid Dynamics), can be used to evaluate the flow characteristics in
        waterjet systems, at least in qualitative manner

                                            Architettura Navale, A.A. 2020/21

Introduction                                                                                waterjet

Impeller and cavitation

           Ï The blade section design requires considerable attention
           Ï Pump impellers work at high rotational speeds
               Thus, the overall design is based on acceptable cavitation and the control
               of its harmful effects (lack of attention to detail con completelly vanish
               this overall concept)

                                        Architettura Navale, A.A. 2020/21

Introduction                                                                                                        waterjet

Tunnel, inlet
               Ï The inlet to the tunnel si frequently fitted with inlet guard to prevent the ingress
                 of large objects
                 The smaller the mesh of the guard the better is the protection. (However, the
                 design of the guard must strike a balance between undue efficiency losses )
               Ï The profile of the tunnel needs to be designed in a carefully in order to provide a
                 smooth uptake of water over the range of vessel operating trims, and
                 consequently avoid any significant separation of the flow or cavitation at the
                 tunnel intake

        NOTE: The form and the character of the wake field immediately ahead of the
        impeller is generally unknown

       Ï Some model tests have been
               performed in the past
       Ï The CFD simulation con be
               employed

                                                             Example of wake survey just upstream of the impeller

                                             Architettura Navale, A.A. 2020/21

Introduction                                                                          waterjet

Boeing - Jetfoil 929

                                        Ï The prototype Jetfoil 929-100 was built in
                                            1974
                                        Ï service speed: 42 knots
                                            maximum speed: 50 knots
                                        Ï foil configuration: canard
                                            - forward single inverted T foil
                                            - three struts, full-span foil aft
                                            - all foils with trailing-edge flaps
                                            Foils and struts retract hydraulically above
                                            waterline
                                            The forward foil assemby can rotate, 7
                                            deg in either direction.
                                        Ï Propulsion system:
                                            - two gas turbines
                                            - water-jet propulsion with axial pumps

                       Architettura Navale, A.A. 2020/21