CHAPTER 69 



The General Design of the Propulsion Devices 



69.1 Introductory Comment 567 



69 . 2 Type and Number of Propulsion Devices . 567 



69 . 3 Positions and Limiting Dimensions .... 568 



69.4 Effect of Type and Design of Propelling 



Machinery 570 



69 . 5 Number and Position of the Engines . . . 570 



69 . 6 Use of Systematic Wake Variations .... 572 



69.7 Rate and Direction of Rotation of Propul- 



sion Devices 572 



69.8 Design to Equalize or to Apportion the 



Powers of Multiple Propellers 573 



69 . 9 Powering Allowances 574 



69.10 Graphic Representation of Powering Allow- 



ances and Reserves 576 



69. H Selection of Feathering, Adjustable, Re- 

 versible, or Controllable Features .... 578 



69. 12 Propulsion Devices to be Used with Contra- 

 Vanes, Contra-Guide Sterns, and Contra- 

 Rudders 579 



69 . 13 Disadvantages of Unbalanced Propulsion- 

 Device Torque 579 



69 . 14 Propulsion-Device Design to Meet Maneu- 

 vering Requirements 580 



69.15 Relation of Propulsion-Device and Hull- 

 Vibration Frequencies 580 



69.1 Introductory Comment. There are con- 

 sidered in this chapter only those features which 

 are common to the design of all mechanical ship- 

 propulsion devices which act on the water. 

 Discussions of specific devices such as screw 

 propellers, paddlewheels, rotating-blade propel- 

 lers, and the like, are found in the two chapters 

 following. The systematic treatment of these 

 design features does not necessarily follow the 

 order in which the characteristics for any one 

 type of propulsion device are worked out or 

 selected in actual practice. 



69.2 Type and Number of Propulsion Devices. 

 Only rarely can lines be sketched for the form of a 

 new ship without first making a tentative decision 

 as to the type, number, and position of the pro- 

 pulsion devices. Too often these features are 

 determined by the types, sizes, and powers of the 

 propelhng units available; admittedly this pro- 

 cedure is inescapable at times. It is proper to 

 remember, however, that the propulsion devices 

 are installed to drive the ship. The function of 

 the propelling machinery is to drive these devices 

 at rates and powers which result in the highest 

 efficiency and the greatest economy for both. 

 Generally speaking, therefore, the type, number, 

 and position of the propulsion devices are deter- 

 mined directly from the ship requirements. The 

 corresponding features of the machinery are 

 selected to suit the devices. 



So many considerations, most of them con- 

 flicting, enter into the selection of the propulsion- 

 device characteristics that it is difficult to set 



down the procedure in systematic form. The 

 outline given here may serve until something 

 better is developed, remembering first, last, 

 and always that no propulsion device, whatever 

 its type and position, can be expected to perform 

 well in a region of poor flow. 



The type is selected first, on the basis of: 



(1) Operating requirements, including limiting 

 draft of vessel and available depth of water 



(2) Maximum ship speed and total power re- 

 quired to be absorbed by the device (s) 



(3) Best general position in the vessel, considering 

 the latter's type, functions, and duty 



(4) Amount of wavegoing anticipated, with 

 consequent change of position of the instantaneous 

 water surface with reference to the region in 

 which thrust is being produced, such as the disc 

 of a screw propeller 



(5) Type of propelling plant available or desired. 

 This boils down generally to the rate of rotation 

 of the output shaft on the last machinery unit, 

 just ahead of the propulsion device. In the past, 

 slow-speed engines drove screw propellers through 

 multiplying gears, and in the present, high-speed 

 engines drive paddlewheels through reduction 

 gears. Therefore there need be few Umitations in 

 this respect. 



(6) Facihties available for repair or replacement 

 of the propulsion devices and their parts in the 

 areas where the ship is to operate, either by the 

 ship's force or by repair crews 



(7) Frequency with which the propulsion devices 



567 



