CHAPTER 73 



The Design of the Fixed Appendages 



73.1 General Rules for Design of Fixed Objects 



in a Stream 675 



73.2 The Design of Leading and Trailing Edges . 675 



73.3 The Stem Cutwater 676 



73.4 Selection of Struts or Bossings 677 



73.5 Strut Design for E.\-posed Rotating Shafts . 678 



73.6 Strut- Arm Section Shapes for Ultra-High 



Speeds 680 



73.7 Appendages for the Arch-Stern ABC Design 681 



73.8 Layout of Contra-Struts Abaft Propellers . 682 



73.9 The Design of Bossings Around Propeller 



Shafts 682 



73 . 10 Design Rules for Defiection-Type or Contra- 



Guide Bossings 686 



73.11 Vertical Bossings as Docking Keels .... 686 

 73 . 12 Design Notes on Fixed Screw-Propeller 



Shrouding; The Kort Nozzle 687 



73 . 13 Shaping and Positioning of Contra- Vanes 



Abaft Paddlewheels 688 



73.14 Design Features of Supporting Horns for 



Rudders; Partial Skegs 690 



73.15 



Selecting the Position, Type, and Number of 



the Roll-Resisting Keels 691 



Bilge-Keel Extent, Area, and Other Features 692 

 Structural Considerations in Bilge-Keel 



Design 694 



Design of Roll-Resisting Keels for the ABC 



Ship 695 



Design of Docking, Drift-Resisting, and 



Resting Keels 695 



The Design of Fixed Stabilizing Skegs or 



Fins 697 



Design of Torque-Compensating Fins . . . 699 



Fixed Guards and Fenders 699 



Design to Avoid Vibration of Appendages . 700 

 Design of Water Inlet and Discharge Open- 

 ings Through the Shell 701 



Partial Bibliography on Condenser Scoops . 703 

 Design and Installation of Galvanic-Action 



Protectors 704 



Design Notes for Locating Echo-Ranging 

 and Sound Gear on Merchant Vessels . . 705 



73.1 General Rules for Design of Fixed 

 Objects in a Stream. In view of the close rela- 

 tion which certain of the fixed appendages bear 

 to other parts of the hull, comments applying to 

 particular design features of these appendages are 

 to be found here and there among the chapters of 

 Part 4. Some duplication is considered justified 

 for the sake of assembling all the appendage- 

 design information in one place. 



Intelhgent positioning and shaping of the fixed 

 appendages can be executed only on the basis of 

 a careful prediction of water flow around the 

 ship hull, based upon detailed knowledge derived 

 from appropriate tests of models or full-scale 

 investigations on ships of similar form. This 

 requirement applies not only to the flow next to 

 the hull surface but at some distance from it, 

 depending upon the projection of each appendage 

 beyond the hull. It applies also to abovewater 

 appendages which may frequently be immersed 

 or submerged during wavegoing. 



Flow in the vicinity of a propulsion device 

 producing thrust is influenced to a certain extent 

 by the induced flow resulting from circulation 

 around the blades. The magnitude of this induced 

 flow varies with the thrust produced but the 

 change in flow direction depends upon the rela- 



tionship of the induced-flow and the ship-flow 

 vectors. When designing appendages that are to be 

 mounted near these propulsion devices the 

 resultant flow needs to be studied carefully. 



The types and numbers of fixed appendages 

 which can be applied to all kinds of ships are 

 almost legion. The discussion in this chapter is 

 therefore limited to the principal or important 

 types, in each of which the hydrodynamic charac- 

 teristics are essentially the same. It is hoped that 

 whatever the nature or shape of the unusual 

 appendage, it can be linked to one or more of the 

 foregoing types for purposes of design. Failing 

 this, its design can be worked out on the basis 

 of its function and a study of the probable flow 

 around it, with check tests in a circulating-water 

 channel. 



73.2 The Design of Leading and Trailing 

 Edges. The less the slope of the sides of the 

 entrance of a streamlined 2-diml body, or of the 

 nose of a 3-diml body, the less is the deflection 

 of the flow and the magnitude of the +Ap in 

 this region. However, there are other design 

 features to be considered: 



(a) The entrance slopes are made small only when 

 the direction of flow is exactly known, and when 



675 



