CHAPTER 77 



The Preliminary Hydrodynamic Design of a 



Motorboat 



77.1 Scope of This Chapter 819 77.23 



77.2 General Considerations Relating to Motor- 77.24 



boat Design 820 



77.3 Special Design Features for Small-Craft Hulls 822 77.25 



77 . 4 Design Notes for Displacement-Type Motor- 



boats 823 77.26 



77 . 5 Semi-Planing and Planing Small Craft . . 823 



77.6 Operating Requirements for Planing Forms 824 77.27 



77.7 General Notes on the Powering of Small Craft 824 



77.8 Principal Requirements for a Preliminary 77.28 



Design Study 825 



77 . 9 Analysis of the Principal Requirements . . 826 77 . 29 

 77 . 10 Tentative Selection of the Type and Propor- 77 . 30 



tions of the Hull 827 



77.11 First Space Layout of the 24-Knot Planing 77.31 



Hull 827 



77.12 First Weight Estimate; Weight-Estimating 77.32 



Procedure 828 



77.13 Second Weight Estimate 831 77.33 



77.14 First Approximation to Shaft and Brake 



Power 832 77.34 



77.15 Selecting the Hull Features; Section Shapes . 835 77.35 



77.16 Rise-of-Floor Magnitude and Variation . . 836 77.36 



77.17 Chine Shape, Proportions, and Dimensions . 837 77.37 



77.18 Buttock Shapes; The Mean Buttock ... 839 77.38 

 77 . 19 Trim Angle and Center-of-Gravity Position ; 77 . 39 



Use of Trim-Control Devices 840 77.40 



77.20 Spray Strips 841 



77.21 Stem Shape 842 77.41 



77.22 Deep Keel and Skeg; Other Appendages . . 842 



Interdependence of Hull-Design Features . 843 

 Layout of the Lines for the ABC Planing- 



Type Tender 843 



Design Check on a Basis of Chine Dimen- 

 sions 846 



Second Estimate of Shaft Power, Based Upon 



Effective Power 847 



Running Attitude and Fore-and-Aft Position 



of the Heavy Weights 850 



First Space Layout of the 18-Knot Round- 

 Bottom Hull 852 



First Weight Estimate for the 18-Knot Hull . 853 

 First Power Estimate for the 18-Knot and 



14-Knot Conditions 853 



Selecting the 18-Knot Hull Shape and Char- 

 acteristics 854 



Layout of the Lines for the ABC Round- 

 Bottom Tender 855 



Example of a Modern Round-Bottom Utility- 

 Boat Design 858 



Design for a Motorboat of Limited Draft . 858 

 Estimate of Screw-Propeller Characteristics . 859 

 Propeller Tip Clearances; Hull Vibration . 859 

 Still- Air Drag and Wind Resistance .... 862 

 Design of Control Surfaces and Appendages . 862 



Third Weight Estimate 863 



Self-Propelled Tests for Models witii Dynam- 

 ic Lift 864 



Partial Bibliography on Motorboats . . . 865 



77.1 Scope of This Chapter. A motorboat is 

 defined in this chapter as a mechanically pro- 

 pelled craft having a length of about 110 ft 

 (33.5 meters) or less, running at a Taylor quotient 

 r, greater than 1.0, F„ > 0.3, and of a general 

 type which may or may not include the fishing 

 vessels and yachts of Sees. 76.11 and 76.19. The 

 treatment here is limited to craft driven by screw 

 propellers. The motorboats may be supported 

 wholly or in part by water buoyancy or by 

 dynamic lift on the hull proper. In other words, 

 they may be of the displacement, the semi- 

 planing, or the fuU-planing type. Hydrofoil- 

 supported craft are in a separate category, 

 discussed in Chap. 31 and partially covered by 

 the limited bibhography of Sec. 53.9. 



As introductory material for notes on the 



design of a planing hull. Chap. 13 discusses basic 

 planing phenomena and Chap. 30 the behavior 

 of actual planing craft. Some quantitative data 

 on dynamic lift and planing and a rather lengthy 

 set of references are given in Chap. 53. Notes for 

 guidance in determining whether a craft designed 

 to meet a given set of requirements is to be of 

 the displacement type, or whether it is to be a 

 semi-planing or a full-planing form, are found in 

 Sec. 77.10 of the present chapter. 



One aim of this chapter is to collect and cor- 

 relate certain small-craft performance and design 

 information applicable to motorboats in general, 

 supplementing the large-ship data set down in 

 earlier chapters. Sec. 77.41 contains a partial 

 bibhography relating to smaU-craft, yacht, and 

 motorboat design. It is to be hoped that, in the 



819 



