Snc. 77.18 



PRELIMINARY DESIGN OF A MO TORBOAT 



839 



other hydrodynamic reasons, the following general 

 design rules are given, based on a 10-station 

 waterline length and the craft at rest at the 

 designed load and trim: 



(1) The chine height at Sta.O, above the designed 

 waterline at rest, should be at least O.OQLwl ■ If 

 the design is for a small pleasure boat the chine 

 can be higher, up to 0.13L,tl above the designed 

 waterline, because the pay load is small and the 

 lost internal volume is relatively unimportant. 

 If it so happens that pay-load capacity is import- 

 ant then a low chine height may be necessary to 

 get additional internal volume, at a sacrifice of 

 rough-water performance. 



(2) Large boats can operate satisfactorily with 

 less chine height forward than can small boats, 

 because the small, steep waves usually encoun- 

 tered have less influence on the rough-water 

 performance of the larger craft 



(3) The chine should He above the at-rest water- 

 line from Sta. at the FP to a point somewhere 

 between Sta. 3 and Sta. 5 



(4) The chine line should be straight from Sta. 6 



or 7 to the stern. There should be no concave 

 curvature in this region. 



(5) The chine Hne from Sta. 6 or 7 to the stern 

 should be parallel to or have only a slight slope 

 upward and forward with respect to the at-rest 

 waterline. Large slopes are to be avoided. 



(6) The chine depth at Sta. 10 should be about 

 O.OlOLfTL to0.030Z/,pi below the designed water- 

 line 



(7) If the boat is intended for smooth-water 

 operation only, a lower chine height forward can 

 be used. Indeed, if smooth-water speed is import- 

 ant, a lower chine height is desirable in that region. 



(8) Spray strips of the general form shown in 

 Fig. 30. A should be fitted along the chine, espe- 

 cially at the forward sections and preferably along 

 the entire length. This matter is discussed more 

 fully in Sec. 77.20. 



77.18 Buttock Shapes; The Mean Buttock. 



The buttock shapes in the bottom of a planing 

 craft are so important that some repetition is 

 justified in emphasizing them. With the straight 

 floor segments customary in V-bottom planing 



Fig. 77.1 Dimensionless Chine-Elevation Diagrams for Six Types and Sizes of Planing Craft 



