Sec. 77.2 



PRELIMINARY DESIGN OF A MOTORBOAT 



821 



that has been devoted to the design of larger 

 ships. Until this time comes, the hydrodynamic 

 and other aspects of semi-planing motorboat 

 design will not be adequately covered, in reference 

 books or elsewhere. 



In the design procedure for a full-planing craft 

 it is necessary to emphasize the large and numer- 

 ous differences between the design problems for a 

 displacement craft, set forth in Chaps. 6G, 67, 

 and 68, and those applying to a planing motor- 

 boat: 



(1) In the first place, the total resistance in 

 pounds per long ton of displacement, 7?,. /A or 

 Rr/W, is higher than for a displacement-type 

 craft, often by two orders of magnitude. Whereas 

 Rt/W for a tanker may be 4 or 5, for a liner 10 

 or 12, and for a destroyer or similar high-speed 

 craft up to 130 or 150, the Rt/W value for a 

 motorboat with a T, of 5 or 6 is 700 or 800, 

 indicated clearly by Fig. 56.M. The latter figure 

 represents a ratio of say 750 to 2,240, or roughly 

 one-third of the total weight. 



(2) In the second place, the buoyancy force 

 corresponds to practically the entire weight of a 

 displacement-type vessel, whereas for a planing 

 craft it may be not more than one-third or perhaps 

 only one-thirteenth of the total weight. The 

 remainder is dynamic lift. 



(3) The shape of the bottom of a displacement- 

 type craft usually affects only its pressure resist- 

 ance but may and often does have an effect upon 

 maneuvering and wavegoing. In a planing craft 

 the bottom shape affects its ability to plane at 

 all, its behavior when planing, its pitching and 

 porpoising characteristics, its wavegoing behavior 

 and its slamming loads, as well as its controll- 

 ability, stability of route, and heel when steering 

 and turning. 



(4) For a destroyer or similar high-speed vessel 

 running on the after side of its own bow wave the 

 slope drag may be as much as 0.01511'", correspond- 

 ing to say 30 or 35 lb per long ton. On a motorboat, 

 before planing is reached, it may be as high as 

 O.lOH^or more, corresponding to say 225 lb per ton. 



(5) The actual wetted surface of a planing craft 

 diminishes rapidly as the speed increases, until at 

 designed speed it may be only one-third or less 

 of the at-rest value 



(6) The still-air resistance of a large ship is 

 rarely a large percentage of the hydrodynamic 

 resistance but in an ultra-high speed motorboat 

 it may approach that resistance in value, and be 



accompanied by large vertical forces. Further- 

 more, the abovewater volume and exposed area 

 of a motorboat increase with the speed, as the 

 craft rises out of the water. 



The planing-boat design problem is not only 

 vastly different from that of the displacement- 

 type craft but is vastly more intricate, with 

 interrelationships and interactions of major effect. 

 In perhaps no other branch of water-craft design 

 is there so great a dependence of each design 

 feature upon all the others. A 10 per cent increase 

 in displacement of a vessel supported only by 

 buoyancy means that it sinks somewhat deeper 

 in the water and runs at slightly reduced speed 

 but it retains essentially the same underwater 

 form and gives about the same performance. A 

 10 per cent increase in weight of a planing craft 

 may prevent it from planing at all, and from 

 reaching more than half of its designed full speed. 

 A slight change in shape of the bottom surface 

 which supports the craft when planing may not 

 only prevent its planing but double its running 

 trim by the stern. Another slight change in 

 bottom shape, hardly large enough to be notice- 

 able, may lift it to planing position but may 

 render it actually dangerous to handle at high 

 speed. 



The planing-craft designer must accordingly 

 reconcile himself to learning, and understanding, 

 what amounts almost to an entirely new science 

 and art. Fortunately for him, dynamic lift and 

 other planing phenomena have also called for 

 intensive study and experimentation on the part 

 of aeronautical engineers and aerodynamicists 

 who have been designing seaplanes and flying 

 boats for the past half-century. Many more 

 minds and hands were concentrated on the 

 problem than would have been the case if the 

 naval architect had had to go it alone. 



The attainment of superior hydrodynamic per- 

 formance is perhaps of greater importance in small 

 pleasure craft than in small utility craft. A person 

 who builds or buys a boat and runs it for sheer 

 enjoyment expects to have fun and not trouble. 

 He is in a position, as when buying an automobile, 

 to pick and choose, and to be satisfied with nothing 

 but the best. 



For the same reasons, appearance may likewise 

 be more of a factor than it is on a larger vessel, 

 although as a rule not more important than per- 

 formance in the water. Even a racing yacht must 

 have pleasing fines. A skipper who is proud of 

 her racing record must also be proud of her 



