810 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.19 



boats the fore-and-aft shapes of the buttocks are 

 largely determined by the shape of the chine, 

 when projected on the centerplane. This is true 

 particularly in the afterbody. 



The shape and position of the buttock at about 

 one-ciuarter beam on each side of the centerhne, 

 often called the mean buttock, is used by some as 

 one of the characteristic parameters of a planing 

 craft [Clement, E. P., "Hull Form of Stepless 

 Planing Craft," SNAME, Ches. Sect., 12 Jan 

 1955, pp. 2-3]. 



Straight buttocks in the wetted region, where 

 the dynamic pressures are generated, reduce or 

 avoid negative differential pressures under the 

 afterbody. These in turn act to prevent bottom 

 suction and excessive trim by the stern. 



The effect of concavity in the buttocks under the 

 afterbody, reckoned with reference to the water 

 underneath, is described in the section following. 



Convex buttock lines under the afterbody act 

 to develop — Ap's, but the convexity is sometimes 

 unavoidable. Successful planing boats have been 

 and can be designed with slightly convex buttock 

 lines in the run, but they require careful attention 

 to other features of the design, such as the center- 

 of-gravity location, because of the negative lift 

 generated under the bottom. 



It is mentioned previously in item (4) of Sec. 

 77.17, it is to be discussed in item (1) of Sec. 

 77.19, and the designer is cautioned here that 

 slight downward hooks in the buttocks near the 

 stern are to be used vnih caution. They often 

 produce erratic and sometimes even dangerous 

 performance. 



77.19 Trim Angle and Center-of-Gravity Posi- 

 tion; Use of Trim-Control Devices. Theoretical 

 and practical considerations relating to the 

 running trim of a planing craft are rather thor- 

 oughly discussed by A. B. Murray [SNAME, 

 1950, pp. 658-692], It is customary, in analytic 

 and design work, to express the trun in degrees; 

 this is the form employed in the present chapter. 

 It would be preferable, if the full-scale data for 

 it could be readily derived, to express the trim 

 as a linear distance over the boat length, as is 

 done for large vessels. Measuring the sinkage or 

 rise at the bow and stern would give the naval 

 architect a direct measure of the amount by 

 which the center of gravity shifts its vertical 

 position. 



For most planing boats, the ideal planing angle 

 for least smooth-water resistance lies between 4 

 to 6 deg by the stern. This much trim often 



results in discomfort to passengers and an un- 

 attractive appearance when viewed from outside 

 the boat. It may actually interfere with the 

 steersman's vision from the control station. 

 Moreover, the possibility of porpoising is greater 

 with the larger trim angles. As a practical com- 

 promise the running-trim angle of a planing craft 

 is usually kept below 2 or 3 deg; the greater 

 ■ resistance and lower speed inherent in this lesser 

 trim are accepted. Limiting the trim to these 

 small angles requires that the center-of-gravity 

 position be farther forward than would otherwise 

 be the case. This in turn acts to prevent porpois- 

 ing. 



There are many advantages in level running 

 aside from those just mentioned, when the force 

 and moment to achieve the smaller trim angle 

 are applied by a trim-control device external to 

 the planing under surface of the hull proper. The 

 slope drag — called by some the induced drag, and 

 illustrated in Fig. 53. A — is diminished, the load- 

 carrying ability of the boat is greatly increased, 

 and it usually behaves better in waves. Trim- 

 control devices to accomplish this are discussed in 

 Sees. 30.11 and 37.24 but the principal kinds are 

 hsted here for the convenience of the designer: 



(1) A wedge or "shingle" applied to the under 

 side of the bottom at its extreme after end, or a 

 controllable flap hinged to the bottom in such 

 manner that it forms a downward "hook" of 

 variable angle at the extreme after ends of the 

 buttocks terminating on it. The thickness of 

 such a wedge, and the angle it makes with the 

 bottom on the full-scale boat, still require to be 

 determined by cut-and-try methods. The wedge 

 must be applied with caution because too much 

 wedge action and vertical lift at the extreme 

 stern are liable to have a disastrous effect upon 

 the steering, especially in a following sea. 



(2) The controllable flap, with its variable 

 "hook" angle, probably requires for best per- 

 formance something better than a shnple me- 

 chanical device which holds it rigidly in any 

 selected position. A yielding device, loaded with 

 a gas or with liquid, similar to the Plum stabilizer 

 mentioned in (3) following, should improve the 

 performance of both the boat and the trim- 

 control device when in waves. 



(3) The Plum stabilizer, whose action is explained 

 briefly in Sec. 37.24. This was developed and 

 described many j'^ears ago [Motor Boating, Mar 

 1928, pp. 16-17, 54, 134] and has benefited by a 



