314 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 57.3 



TABLE 57.a — Classification and Subdivision of 

 THE Resistance op a Ship to Steady, Straight- 

 Ahead Motion 



I. Pressure Drag or Resistance, due to Normal Pressure 

 on the Ship 



(a) Deflection drag and closing thrust for the hull proper 



(b) Deflection drag and closing thrust for the hull 

 appendages 



(c) Separation or eddying drag Rg and cavitation drag, 

 for the hull proper and for the appendages 



(d) Wavemaking drag Rw , for the hull proper and for 

 such appendages as may be near enough to the surface to 

 generate waves 



(e) Drag due to the generation of spray roots and spray. 



II. Friction or Tangential Resistance Rp on the Wetted 

 Area. This is considered to be either: 



(a) Tanvis resistance, varying as Z7 or F to the first 

 power 



(b) Tanqua resistance, varying as U^ or V^ 



(c) Some unknown combination of (a) and (b), varying 

 as an unknown (and probably varying) power of f7 or F 

 between 1 and 2. 



A different classification could be used here, embodying a 

 subdivision into: 



(c) Friction resistance on such hydrodynamically 

 smooth surfaces, flat or curved, as may be incorporated in 

 the ship 



(d) Friction resistance due to roughness superposed on 

 the hydrodynamically smooth surfaces, flat or curved. 



III. Interactions between I. and II., as follows: 



(a) Interaction effect of viscous or friction flow on the 

 pressure resistance due to wavemaking, and the reverse, 

 symbolized by Rwf 



(b) Interaction effect of separation or eddying on the 

 pressure resistance due to wavemaking, or the reverse, 

 symbolized by Rws 



(c) Interaction effect of viscous or friction flow on the 

 pressure resistance due to separation or eddying, or the 

 reverse, symbolized by Rsf • 



IV. Air and Wind Drag and Resistance, embodying: 



(a) Still-air resistance Rsa > due to ship motion alone, 

 with a true or natural wind of zero 



(b) Wind drag D,f, , exerted always downwind from the 

 relative-wind direction. This drag has both transverse 

 and axial components, due to aerodynamic lift and drag. 



(c) Wind resistance iJwind > composed of the net axial 

 force imposed by the wind, acting opposite to the direction 

 of ahead motion. The definitions of and distinctions 

 between these terms are explained in Sees. 26.15 and 54.1 

 and illustrated in Figs. 26.G, 26.H, and 26.1. 



V. Gravity Forces Acting on the Ship: 



(a) Slope drag Ds , due to the inclination of the buoy- 

 ancy-force vector to the weight-force vector, with a force 

 component opposing motion 



(b) Slope thrust Ts , similar to (a) preceding but with 

 a force component assisting motion. 



0,20 0.25 0.30 0J5 O.40| 0.45 



OA 0.6 0.6 1,0 1.2 1.1 I. 



I.S 20 



Fig. 57. a Typical Percentages op Friction and 

 Residuart Resistances for a Range op Speed- 

 Length Quotients 

 The significance of the two graphs is explained in the 



text 



The circled spots are values for the ABC transom- 

 stern ship designed in Part 4, for the sustained 

 sea speed and trial speed, respectively. 



Both curves are loci of division points for de- 

 signed speeds at the various T^ and F„ values 

 given. It is to be noted that the ratios of Rk to 

 Rf vary rather widely over the speed-length range 

 indicated. If ships are overdriven the percentage 

 oi Rii may be up to twice as great as that shown 

 in the figure. If underdriven, it may be only 

 two-thirds as large. 



0.4 0.6 08 1.0 1.2 1.4 1.6 1.6 2.0 



Fig. 57.B Percentages of Friction and Residuary 

 Resistances for Three Specific Cases 



Fig. 57. B indicates, for an EMB research 

 model, for the transom-stern ABC ship design, 

 and for destroyers as a class, the variation of 

 friction and residuary resistances throughout the 

 intermediate and upper speed ranges of those 

 ships. EMB model 2861 is the subject of the 

 first model pressure-distribution tests made by 

 E. F. Eggert [SNAME, 1935, pp. 139-150]. As 

 expected, in the low-speed ranges before appre- 

 ciable wavemaking begins, the total resistance in 



