576 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.10 



then of the order of 20 to 30 per cent [ME, 1942, 

 Vol. I, p. 28; Troost, L., SNAME, 1953, p. 576]. 



It is to be noted that item (6) of the tabula- 

 tion above does not include a sort of average 

 allowance for appendages, as is customary in 

 some quarters. This is taken care of by the model- 

 testing establishment, on the basis of the kind, 

 number, shape, size, and location of the append- 

 ages, relative to the hull and to each other. 



When estimating and applying the power 

 percentages to the predictions derived from 

 model tests it is most important to insure that 

 an allowance corresponding to one or more of the 

 foregoing factors has not already been worked 

 into the model-basin predictions. In America 

 it is customary to omit all the allowances listed 

 except the S(ACi?) for plating, structural, and 

 coating roughnesses to be expected on a clean, 

 new vessel under trial conditions; see Sec. 45.18. 



It is generally necessary, at some stage in the 

 formulation of requirements or in the preliminary 

 design, to know the speed-power relationships 

 when some or all of these increases in resistance 

 and power are in effect. For example, when the 

 bottom is dirty the wake fraction becomes 

 greater and the thrust loading of the propeller is 

 increased. It must also be decided at what 

 power the propelling plant is to operate at maxi- 

 mum efficiency. At a later stage the detail pro- 

 peller design calls for an estimate of the propulsion 

 factors at what might be termed the propeller- 

 design point, to be explained presently. 



Model-testing techniques [C and R Bull. 7, 

 1933, p. 32] permit running a self-propelled 

 model under conditions in which the model 

 propellers develop thrust under or over that 

 necessary to push the model through the water. 

 The auxiliary towing or retarding force is adjusted 

 to provide the equivalent of underwater body 

 resistance, additional drag due to roughness of 

 the hull surface, and any overload that may be 

 expected on the ship due to fouling, adverse 

 weather, and the like. This procedure admittedly 

 does not change the velocity profile, the thickness, 

 and other features of the boundary layer cor- 

 responding to the effects of the roughnesses which 

 produce the additional drag but it does increase 

 the model propeller thrust loading. Any desired 

 thrust overload can be applied to the model or 

 runs can be made \vith varying overload to give 

 predictions for any estimated power increase at a 

 given speed. One method successfully used for 

 many years predicts ship and propeller operating 



conditions at the designed trial speed but with a 

 power increase of 12.5 per cent above that 

 required for trial conditions. This 12.5 per cent 

 increase is a sort of selected average between 

 clean-bottom trial conditions with no adverse 

 forces acting on the ship and a 25 per cent 

 increase to be expected toward the end of the 

 docking interval, \vith some adverse weather and 

 other overloads thrown in. 



Data derived from this procedure are definitely 

 to be preferred, for predicting service performance 

 and for design of the ship propellers, to data 

 derived from driving a smooth model faster than 

 the sustained speed by the use of the maximum 

 power that it is proposed to put in the ship. 



It is pointed out in Sec. 65.3, and it is again 

 emphasized in a discussion of speed reduction in 

 wavegoing in Part 6 of Volume III, that a ship 

 is in much better position to maintain a high 

 sustained speed if it has a speed margin designed 

 into it rather than a power margin designed into 

 the propelling machinery alone. The speed 

 margin may be a percentage above the sustained 

 speed or it may be a speed increment, as men- 

 tioned in (a) and (b) preceding. It may be 

 determined by a graphic method such as that 

 described in Sec. 69.10. Whatever the method 

 employed to determine the speed margin, the 

 designer has more assurance of achieving the 

 extra speed required to make up for lost time if 

 the ship is fashioned to make that extra speed 

 easily. 



69.10 Graphic Representation of Powering 

 Allowances and Reserves. Assuming a no-over- 

 load condition for the ship, involving only the 

 unavoidable plating, structural, and coating 

 roughnesses to be expected in the clean, new 

 condition, a typical speed-power curve is as 

 indicated by AGB in Fig. 69. A. If the clean, new 

 ship were run at the maximum designed power 

 f Mas , under perfect trial conditions, the speed- 

 power point would be at B and the speed would 

 be Fmsx • If the power were limited to 0.95 of the 

 maximum designed value, the speed-power point 

 would be at G and the speed would be Firiai • 



Running the vessel at the power Pmsi with A;, 

 per cent of increased resistance due to adverse 

 elJects, along the curve DGjC marked "AVER- 

 AGE OVERLOAD" on the figure, gives the 

 speed-power point C for a speed somewhat less 

 than the trial speed Ft rial • The ship can now 

 run at this speed with maximum designed power 

 ■Pmox or it can run at a reduced speed Vt with 



