578 



IIYDROnYNAMICS IN SHIP DESIGN 



Sec. 69.11 



designs, Pm.i is about 1.25 or 1.30 times PNorm ■ 



One means of making this procedure more 

 precise is to have the self-propelled model run 

 with an estimated average overload, say 1.25 

 times the no-overload ship resistance, as men- 

 tioned in the preceding section. Then curve 

 DGiC as well as curve AGB is obtained and the 

 speeds Fj and V^ are predicted reasonably well. 



The next problem, equally as important as the 

 establishment of these speed-power-overload rela- 

 tionships, is selecting the condition for which the 

 propeller is to be designed. First, it must be 

 capable of absorbing the power Pm.i at some rate 

 of rotation n which can be acliieved by the engine 

 when delivering that power. Second, it must 

 operate efficiently at either P95 or PNorm , which- 

 ever the owner and operator thinks most im- 

 portant, and at a ship speed corresponding to 

 the selected power and to an overload condition 

 at which the owner and operator wishes the 

 ship to do its best. This is usually but not neces- 

 sarily the power for which the propelling plant is 

 designed to run most efficiently and economically. 

 At the power, ship speed, and overload condition 

 selected the rate of rotation of the propeller and 

 the propelhng plant must also correspond. 



In general, the propeller-design point may be 

 taken as Gi in Fig. 69.A. If so, a check is made to 

 insure that the propeller efficiency does not fall 

 off appreciably at the points D and H. This is 

 one reason why some propeller designers ai'e 

 reluctant to work a propeller near the peak of 

 its efficiency curve, for fear that at the lower 

 loadings and real-slip values the so-called working 

 point will pass over the maximum-efficiency 

 hump and slide down the steep side of the r/-curve. 



If internal-combustion propelling machinery is 

 utilized the maximum power Pmsi can be devel- 

 oped only at the exact rpm for which the engine 

 is designed. This rate of rotation must in turn 

 correspond to a certain ship speed for that power. 

 If maximum efficiency at a sustained speed cor- 

 responding to full overload is desired, the speed- 

 power point G2 should be the propeller-design 

 point. Although F is also a point on the full- 

 overload speed-power curve it may not represent 

 the power developed by the internal-combustion 

 engine at a rate of propeller rotation correspond- 

 ing to the speed V^ . However, if the engine- 

 propeller combination can drive the ship at full 

 overload at the speed corresponding to the 

 point G2 it almost certainly can, with some lesser 

 power, maintain the slower speed at F. If a 



higher speed is aimed for, at maximum designed 

 power corresponding to an average overload at 

 the point C, the speed-power and the propeller- 

 design points are then represented by that point. 



It is again emphasized that speed-power 

 points such as G2 or C are only achieved acci- 

 dentally on a ship, when the overload from all 

 causes happens to be either the average or the full 

 value assumed by the designer. It is therefore not 

 possible to check these points on an actual ship 

 by full-scale tests. On the other hand, the speed- 

 power points B, G, and A are easily reached and 

 checked under planned trial conditions. It is 

 therefore logical to embody one of these points, 

 say G, as one of the design requirements and to 

 call for a check of it as a sliip-contract stipulation. 



This is essentially what was done in Table 64. d 

 and Sec. 66.9 for the ABC design, when the trial 

 speed of 20.5 kt was required at a power expendi- 

 ture of 0.95PMai , corresponding to the point G 

 on Fig. 69. A. To prevent the problem from be- 

 coming too complicated, especially as overload 

 values are not accurately known, the ABC 

 propeller is also designed in Chap. 70 for the 

 speed-power point G. The data from the model 

 self-propulsion test, without overload, correspond 

 to that point. The additional shaft power from 

 P95 to Pjnax is taken care of by the macliinery 

 designer. 



Under other circumstances the propeller could 

 be designed equally well for the points Gi or G2 , 

 depending upon advance knowledge as to over- 

 loads, the type of machinery to be installed, the 

 judgment of the designer, and the wishes of the 

 owner and operator. 



69.11 Selection of Feathering, Adjustable, 

 Reversible, or Controllable Features. Feathering 

 features on paddlewheels form an integral part 

 of the design of these devices. As such they 

 are discussed in Sees. 71.6 and 71,7. 



Feathering and folding propellers are fitted 

 almost exclusively on sailing yachts with auxiliary 

 power, as a means of reducing the drag of the 

 stationary propeller. Notes relative to their use 

 are found in Sec. 71.13. 



Adjustable sci'ew propellers, described briefly 

 in Sec. 32.19, carry blades whose position relative 

 to the hub can be changed only when the adjust- 

 ing mechanism is out of water. They permit 

 pitch changes in the event that the ship resistance 

 in service is found not to agree with that predicted 

 in the design stage. Altering the pitch in this 

 manner is one means of insuring that internal- 



