CHAPTER X 



EFFECT OF VARIATIONS IN PROPELLER DIAMETER 

 AND SHIP DRAFT AND TRIM 



A propeller diameter equal to 0.70 of the designed load draft was adopted as a standard 

 in the LCB and geometrical variation series. Although this is fairly representative of average 

 practice, there will be many occasions on which a different diameter will be necessary because 

 of the design of machinery used or for other reasons. In order to give some guidance on this 

 matter, each of the five parent models of Series 60 was run with additional propellers having 

 diameters smaller and larger than the standard. 



Also, the main test program covered the models only at the full load draft and level 

 trim. To get some information on the performance at other displacements and trims, additional 

 experiments were made on three of the parent models, those with C g of 0.60, 0.70, and 0.80. 



The stern arrangement was identical with that already described in Chapter VII and 

 shoY^'H in Figure 36. The vertical dimensions are given there as functions of the designed 

 draft or propeller diameter, and all longitudinal dimensions are given as functions of LBP . 

 The propeller position is so defined that the generating line at 0.70 radius is 0.94 percent of 

 the LBP forward of the after perpendicular. The stern details are therefore defined completely 

 regardless of the selection of design draft or propeller diameter, so that Figure 36 defines the 

 arrangement for all the models. The clearances were rather larger than normal practice at the 

 time, but this was considered desirable in view of the ever-increasing horsepower of single- 

 screw ships, and th6ir use has been justified by later developments. 



One method of achieving larger clearances or, alternatively, of using a larger diameter 

 propeller without sacrificing clearance is to fit a semi-balanced rudder and no rudder shoe. 

 This arrangement was fitted to the MARINER ships, and has become known as a "clearwater 

 stern," which is now used on many seagoing ships. In the course of the propulsion tests, 

 the opportunity was taken to run the 0.60 C g model with such a stern arrangement for com- 

 parison with the normal streamlined rudder results. 



The standard propellers for the parent models had a diameter equal to 0.7 of the draft 

 and have already been described in Chapter VII. For the experiments with larger and smaller 

 diameters, propellers as similar as possible to the Troost type were selected from stock. 

 The selection was made on a basis of general similarity, and the actual diameters of the 

 propellers departed somewhat from the desired values. 



The selection of propeller characteristics was based on the assumption that diameter 

 was fixed and revolutions could be chosen to obtain maximum efficiency. The values of 

 expanded-area ratio were selected on the basis of current design practice and checked for 

 suitability as to cavitation by Lerb's data."*^ 



X-1 



