Propeller Excitation and Response of 230000 TDW Tankers 



mental results from tests in grid wakes, as well as in the actual be- 

 hind conditions. Two parallel methods of approach have been out- 

 lined : 



1. Unsteady lifting line technique 



2. Unsteady lifting surface technique 



The steady part of l,i.e, study of open water performance has 

 been completed |_13, 27j . For 14 propellers tested the method des- 

 cribed in |_13J reproduced recorded open water characteristics within 

 experimental accuracy for all values of advance ratio. The method 

 employs results from an experimental study of pressure distribution 

 across a propeller surface [l4jas the basic empirical "tool" for 

 obtaining a realistic lift distribution along a lifting line. 



Preliminary results from use of 2(unsteady lifting surface 

 theory) for open water work indicate that the method works poorly 

 for off design cases (low advance ratios) P 1 5~j . 



Combined with effects indicated above, the lifting line tech- 

 nique in use at DnV has been advanced to a stage where it has been 

 possible to reproduce the experimental results in behind condition for 

 the relatively few experimental results available. In Figure 24 is 

 shown how both the lifting line and lifting surface technique may work 

 poorly, when interaction between propeller and hull wake field is not 

 considered. The propeller model in question works behind a 220 000 

 TDW tanker. Clearly, our method of approach, which includes in- 

 teraction corrections, based on simple continuity of flow, reproduces 

 the experiments "within experimental accuracy" [_16J. 



Local advance ratios, as may be experienced in the tip region 

 of the blade when passing a wake peak, will lead to a significantly non- 

 linear Ct - a relationship. This effect has been approximated by 

 use of results of experiments for low aspect ratio wings described in 

 [17] and [18] . 



To decrease the risk for a "happy coincidence to occur" we 

 have performed other comparisons with experiments. Thus in Fig. 25 

 results from a one -blade dynamometer test are compared with cal- 

 culated values, obtained by using our approach ; the calculations 

 being based on the nominal wake field and the detailed propeller geo- 

 metry. As far as we understand, the practical implications of the ob- 

 servations given above, are the following : 



1. The wake survey should be performed in the propeller plane of the 



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