Propeller -Hull Interaction 



HULL EFFICIENCY 



RELATIVE ROTATIVE EFFICIENCY 



PROPULSIVE EFFICIENCY 



OPEN PROPELLER EFFICIENCY 



WAKE FRACTION W 



THRUST DEDUCTION FRACTION T 



5 10 I 5 20 2.5 



LCB POSITION AS % Lrp FORWARD OF MIDSHIPS 



Fig. 1 - LCB position as % Lpp forward of 

 midships; propulsion factors 



section to allow for the effect of shearing stresses on deck and double bottom. 

 The iteration necessary in the calculation was carried out on an IBM 360 com- 

 puter. The calculated frequency was 45.3 per minute as compared with the 

 measured value of 44 per minute. The effect of shear deflection on the total 

 deflection was 15%, 



Professor Aertssen's second point dealt with the problem of wake scale 

 effects. There has been much discussion about a 1 or 2% reduction of ship rpm 

 due to wake scale effect between model and ship. There is some evidence that 

 this allowance on rpm is correct for a welded ship of about 200-250 m in length 

 with the hull in best trial condition on the measured mile. When, however, the 

 power allowance necessary on model-test results increases due to fouling or 

 the state of the sea is not calm but moderate, this 1 or 2% allowance on rpm 

 may disappear. The fouling is a result, for instance, of the ship going on 

 trials (and this occurs frequently) a fortnight or even more after undocking. 

 Hull fouling resulted in an increase of wake on the "Lubumbashi," where thrust 

 was carefully measured. Professor Aertssen believed she is the only fouled 

 ship where thrust has been measured carefully, and the measurements of thrust 

 were good. On the "Lubumbashi" the wake was deduced from these thrust meas- 

 urements, the increase being 0.027 after six months service. The corresponding 

 increase in power, due to fouling, was 9%. 



On large ships with high block coefficients, a substantial scale effect on 

 wake has been found. The ore-carrier "Min Seraing," 218 m in length, with a 

 block coefficient of 0.80 in loaded condition, at a speed of 16.5 knots at sea in 

 the newly built condition, had a wake of 0.345 for the ship as compared with 



1646 



