Prediction of Steering and Manoeuvring of Ships 



Fig. 26 - Model suspended on a torsional 

 pendulum for measurement of the polar 

 moment of inertia 



The inertia of the model is found by first measuring the period of oscillation 

 of the model and yoke, and then the period of the yoke without the model. The 

 period of oscillation can be measured with sufficient accuracy using a stopwatch, 

 as the time for one complete oscillation is of the order of 2 minutes. The very 

 slow movement of the model precludes the possibility of significant aerodynamic 

 damping. 



The inertia of the full-scale ship is normally computed on the basis of the 

 longitudinal weight distribution and the main dimensions. 



The ability to account for differences in inertia between the model and the 

 ship is convenient, as it permits the model to be constructed without paying any 

 regard to its inertia. Model testing can be executed for any value of model 

 inertia, and the appropriate ship value introduced in the analysis of the force 

 measurements. 



Scale Effects 



Most of the hydrodynamic coefficients are obtained from model tests; hence 

 it is reasonable to give some consideration to correlation between model and 

 full-scale results before applying the coefficients to the prediction of full-scale 

 manoeuvres. 



The model tests are conducted according to Froude's law; consequently 

 Reynolds' number is not satisfied, and the possibility of scale effects due to dif- 

 ferences in Reynolds' number must be considered. 



Results from airfoil testing are relevant in a discussion of scale effects. 

 Such tests, covering a wide range of Reynolds' numbers, indicate that change of 



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