Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014


Experimental tests in the cavitation tunnels are performed according to the planned cavitation numbers, which are presented in Table 3.


Table 3: Cavitation numbers  :


1   ... s


When experimental tests are executed in the towing tank or the cavitation tunnel, then the values of the thrust of the model propeller T, the values of the torque on the model shaft Q, the values of velocity of the water inflow va , and the number of rotations (revolutions) of the shaft per second n, are measured and recorded. The condition of the environment is determined by the cavitation number . The results of the tests present raw results of the measurements. They are presented in the form of non-dimensional coefficients


K ρnD 


T T 24


K ρnD 


Q Q


J nD 


va 25 thrust coefficient torque coefficient advance coefficient


The values of these coefficients are the points that are entered into the coordinate systems KT-J and KQ-J. The curve that passes through the points (KT, J) of


the


coordinate system KT-J represents a law according to which the quantity KT depends on the quantity J. The same applies for the coordinate system KQ-J. It is usual to show the


both coordinate


η K J K π





T Q


systems in one graphic.


Propeller efficiency is calculated by the formula 2


(11) Figure 4: Cavitating conditions


To get smooth and logical curves representing the dependence of KT on J and the dependence of KQ on J, it is necessary to move some points


in the coordinate


systems. This action is called the fairing of curves. Faired curves present official results of empirical tests in the towing tank and the cavitation tunnel.


There are as many curves of KT and 10KQ against J as there are model propellers and conditions of environment covered in the tests performed. This means that, with each plot of KT and 10KQ against J, there are associated values of Z, Ae/A0, P/D and , designating some specific model and condition of environment with respect to cavitation.


The results of the measurements are presented in a nondimensional form in order to be used in the propeller design process.


All graphics showing the results of measurements of a series of model propellers form a database presented in a graphic form.


6. MATHEMATICAL MODELS OF PROPELLER SERIES


When a propeller series is used in the design process of a real propeller, the task is


to select a propeller, i.e.


determine the geometric quantities of one propeller which will be


somewhere between the geometric


quantities of propellers making up the series. The selected propeller should have such properties as to satisfy the design conditions. Thus, the task of selection leads to the task of interpolation. This is depicted in Figure 2, where the selected propeller is marked by the bold outline. How is the interpolation done? There are


Figure 3: Noncavitating conditions


KT 10KQ


Ae /A0, P/D,  10KQ  KT J 


The results of model tests are graphically presented in Figures 3 and 4. Figure 3 shows the properties of a propeller model operating in an environment with no cavitation or where its influence is negligible. Figure 4 shows how an environment where the cavitation occurs has a significant impact on the performance of a propeller.


KT 10KQ


10KQ KT 


 Ae /A0, P/D


J


©2014: The Royal Institution of Naval Architects


B-47


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