Design of "Z" Drives for Propulsion of Hydrofoil 



To preclude cavitation on the strut its maximum relative thick- 

 ness in the first approximation may be determined according to the 

 following empirical formula : 



C 6 



^ =— = 



max 2, 5 



where 6 is the cavitation number . 



This formula holds for the profiles with a fairly unique pressure 

 distribution . Specifically , NACA-16 and Mandel profiles comply with 

 these requirements . At the place of the strut-free surface intersec - 

 tion the strut profile usually has the form of a symmetric segment de - 

 veloping smoothly into the parent profile of the part immersed . The 

 most critical element of "Z " drive is the junction of the pod with the 

 strut because here , and particularly in its bow part , the cavitation 

 inception is possible . The radius of the fillet at the place of junction 

 is chosen so that the thickness -chord ratio should not exceed the value 

 h max . 



It is obvious that with the proper choice of "Z " drive forms 

 the cavitation can be avoided only in a comparatively narrow range of 

 incident angles . The critical cavitation number versus the angle of 

 incidence is shown in Fig. 1 . 



For designing the propeller to be mounted on " Z " drive the 

 knowledge of the hull -propeller interaction factors is necessary . 

 The distinguishing feature of the propulsion system in question is the 

 large value of D/d ratio where D is the propeller diameter , d is the 

 pod diameter . In these conditions the decisive role in the formation of 

 the nominal wake in way of propeller belongs to a potential component . 

 Replacing the propulsion pod with a system of singularities makes it 

 possible to calculate in the first approximation the value of nominal 

 wake . It is of interest to note that in the range of diameter relation 

 (1 . 5 < -7- < 2 . 0) under discussion the wake value is practically inde - 

 pendent of this relation . This is supported by the data in Fig. 2 where 

 the values of the nominal wake are plotted against the pod length-to- 

 diameter ratio at different -2— 



As is known , "Z "drive propellers operate at comparatively 

 small load factor values Cp ranging from 0. 3 to 0.6 . It makes pos- 

 sible in practical calculations to consider the effective wake to be 

 equal to the nominal one . To proceed from the assumption that the wake 

 is potential , the thrust deduction factor can be calculated from the 

 known formula . 



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