Rouse tsky 



qualities requires the increase in clearance between the ship bottom 

 and free surface and this dictates the increase in the propeller shaft 

 inclination . That is why the propulsion system known as "Z" drive 

 came into use on seagoing hydrofoil ships . There are different va - 

 riants for "Z"drives distinguished by the arrangement of ship propel- 

 lers : (a) single propeller "Z" drive with one propeller at the forward 

 end or at the after end of the propulsion pod and (b) twin propeller 

 "Z" drive where one propeller is at the forward end of the propulsion 

 pod and the other is at its after end . The disadvantages of the latter 

 "Z" drive are due to the forward propeller effect upon the flow around 

 "Z" drive and the after propeller operation . This effect can result in 

 intensive erosion damages on Z -drive elements situated in the forward 

 propeller wake . As a consequence such type of the propeller arrange- 

 ment is used only for "Z" drive intended for high power transmission 

 where the decrease in dimensions of the gear assembly and propulsion 

 pod is required . From the point of view of providing the uniform velo- 

 city field in the propeller disk , (a)-variant with a propeller at the for - 

 ward end of the propulsion pod is preferable ; however , the propulsion 

 efficiency of this "Z" drive is somewhat below the efficiency of "Z" 

 drive with a propeller at the after end of the propulsion pod . Besides 

 there is a danger of erosion damage on "Z" drive hull . Thus at pre - 

 sent"Z" drive with a single after propeller is considered to be the most 

 attractive ; a number of hydrofoil ships are equipped with such " Z " 

 drives . 



The design of propeller "Z " drive propulsion system involves 

 a series of problems which can be divided into two groups : the first 

 group deals with choosing the geometry of the propulsion pod and strut; 

 the second group deals with designing ship propellers adapted to the 

 velocity field generated by "Z " drive hull . 



Generally the propulsion pod-and- strut cross sections are pre- 

 determined taking into account the arrangement of the gear assembly 

 and bearings of the vertical shaft . Since the increase in the wetted 

 surface is unfavourable , from the point of view of resistance , the 

 problem is reduced to the choice of minimum length-to-diameter ratio 

 providing the absence of cavitation on these elements . 



As the calculations and experiments show , length-to-diameter 

 ratio providing the minimum resistance lies in the range of 4-5 ; in 

 this case no cavitation occurs on the propulsion pod up to cavitation 

 numbers 0.2-0.3 . Circular-arc cross sections for the propulsion 

 pod are preferable provided that there are no structural impediments . 



402 



