Davis and English 



to transmit 25,000 hp through two Zed drives in the struts has required consid- 

 erable development effort by the General Electric Company of the USA. Rather 

 similar drive systems of comparable complexity have been used in the US on the 

 hydrofoil ships Denison and Plainview (AGEH-1) (Refs. 3 and 18), and no doubt 

 the complexity expense, and vulnerability of this particular component, more 

 than any other factor, has motivated the search for alternative means of propul- 

 sion. However, in the authors' opinion it is extremely unlikely that a more effi- 

 cient means of propulsion will be found. 



The point is frequently made (Refs. 19 and 20, for example) that the effi- 

 ciency of the propulsion device is not the only factor to be considered when se- 

 lecting a method of propulsion for a hydrofoil ship, and that when all the relevant 

 factors are considered, preference for an alternative device with a lower pro- 

 pulsive efficiency may result. It might be expected that the overall system effi- 

 ciency obtainable with the alternative device will be higher, but even this is not 

 an essential prerequisite if the alternative is more reliable and requires less 

 maintenance. At the present time, however, and in relation to the Bras d'Or 

 power requirement, no alternative propulsion system can be said to provide a 

 satisfactory alternative to gas -turbine-driven fully cavitating propellers. Never- 

 theless, it might be of interest to make a brief assessment of the possible future 

 means of propulsion. 



In basic terms, hydrofoil ship propulsion, as with all ships, requires the 

 rearward acceleration of fluid, whether it be water or air, or a mixture of both. 

 This requires the generation of energy onboard, the transmission of this energy, 

 and finally the transference of the energy to the fluid used for propulsion. Some 

 methods of propulsion such as rockets, both above and below water, do not re- 

 quire the transmission and transference stages, but these fall outside the scope 

 of devices that accelerate the ambient mass of fluid to obtain propulsion. Other 

 devices such as jet engines without additional equipment only utilise the first 

 and last stages, omitting the energy transmission stage. They lead to a me- 

 chanically simple system, but unfortunately, due to the high jet velocities, the 

 efficiencies are much too low. In fact, all forms of air propulsion, including air 

 propellers, can be excluded on the grounds that the efficiency will be too low or 

 the device will be too large. 



The possibility of extending air propulsion by mixing water with airjets 

 after the compression stages as described in Ref. 21 appears very attractive. 

 This would increase the density of the fluid being accelerated and reduce its jet 

 velocity, leading to an increase in efficiency over that of the plain airjet. The 

 performance of such a device depends largely on the effectiveness with which 

 the water particles can be accelerated and the air decelerated in the energy 

 transference stage, and also on the efficiency of this transference. It seems to 

 the authors that this method of propulsion will fail in the context of the Bras 

 d'Or requirements precisely on these grounds, although this statement is based 

 on the performance of the airblown ramjet and airlift pumps, and not on hard 

 facts obtained from a water-augmented airjet. The airblown ramjet itself (Refs. 

 22, 23 and 24), appears doomed to failure on the grounds that the efficiency is 

 too low or the device will be too large. Also, auxiliary starting is required and 

 acceleration is poor. 



1006 



