Unconventional Pro puis ion — Silver leaf 



The resulting "minimum" curve indicates that, for a given speed-displacement 

 ratio, there is often one type of marine craft with a significantly better hydro - 

 dynamic performance than others and gives an estimate of the minimum power 

 required by such a craft. It also demonstrates the penalties in power incurred 

 by design constraints or by a decision not to adopt the most favorable type of 

 craft. Some simple diagrams illustrate the general guidance which can be di- 

 rectly derived in this way. Thus, Fig. 3 shows that the minimum values of P/A 

 (hp per ton displacement or all-up weight) rise steeply with speed but fall stead- 

 ily as displacement increases, while Fig. 4 shows the rapid rise in minimum 

 power needed as either speed or displacement are increased; since Fig. 2 shows 

 that for many high-speed displacement craft the power requirements are be- 

 tween two and three times the minimum, it is clear that there are serious limi- 

 tations on speed-displacement values which are likely to be achieved in prac- 

 tice, and that even significant improvements in propulsive efficiency, however 

 obtained, can have little effect in raising the practical speed-displacement 

 boundaries. 



The concept of specific power is also useful in assessing the prospects of 

 different types of propulsion plant and propulsion device. Figure 5 illustrates 

 the dependence of the ratio MA on specific power and on speed; here M is the 

 total weight of the propulsion system, and typical, reasonably representative 

 values of 15 hp/ton and 20 hp/ton have been taken for diesel and steam turbine 

 installations respectively (Ref. (5)), and 300 hp/ton taken for gas turbine instal- 

 lations based on mean values for known installations. Figure 6 shows the mini- 

 mum values of the machinery weight ratio ma for a range of speeds and dis- 

 placements, corresponding to the minimum specific power values in Figs. 2 

 and 4. a - ; ,• ,■ /v 



It is also useful to examine fuel requirements in a similar general way. 

 Figure 7 demonstrates the dependence of fuel weight ratio (FA) on specific 

 power and on range, while Fig. 8 is a guide to the minimum values of f/A needed 

 for any given displacement and speed for a fixed range of operation. 



Cavitation and Vibration r'""^' 



Almost all marine propulsion devices, particularly those dependent on screw 

 propellers or pumps to impart energy to the fluid, are affected by cavitation or 

 similar fluid-flow phenomena. Almost invariably, cavitation has two undesirable 

 effects: It produces radiated noise, and it causes erosion of rotor blades and 

 other parts of the propulsion device. Further, extensive cavitation may ad- 

 versely affect the hydrodynamic performance of a propulsion device unless posi- 

 tive steps are taken to prevent this. 



Many different criteria have been proposed and used to define the likelihood 

 of cavitation occurrence and its extent; in general these can be divided into those 

 which take account only of the ahead speed of the device, and those which also 

 take some account of the rotational speed of the rotor or pump blade. The sim- 

 ple forms of cavitation index such as a^ , which involve only ahead speed and 

 depth of immersion, can be misleading and are almost always more inadequate 

 than those, such as a^ , which attempt to take account of blade resultant velocity. 



894 



