Table II 

 PHYSICAL CONSTANTS FOR WATER VAPOR AND WATER 



T C C 



Water Vapor 



/i x 10 4 gm/cm • sec 



k x 10" 3 erg/cm • sec • °C 



c r x 10 7 erg/gm • °C 



p (g x 10 4 gm/cm 3 



D = k/p eg c v cm 2 / sec 



Gas constant/gm of water vapor: 



Water 



/i x 10 ! gm/cm «sec 



k x 10~ 4 erg/cm • sec • C C 



c,. x 10" 7 erg/gm • °C 



L x 10 10 erg/gm 



p gm/cm 3 



P v x 10"* dynes/cm 2 



eq 



a dynes/cm 



D x 10 3 cm 2 /sec 



dP v /dT dyne cm 



A x 10- 3 = 



P gm °C 



15 



1.10 



100 



210 



37.7 



447. 



DISCUSSION 



R. W. L. Gawn 



I hope you won't think I have gone too far from the strictly physical aspects of 

 Professor Plesset's excellent lecture if I talk about naval architecture and the physical 

 cavitation on ships. 



You probably know the physical effects of cavitation as far as a ship is concerned 

 are objectionable in every sense: speed is lost, propeller blades vibrate or sing, noise is 

 emitted, and the material of the propeller is eaten away. 



Now despite many years of endeavor and all sorts of research, these defects are 

 still manifest in different degrees depending on the class, of the ship and its speed. 



The propeller designer is guided in the suppression or limitation of the cavitation 

 by model experiments, but the value of the information thus obtained depends on the 

 extent to which the tests can be relied upon as representative of conditions on the ship. 



In the past, models have been tested in axial flow water tunnels at various speeds 

 and pressures, but there is a growing recognition of the necessity of reproducing the 

 velocity field of the ship in magnitude and direction, and facilities are being provided 

 accordingly, one aim being the test of an actual hull-propeller combination. And this 

 we have been doing at Haslar. 



This may prove a valuable step forward, although similitude between model and 

 ship conditions will not be completely realized. 



Generally, velocity, pressure and density are correlated in the parameter known 

 as the cavitation number. Other physical qualities are involved, such as gas and solids 

 in solution and in suspension, surface tension and viscosity in sea water. I am not sure 

 we know much about the surface tension of sea water. We know a little bit about 

 viscosity. 



These may not all be sufficiently significant to call for close matching, but there 

 is at present little information on this, and research is essential for clarification. 



Notable researches have been carried out with fresh water and with pure water, 

 but ocean water has received scant attention. Until the relevant physical properties of 



318 



