Scale Effects on Propeller 

 Cavitation Inception 



G. Kuiper 



Netherlands Ship Model Basin 



Wageningen, The Netherlands 



ABSTRACT 



The boundary layer of four propeller models in 

 uniform flow is investigated and related with cavita- 

 tion inception. Laminar separation is found to be 

 an important phenomenon on model propellers. The 

 radius where laminar separation starts is found to 

 be a limit for the radial extent of cavitation. 

 No inception takes place in regions of laminar flow. 

 The effect of nuclei in the flow is investigated 

 using electrolysis. Nuclei seem to be important 

 for cavitation inception when laminar separation 

 occurs, but they do not initiate sheet cavitation, 

 when the boundary layer flow is laminar. When the 

 boundary layer on the blades is tripped to turbu- 

 lence by roughness at the leading edge it is shown 

 that this changes the cavitation by restoring cavita- 

 tion inception at the vapour pressure. The effect 

 of electrolysis on cavitation becomes very small 

 when the propeller blades are roughened. Calcu- 

 lations of the pressure distribution and the laminar 

 boundary layer were made and related with test 

 results . 



1 . INTRODUCTION 



When cavitation patters, observed on full scale 

 ship propellers, are compared with observations on 

 model scale, differences are often found [e.g., 

 Bindel (1969), Okamoto et al. (1975)]. These 

 differences are caused by two main factors: in- 

 correct scaling of the incoming flow of the 

 propeller, including propeller-hull interaction, 

 and incorrect scaling of cavitation. 



Considerable efforts have been made to improve 

 the simulation of the incoming flow by testing the 

 cavitating propeller model behind the ship model 

 in a large cavitation tunnel or in a depressurized 

 towing tank, or by correcting the measured model 

 wake to simulate the full scale wake in a cavitation 

 tunnel [Sasajima and Tanaka (1966), Hoekstra (1975)]. 



In this paper the problem of proper scaling of 

 cavitation will be investigated. 



Scaling rules for cavitating propellers can be 

 formulated using dimensional analysis when the 

 relevant parameters are known. This results in the 

 following well-known dimensionless quantities: 



V, 



the advance ratio 



the cavitation index a. 



A 



nD 



p -p +pgh 

 o V 



^pn^D^ 



the Froude number 



Fr 



nD^ 

 the Reynolds number Re = — — 



N V 



(1) 



(2) 



(3) 



(4) 



where V = advance velocity of the propeller 



n = number of propeller revolutions 



D = propeller diameter 



p = pressure at some reference level 



p = vapour pressure 



p = density of water 



g = acceleration due to gravity 



h = vertical distance from reference level 



V = kinematic viscosity 



When these dimensionless parameters are kept the 

 same for model and prototype, the cavitation 



400 



