Nowaekt and Sharma 
I. INTRODUCTION 
The purpose of this research was to clarify by analysis, com- 
putation and experiment the quantitative role of wavemaking at the 
free surface in the phenomenon of hull propeller interaction and con- 
sequently its contribution to the hydrodynamic propulsive efficiency of 
the system hull and propeller. 
Following Froude (1883), hull propeller interaction is con- 
veniently studied in terms of three propulsion factors : wake, thrust 
deduction and relative rotative efficiency. The wake is caused by the 
presence of the hull and the free surface and is a simple measure of 
the change in propeller inflow as compared to an equivalent open- 
water condition (free running propeller in an infinite parallel stream). 
The thrust deduction is really an indirect expression of the fact that 
the force of resistance acting on the hull is modified (usually augment- 
ed as compared to the towed condition) as a result of propeller action. 
With the present state of our knowledge, only wake and thrust deduc- 
tion are amenable to rational analysis, the relative rotative efficiency 
being an empirical catch-all for various unclarified effects of relative- 
ly insignificant magnitude. 
Since the fundamental work of Dickmann (1938, 39), it has 
been customary to study hull propeller interaction as a superposition 
of three basic effects : ''potential'' effects due to an ideal displacement 
flow about a deeply submerged double body (the zero Froude number 
approximation), viscous effects due to the boundary layer and viscous 
wake, and wave effects due to the presence of the free surface. Using 
standard symbols w and t for wake and thrust deduction fractions 
respectively, one may write formally 
= + + 1 
w w Ww (1) 
a ge 2 eee: (2) 
where the subscripts p,v and w denote potential, viscous and wave 
respectively. By comprehensive theoretical analysis and careful ex- 
periments Dickmann demonstrated that the most significant compo- 
nents were Wp? tb and Wy - 
Among Dickmann's most impressive achievements were 
1) a theoretical relation between potential wake and trust deduction in- 
volving the thrust loading coefficient, and 2) a reasonable explanation 
of the effect of the free surface on propulsive efficiency. His main 
analytical tools were a simple actuator disk model of the propeller 
1846 
