PANEL DISCUSSION-PROPELLER-HULL 

 INTERACTION 



F.H.Todd, Panel Chairman 

 Office of Naval Research Branch Office 



Londo7i, England ' 



The chairman, in opening the discussion, pointed out that the subject of 

 propeller- ship interaction involved many different aspects of our disciplines. 

 A balanced design for a ship needs not just an optimum hull shape from a re- 

 sistance point of view and a propeller with the highest open-water efficiency, 

 but requires that we have the optimum combination of the two. It is not always 

 the case that the best hull and the best open propeller together lead to the best 

 combination, this last being a function of the interaction between the propeller 

 and the ship, the subject of this panel discussion. 



The chairman went on to outline the principal headings under which the panel 

 members might wish to contribute their ideas. The first of these is the ivake , 

 because a propeller behind the hull is not operating under open-water conditions 

 but in the somewhat confused flow field that exists behind the ship and, therefore, 

 a knowledge of the wake distribution is an essential factor in the interaction be- 

 tween hull and propeller. The second factor is the resistance augment or thrust 

 deduction due to the effect of the propeller in accelerating the water ahead of it, 

 which can have a number of effects on the hull— it reduces the pressure over the 

 stern compared with that in the towed condition, thus increasing the resistance; 

 it can cause an increase in the skin friction, because a greater part of the after- 

 body is subject to higher-velocity flow; also, by moving any point of separation 

 aft, it may result in a decrease of separation resistance. A third heading is the 

 propeller performance. Model propellers are standardized in open water, but 

 when operating in the wake behind the hull the efficiency is in general different 

 from that realized in open water. The variable wake will also induce cavitation 

 at an earlier stage than that at which it will occur in open water. The operation of 

 the propeller behind the hull also gives rise to propeller forces. Each blade as it 

 rotates has a pressure field around it, and as this pressure field passes the hull, 

 rudder, or bossings, the varying pressure gives rise to forces both on the pro- 

 peller shaft and on the hull surface— forces referred to as bearing forces and 

 surface forces, respectively. In addition, the pressure fields are themselves 

 varying, due to the effect of the wake, which in turn modifies the transmitted 

 forces. These forces can excite hull and shaft vibration, and it is desirable that 

 they be kept as small as possible. 



For many of the items mentioned, difficulties arise in applying model re- 

 sults to a ship because of scale effect, and Dr. Todd suggested that the panel 

 might well discuss our present knowledge on scale effect upon the propulsive 

 factors, and the overall problem of extrapolation from model to ship in which 

 these factors play a most important part. Other items suitable for attention 

 were the effect of fully cavitating propellers on thrust deduction and the relative 



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