Analysis of these results enabled the optimum propeller to be designed and we have subsequently confirmed the break¬down of this overall propulsive coefficient as a result of an interesting and realistic experiment in our cavitation tunnel where the shaft has been inclined to 130 to the tunnel axis, and is held by a propeller bracket with a rudder realistically placed in the slipstream. We have been able to gain an insight into the various factors involved, as we are able to measure the axial thrust and torque, also the forces acting on the propeller bracket and rudder. A study of the cavitation pattern shows marked difference between one side of the propeller and the other, the chief difference occurring at 3 o’clock and 9 o’clock. (See Figs. 16 (a), (b) and 17.)
It is of passing interest that in fact the propulsive efficiency of the propeller is fractionally greater for the case of the inclined shaft, though a satisfactory explanation of this phenomenon has not so far been arrived at. It is possible that after further investigation more may be gained in the future by using one of the specially designed sections (Refs. 5, 10).
As an aid to reducing the appendage drag, Vosper, in conjunction with Messrs. S.K.F., developed a technique for avoiding the necessity for cutting a keyway in the shaft in way of propeller boss.
This allows of a half-inch reduction in the diameter of shafting, which assists the general problem of appendage drag reduction. Though the diameter is reduced we are not helped over the matter of critical whirling speeds to meet the problem of which we have to incorporate an intermediate shaft bracket. (See Fig. 18.)