PROPULSION

For the case of the high-speed craft of the F.P.B. type, where a speed of ~o knots or more is the requirement, the problem of propulsion requires special consideration.

While it cannot be doubted that in the interests of avoiding cavitation a large blade surface running at relatively low speed associated with quite a high pitch ratio (P/D) of between 1.5 and 2.0 will give optimum results, certainly so long as the flow to the propeller is axial or nearly so.

However, this conclusion is compromised in practice in certain respects which tends towards the adoption of a high-speed propeller.

While as above stated the propeller efficiency will possibly be greater for the low-speed high-pitch ratio propeller, so long, as cavitation can be avoided, it is the overall propulsive efficiency which is the parameter of overriding importance so far as it affects the speed obtainable from the horse-power available.

Low speed of revolution and high torque will involve a large shafting diameter. Larger propeller diameter will involve large running shaft inclination to the ambient flow to propeller. These two factors taken together involve considerable appen¬dage drag when it is appreciated that the shafting inclined at an angle to the flow involve a drag equivalent to that set up by an appendage of depth=length of wetted shaft sin 6, where 6 is angle of shaft axis to local flow, and width= shaft diameter. Add to this the additional drag of shaft brackets and probably increased rudder dimensions, and it is not difficult to envisage the probability that a relatively high-speed propeller will involve very much less appendage drag while losing only fractionally less in propeller efficiency.

It must be taken into account that owing to the running angle of inclination, which is severe (circa 13°-15°), the large propeller will not be able to avoid cavitation in any case due to the effect of change in effective angle of incidence throughout the disc swept by a propeller blade (Ref. 4). (See Fig. 15.)

Certainly the recorded and analysed results from the trial running of H.M.S. Brave Borderer have justified the selection of a high speed propeller. An overall propulsive coefficient of 0.52 has been achieved at a speed in excess of 50 knots. It is in fact 0.54 if we take the shaft power reading.* This definitely represents an advance over previous similar naval craft. A good wartime boat at 40 knots maximum would be lucky to have an overall propulsive coefficient more than 0.47.

In achieving this performance there can be no doubt that the propeller design was very much helped by the action of the Director of Naval Construction in putting in hand a methodical series of high speed propeller tests in the Vosper high speed cavitation tunnel.

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.)

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