6 Propeller performance characteristics
6.4 Specific propeller open water characteristics
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98 Marine propellers and propulsion
process from model to full scale to become unreliable since only averaged amounts of laminar flow are taken into account in the present estimation procedures.
To try and overcome this difficulty a number of tech- niques have been proposed, particularly those involving leading edge roughness and the use of trip wires, but these procedures still lack rigour in their application to extrapolation. Bazilevski (Reference 41), in a range of experimental conditions using trip wires of 0.1 mm diameter located at 10 per cent of the chord length from the leading edge showed that the experimental scatter on the measured efficiency could be reduced from 13.6 to 1.5 per cent with the use of turbulence stimulation. It was found that trip wires placed on the suction surface of the blades were more effective than those placed on the pressure face and that the effectiveness of the trip wire was dependent upon the ratio of wire diameter to the boundary layer thickness. Boorsma in Reference 42 considered an alternative method of turbulence tripping by the use of sand grain roughness on the leading edge based on the correlation of a sample of five propellers.
In his work he showed that the rotation rate correlation factor at constant power could be reduced from 2.4 to 1.7 per cent and, furthermore, concluded that turbulence tripping was not always effective at the inner blade radii.
It is often considered convenient in model experi- ments to perform model tests at a higher rotational speed than would be required by strictly adhering to the Froude identity. If this is done this then tends to minimize any flow separation on the trailing edge or laminar flow on the suction side of the blade. Such a procedure is particularly important when the propeller is operating in situations where relatively low turbulence levels are encountered in the inflow and where stable laminar flow is likely to be present. Such a situation may be found in cases where tractor thrusters or podded propulsors are being investigated. Ball and Carlton in Reference 43 show examples of this type of behaviour relating to model experiments with podded propulsors.
Clearly compound propellers such as contra-rotating screws and ducted propellers will present particular problems in scaling. In the case of the ducted pro- peller the interactions between the propeller, the duct and the hull are of particular concern and importance.
In addition there is also some evidence to suggest that vane wheels are particularly sensitive to Reynolds num- ber effects since both the section chord lengths and the wheel rotational speed are low, which can cause difficulty in interpreting model test data.
Holtrop (Reference 44) proposed that the scaling of structures like ducts can be addressed by considering the interior of the duct as a curved plate. In this analysis an assumed axial velocity ofnPtipis used to determine a correction to the longitudinal towing forceFgiven by
F=0.5ρm(nPtip)2(CFm−CFs)cmDm
wheren,Ptip,candDare the rotational speed, pitch at the blade tip, duct chord and diameter, respectively.
In the case of podded propulsor housings the problem is rather more complicated in that there is a dependence upon a number of factors. For example, the shape of the housing and its orientation with respect to the local flow, the interaction with the propeller wake and the scale effects of the incident flow all have an influence on the scaling problem.
6.4 Specific propeller open water
Propeller performance characteristics 99
Figure 6.6 Typical controllable pitch propeller characteristic curves
three components as follows:
Qs(J,θ)=QSH(J,θ)+QSC(n,θ)+QSF(J,θ) (6.14) where Qsis the total spindle torque at a given value
ofJandθ;
QSHis the hydrodynamic component of spindle torque due to the pressure field acting on the blade surfaces;
QSCis the centrifugal component resulting from the blade mass distribution;
QSFis the frictional component of spindle torque resulting from the relative
motion of the surfaces within the blade hub.
The latter component due to friction is only partly in the domain of the hydrodynamicist, since it depends both on the geometry of the hub mechanism and the system of forces and moments generated by the blade pressure field and mass distribution acting on the blade palm.
Figure 6.7 shows typical hydrodynamic and cen- trifugal blade spindle torque characteristics for a controllable pitch propeller. In Figure 6.7 the spindle torques are expressed in the coefficient form ofKQSH
andKQSC. These coefficients are similar in form to the conventional propeller torque coefficient in so far as
Figure 6.7 Typical controllable pitch propeller spindle torque characteristic curves
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100 Marine propellers and propulsion
they relate to the respective spindle torques as follows:
KQSH= QSH
ρn2D5 KQSC= Qsc
ρmn2D5
(6.15)
whereρis the mass density of water andρmis the mass density of the blade material. Clearly, since the centrifu- gal component is a mechanical property of the blade only, it is independent of advance coefficient. Hence KQSCis a function ofθonly.
6.4.3 Ducted propellers
Whilst the general aspects of the discussion relating to non-ducted, fixed and controllable pitch propellers apply to ducted propellers, the total ducted propulsor
Figure 6.8 Open water test results of Ka4–70 screw series with nozzle no. 19A (Courtesy: MARIN)
thrust is split into two components: the algebraic sum of the propeller and duct thrusts and any second-order interaction effects. To a first approximation, therefore, the total propulsor thrustTcan be written as
T=Tp+Tn
whereTpis the propeller thrust andTnis the duct thrust.
In non-dimensional form this becomes
KT=KTP+KTN (6.16)
where the non-dimensionalization factor isρn2D4 as before.
The results of model tests normally present values of KTandKTNplotted as a function of advance coefficient Jas shown in Figure 6.8 for a fixed pitch ducted propul- sor. The torque characteristic is, of course, not split into components since the propeller itself absorbs all of the torque of the engine. In general the proportion of thrust
Propeller performance characteristics 101 generated by the duct to that of the total propulsor thrust
is a variable over the range of advance coefficient. In merchant practice by far the greater majority of ducted propellers are designed with accelerating ducts, as dis- cussed previously in Chapter 2. For these duct forms the ratio ofKTN/KT is of the order of 0.5 at the bol- lard pull, or zero advance coefficient, condition, but this usually falls to around 0.05 or 0.10 at the design free running condition. Indeed, if the advance coeffi- cient is increased to a sufficiently high level, then the duct thrust will change sign, as seen in Figure 6.8, and act as a drag; however, this situation is unlikely to arise in normal practice. When decelerating ducts are used, analogous conditions arise, but the use of these ducts is confined to certain specialist cases, normally those having a low radiated noise requirement.
6.4.4 High-speed propellers
With high-speed propellers much of what has been said previously will apply depending upon the application.
However, the high-speed propeller will be susceptible to two other factors. The first is that cavitation is more likely to occur, and consequently the propeller type and section blade form must be carefully considered in so far as any supercavitating blade section requirements need to be met. The second factor is that many high- speed propellers are fitted to shafts with considerable rake angles. This rake angle, when combined with the flow directions, gives rise to two flow components act- ing at the propeller plane as seen in Figure 6.9. The first of these is parallel to the shaft and has a magni- tudeVacos (λ) and the second is perpendicular to the shaft with a magnitudeVasin (λ) whereλis the relative shaft angle as shown in the figure. It will be appreciated
Figure 6.9 Inclined flow velocity diagram
that the second, or perpendicular, component immedi- ately presents an asymmetry when viewed in terms of propeller relative velocities, since on one side of the propeller disc the perpendicular velocity component is additive to the propeller rotational velocity whilst on the other side it is subtractive (see Figure 6.9). This gives rise to a differential loading of the blades as they rotate around the propeller disc, which causes a thrust eccentricity and side force components. Figure 6.10 demonstrates these features which of course will apply generally to all propellers working in non-uniform flow but are more noticeable with high-speed propellers due to the speeds and inclinations involved. The magnitude of these eccentricities can be quite large; for example, in the case of unity pitch ratio with a shaft rake of 20◦, the transverse thrust eccentricity indicated by Figure 6.10 may well reach 0.40R. Naturally due to the non-uniform tangential wake field the resulting cavitation pattern will also be anti-symmetric.