15 SE 172
bhotheek van de
OnderafdeUn. -:.sbouw .ncie
DATUM: j OKt. 1973
The Ninth Symposium on Naval Hydrodynamics
ON THE DESIGN OP THE PROPULSION SYSTEMS WITH "Z" DRIVES POR HYDROFOIL SHIPS
The propeller powered through an inclined shaft is the. most extensively employed propulsion system forhydrofoil ships. Such propulsion systems distinguished by simple design are used
for most hydrofoil ships now in operation. However, this system
has some disadvantages. The oncoming flow obliquity due to the
propeller shaft inclination resultà in the periodic change of
the incident angles of the propeller elementé, th amplitude
being increased with the decrease in the relative radius. The
variation of the incident angles prevents fròm designing the optimum propeller, from the point of view of propulsion quali-ties, and forces the designers to make. a certain compromise, while choosing the propeller elements, to avoid the intensive
erosIon damages on the prdpeller blades. The methód of
de8ign-Ing the propellers adapted to the oblique flow is described in
The experience obtained in calculating such propellers shows that with the oblïque flow angles exceeding l4°-15° the design of the ship propeller displaying satisfactöry erosion
characteristics has failed. At the same time the improvement
of the hydrofoil ship seakeeping qualities requires the increase in clearance between the ship bottom and free surface and this
is why the propulsion system known as "Z" drive came into use
on seagoing hydrofoil ships. There are different variants for
"Z" drives distinguished by the arrangement of ship propellers: (a) single propeller "Z" drive with one propeller at the forward end or at the after end of the propulsion pod and (b) twin pro-peller "Z" drive where one propro-peller is at the forward end of
the propulsion pod an4 the other is at its after end. The
dis-advantages of the latter "Z" drive are due to the forward peller effect upon the flow around "Z" drive and the after
pro-peller operation. This effect can result in intensive erosion
damages on Z-drive elements situated in the forward propeller
wake. As a consequence such type of the propeller arrangement is used only for "Z" drive intended for high power transmission where the decrease in dimensions of the gear assembly and
pro-pulsion pod is required. Prom the point of view of providing
the uniform velocity field in the propeller disk, (a)-variant with a propeller at the forward end of the propulsion pod is
preferable; however, the propulsion efficiency of this "Z" drive is somewhat below the efficiency of "Z" drive with a propeller
at the after end of the propulsion pod. Besides there is a
danger of erosion damage on "Z" drivò hull. Thus at present "Z"
witha single after propeller is considered to be the most attractive; a number of hydrofoil ships are equipped with such
The desii of propeller "Z" drive propulsion system involves a series of problems which can. be divided into two groups: the first group deals with choosing the geometry of the propulsion pod and strut; the second group deals with
designing ship propellers adapted to the velocity field generat-ed by "Z" drive hull.
Generally the propulsion pod-and-Strut croas sections are predetermined taking into account the arrangement of the gear
assembly and bearings of the vertical shaft. Since the increase
in the wetted surface is unfavourable, from the point of view of resistance, the problem is reduced to the choice of minimum length-to-diameter ratio providing the absence of cavitation on these elements.
As the calculations and experiments show, length-to-diameter ratio providing the minimum resistance lies in the range of 4-5; in this case no cavitation occurs on the propulsion pod up to
cavitation numbers 0.2-0.3. Circular-arc cross sections for the
propulsion pod are preferable. provided that there are no struc-tural impedimenta.
To preclude cavitation on the strut its maximum relative thic1iesa in the first approximation may be determined according to the following empirical formula:
max - 2,5 (1)
where 5 is the cavitation number.
This formula holds fOr thé profiles with a fairly uxique pressure distribution. Specifically, NACÂ-l6 and Mandel profiles
with theserequirements. At the place of the strut-free surface intersection the strut profile usually has the form of a symmetric segment developing smoothly into the parent profile of
thepart immersed. The moat critical element of "Z" drive is
parti-cularly in its bow part,.the cavitation
inceptionis possible. The radius of the fillet at the place of junction is chosen so
that the thickne 88-Chord ratio should not exceed the value
It is obvious that with the proper choice of "Z" drive forma the cavitation can be avoided only in a comparatively narrow range of inci4ent angles. The critical cavitation number versus the angle of incidence is shown in Pig.l.
Por designing the propeller to be mounted on "Z" drive the knowledge of the hull-propeller interaction factors is necessax The distinguishing feature of the propulsion system in question
is the large value of Did ratio where D is the propeller
dia-meter, d ja the pod diameter. In these conditions the decisive
role in the 'formation of the nomina]. wake in way of propeller
belongs to a potential component. Replacing the propulsion pod
with a system of singularities makes it possible to calculate
in the first approximation the value of nominal wake. It is of
interest to note that in the range of diameter relation
(1.5 .< <2.0) under discussion the wake value is practically
independent of this relation. This is supported by the data in
Pig.2 where the values of the nomina]. wake are plotted against
the pod length-to-diameter ratio at different
-s-A8 13known, "Z" drive propellers operate at comparatively
small load factor values
Gpranging from 0.3 to 0.6. It make.a
possible in practical calculations to consider the effective wake to be equal to the nominal one. To proceed from the
assump-tion that the wake. is potential, the thrust deduction factor can
where p- propeller advance ratio,
- apparent advance ratio, p - propeller thrust,
- gain in resistance of "Z" drive due to the presence of propeller.
The tests with various types of "Z" drives showed that the wake value was independent of the propeller loading. This gives grounds to consider that the wake value will neither be influ-enced by the propeller loading changes due to cavitation.
where Wpis the nominal potential wake factor.
The thrust deduction factors calculated by formula (2) hold, strictly speaking, for the perfect fluid only, however, the er-rors due to this assumption are essential but at very small load
factors. The interaction factors can be refined through the
tests in a cavitation tunnel. During the tests "Z" drive
resist-ance and propeller performresist-ance curves are recorded. Apart from
the refinement of the interaction factors, the tests make it
possible to reveal the propeller cavitation effect upon the value
of these factors. When analysing these experimental result8
the wake and thruSt deduction factors may be determined by the following formulae:
Therefore the wake value obtained by the comparison of the
pro-peller performance curves for eubcavitating regimes may be used.
in further calculations. Por the agreement of the propeller
performanoe curves in open water and downstream of "Z" drive under cavitation, use is made of the correction factors taking into account the effects of the flow nonuniformity and pressure
change in the propeller disk upon the propeller thrust and.
The dependence of the thrust deductión factor upon the
pro-peller loading and cavitation nuniber for the propulsion system
with the pod length-todiaineter ratio .-- = 5 and = 1.5 is
shown in Pig.3. As is Been, with the development of cavitation
the thrust deduction factor decreases. This result is ïn
quálitative agreement with the conclusions of the paper .
By way of comparison the thrust deduction factor calculated by Eq.(2) is plotted on the saine curve against propeller loading
in the absence of cavitation. The value k40mfoi' the propulsion
system under study amounts to 0.045. The agreement of t-values
in the range of moderate and high. loads is considered to be,
The propeller mounted on "Z" drive operates in a rada1ly
nonuniform axial flow. Besides this the propeller oncoming
flow exhibits circumferential nonuniformii,y due to the strut effect and sometimes due to the foils adjacent to the pod. Por
the purpose of adapting the propeller to the wake the results of the experimental investigation of the velocity field are
used. The typical results of the measurements for "Z" drive
with length-to-diameter ratio = 5 are given in Pig.4 (a)
by integratingthe diagrams
of velocitydis-tributions are in good agreement with the theoretical data and also with the desii wake factor defined in terms of propeller characteristics.
In màking calculations use is made of the wake values averaged over the circumference; however, after the propeller desii is finished it is necessary to verify the possibility of cvitatiòn inception on the face of propeller blade in the
positions corresponding to the minimum wake values. The most
effective method of verification Is the visual observation during the tests in a cavitation tunnel.
In choosing the optimum,values of the propeller diameter and the number of revolutions for the propulsion systems with "Z" drives it should be taken into account that the increase in,
the number of revolutions makes it possible to decrease the
dimensions of the reduction gear and consequently the propulsion pod dimensions and, as a result, to decrease "Z" drive
resist-ance. In this case the number of revolutions may increase in
comparison with the optimum number specific for an isolated
.A.Titoff, A.A.Rouaetsky, and E.P.Georgievskaya, "Principles of cavitating propeller desii and development on this basis of screw propellers with better resistance to erosion for hydrofoil vessels "Raketa" and "Meteor", Seventh Symposium on Naval Hydrodynamics, 1968, Rome.
2 E.P.Geoxgievskaya, "Propeller cavitation ero8ion and methods
of protection" (in Russian), Sudostroenie, 1970.
3 V.P.Bavin, I.J.Miniovich, "Experimental investigation of.
interaction between hull and cavitating propeller", 10th ITTC, 1963, London.
Critica]. cavitation number versus incident
L 0ß3 Q,O2 ao'i o 10 20
Nominal wake factor versus pod
Thruat deduction factor of the propeller aituated. at the after part of the propuleion pod.
-- - calculation by formula (2)
e-Q -%O 'II
---IG Q G
1,0 V.. vo 0,9 0,6 0,4 0,5 0,6 0,7 0,8
(a) along the radius
- - in the atrxt plae