Shipbuilding and Shipping Record, !one 12, 1958 769
AT
tution of Naval Architects, the
the Spring Meetings of the Insti-discussions on the paper by Dr. Corlett,Mr. Venus and Mr. Gibson on twin-screw tugs, and on Mr. Sweet's paper
ion progressive towing trials on
shallow-draught vessels, revealed considerable
reservations on the part of U.K. ship-builders on the application of the
" screw + nozzle " propeller.
This attitude is based mainly on
two features of this propulsion system, i.e. the increased chance of damage to
the screw in the nozzle, and the prob-able deterioration in the manoeuvring
qualities of a ship
propelled by a
screw+ nozzle.During the discussion Mr. Lederer,
of Otraco, Brussels, quoted his experiences in the Belgian Congo, which
were favourable to the nozzle screw,
and Mr. W. P. Walker, of William
Denny 8c Brothers, pointed out that,in general, the design of the " screw +
nozzle " could be improved by apply-ing the results of recent research on
nozzles.
So from the Spring Meetings it was
again clear that conflicting opinions are
held as to the application of the
screw + nozzle."
In the opinion of the Netherlands
Ship Model Basin, the lack of a prac-tical design method for the screw +Fig. 1 (above)Schematic
representa-tion of the screw ± nozzle propeller
Fig. 2a (right)Results of open wate r
,tests with the B 4-70 screw series innozzle No. 7
he "screw + nozzle"
propeller
By DR. IR. J. D. VAN MANEN
(Assistant Director, The Netherlands Ship Model Basin.Wageningen)
nozzle propeller has retarded the development which it rightly deserves. Even before 1940, German scientists had supplied important thoeretical
con-tributions to the framing of a nozzle theory, but it was only after 1950 that the hydrodynamic principles of the
" screw +nozzle " were defined.
The principal conclusions drawn from
the theoretical observations made in
the period from 1935 to 1955 are :
The action of the screw within
a nozzle is far more like that of an
axial pump than of a screw without a,nozzle.
Through the effect of the
nozzle wall, the occurrence of the trail-ing vortices at the blade tips is restricted
or, with a sufficiently small clearance
between blade tip and nozzle, even
avoided.
The forces acting on the. nozzle are caused by the contraction of the
screw race and by the screw loading.
It will be obvious from these
remarks that the system of nozzle andscrew forms a unit There would be no sense in considering the nozzle
with-out the influence of the screw, or vice
versa.
The thrust on the nozzle does not
contribute to an improvement in its
efficiency. When the screw is
sur-rounded by a nozzle, the velocity in
the vicinity of the screw increases, and
consequently, with equal output, the
thrust decreases. The force acting on
the nozzle as a result of the induction of the higher velocity of the water in the vicinity of the screw is, according to the law that action =reaction, equal
to the decrease in the thrust of the
screw.
The most plausible explanation for
the increase in efficiency with the
nozzle is that part of the trailing
tices of the screw become bound vor-tices around the nozzle. Because of
this, a decrease occurs in the exit
losses. As long as this decrease of exit
losses exceeds the drag losses of the
nozzle, the screw +nozzle propeller will lead to an increase in efficiency.
Fig. 1 shows the screw +nozzle system schematically. In this figure :
the circulation around the
nozzle profile;
ye
= the
intake velocity in the nozzle system ;Vi) = the velocity in the nozzle at the
screw;
dA = the lift force of an elementcif
the nozzle ring;
dRnozzle = the resistance of an element of the nozzle ring;
dSnozzle the thrbst of an element of the
nozzle ring; f/1 the camber ratio;
sll the thickness ratio; and = the angle of the nozzle profile
relative to the shaft line.
By starting from the
above-men-tioned conceptions of nozzle action, the Netherlands Ship Model Basin, Wagen-ingen, has carried out model tests withsystematically varied nozzle shapes. The
results obtained from these experiments.
were published in Jalzrbuch STG,
1953, International Shipbuilding
Pro-gress, No. 3, 1954 and No. 36, 1957and
The Journal of Ship Research, Vol. 2, 1957.
The results of the open-water tests with nozzles in combination with the
Wageningen B-screw series have been
presented in K,,tc,tat A diagrams
and K,,,,oizie diagrams, where
K'total= K'screw 4- icnozzle (See Fig. 2a,'
b).
The problems arising in designing a
screw with nozzle can be solved for the
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Shipbuilding and Shipping ReCordr, June 12,1795g
different nozzle types by means of dia-grams as shown in Figs. 2 a and b in the same manner as is used to solve screw
problems in the well-known screw-series diagrams Moreover, it will be possible
to determine in most cases whether the adoption of a nozzle is to be preferred from! an efficiency standpoint, and, if so, what form the nozzle should have. With regard to the design of a nozzle
the following general remarks may be
made :
The optimum diameter of the
screw with nozzle is considerably
smaller than the optimum diameter of
the screw without nozzle.
Systematic experiments have
proved that if the nozzle is of
suffi-ciently short length, an increase in
efficiency by means of a nozzle can be obtained even at very low screwload-ingssee Fig. 3. From this Fig. 3 it
is clear that the use of thescrew +nozzlepropeller need no longer be restricted to heavily loaded screws.
Making allowance for the radial distribution of the translational velocity
in the nozzle leads to an increase in
pitch at the blade tip. Neglect of this radial distribution caused by thecir-culation around the nozzle profile may
result in face cavitation at the blade
tip.
The favourable characteristics of the screw +nozzle propeller with regard to efficiency manifest themselves only
if the clearance between blade tip and
nozzle is as small as possible.
For the adaptation of the nozzle for the ship form, it is recommended that the complete nozzle ring be left
'intact as far as possible.
Finally it must be noticed that, with
a view to avoiding cavitation, the blade tip is given a wide span (Kaplan type).
Round blade tips show strongly
developed gap cavitation and tip
vor-tices.
The results of recent tests with
aKaplan-type screw series in nozzles, car-ried
out by the
Netherlands ShipModel Basin, will be published in
ship-building and Shipping Record's
Inter-national Design and Equipment, 1959.
Wodd shipping coloured wall
map
A large wall map in two parts, pub-lished by the Institut fur
Schiffahrts-forschung (Institute for Shipping
Research), Bremen, by an ingenious
system of varying colours and shades, gives an overall picture of international
sea traffic in the various trades and
services, cargo and passenger.It is believed to be the first of its!
kind to be published. Some 2,500sea-ports have been included with English
names, and the quantitative share in seaborne trade of individual countries
is distinguished by colour.
It is pointed out that the traffic in-_
tensities shown applicable to 1956 do
not take into account diversions caused
26--Axial forces acting on nozzle No. 7 with the.. screw seried' I
by the blockage of the Suez Canal. As
stated, the various countries are col-oured according to their share in
sea-borne trade, the graduations in shade
'corresponding to shares of less than 10 million metric tons up to over 100 mil-lion tons.
In effect, the publication amounts to
two very large companion wall maps
one being largely concerned with the
U.S.S.R., China and other Far Eastern territories and Australasia.
These maps may be obtained from
the Institut
fiir Schiffahrtsforschung, Bremen, Osterdeich 27. Drawn on linen and fitted with wooden battensand hooks, the price is DM.
68.50,a bipartite paper edition, without these fitments being available at a cost of DM. 46.50.
Fig. 3Comparison of nozzle systems of different length-diameter ratios and th B 4-55 screw series withouta nozzle
,
Decca Navigator to help
Dutch scheme
A Decca Navigator survey chain
is to be used in the reconstruction of
the Dutch dykes damaged in the flood disaster of 1953. The Decca equipment is already under construction and will I
-come into operation early in September.,
The Delta plan, the greatest under-taking in Holland's long battle against the sea, is vital in the prevention of a !
recurrence of the flood disaster of 19531
which brought about the flooding of
375,000 acres of land.
Estimated at present cost of materials II
and labour to cost :C200 million, the plan will take 20 to 25 years to com-plete but many of its protective
bene-fits will take effect long before that.
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