ISTITUTO DI ARCHITETTURA NAVALE DELL UNIVERSITA DI TRIESTE
6° CONVEGNO TRIESTINO DI TECNICA NAVALE
-TRIESTE, 3 - 5 GIUGNO 1963 TECHNISCHE UNIVERSITET Laboratorium voor Scheepshydromechanica Archlef Mekelweg 2,2825 CD Delft TOL' 015-788873 . Fax 015. 781838 W. P. A. van LAMMERENSOME NEW AFTERBODY FOR SI1NGLE SCREW SHIPS
EDIZIONE: TECNICA ITALIANA - :RIVISTA D'INGEGNERIA E SCIENZE
SOME NEW AFTERBODY FOR SINGLE SCREW SHIPS
SUMMARY: The increase of engine power on single screw ships, especially on tankers, is often
accompanied by two annoying phenomena, viz, cavitation and vibrations, which may result in damage of propeller, propeller shaft and shaft bearings.
These phenomena are closely associated with the non-uniform flow into the propeller. They can be reduced to a great extent by applying afterbody hull forms which produce a more uniform flow.
The cavitation and vibration problems, which can successfully be studied now by model research, are discussed.
New constructions of the afterbody are introduced such as the combination of a Hogner hull form with a shrouded propeller, the combination of a Hogner hull form vith a Haselton propeller and the asymmetric afterbody.
SOMMARIO: Un aumento di potenza su navi monoeliche, specialmente su petroliere, e spesso
accompagnato da due fenomeni, cavitazione e vibrazioni, che possono danneggiare l'elica, l'albero
dell'elica ed i cuscinetti dell'albero.
Questi fenomeni sono strettamente legati al flusso non uniforme in cui lavora l'elica e possono essere sensibilmente ridotti con opportune forme poppiere di carena.
Nella rnemoria si esaminano i problemi della cavitazione e delle vibrazioni attraverso
studi compiuti su prove di modelli e si considerano talune nuove forme poppiere applicate
sin golarmente o in combinazione.
1. SOME PROBLEMS ON SINGLE
SCREW SHIPS
The increase of engine power on single screw
ships, in particular on tankers, is often
accom-panied by two inconveniences, viz, cavitation and
vibrations. In many cases erosion due to
cavi-tation occurs, which may lead to serious damage
of ,the propeller and thus to expensive repair
and loss of time.
The occurrence of longitudinal, torsional and
bending vibrations in the propeller shaft may
also lead to damage of the propeller shaft, the
thrust block, the propeller shaft bearings and
even tb crack of the tail shaft.
These phenomena must a.o. be due to the
considerable nonuniformity of the velocity field
in the screw disk especially for single screw
ships.
The influence of these phenomena can never
be eliminated completely, but it may be reduced
by a suitable propeller design (correct number of blades, sufficiently large blade area, blade
sections having a wide range of shock free entry,
application of material with a good resistance
against erosion) and a specific shape of the
afterbody.
It is more effective to look for constructions
of the afterbody which homogenize the velocity
field.
The Netherlands Ship Model Basin has
latter-ly tried and is still going on to introduce some of these constructions. The discussion of these
new constructions is the object of this paper.
2. SCALE RESEARCH a. Cavitation.
Cavitation phenomena can successfully be
studied by scale research. Cavitation and erosion
research is carried out in the cavitation tunnel, a vertically placed closed channel, in which the water is pumped round by an impeller located in the lower leg. In the upper horizontal leg is the rotating propeller. The air present above the free water surface is connected to a suction pump and the pressure can be reduced at will.
Some of the laws of similitude, which should be met (equal Froude-, Reynolds-, Weber- and Tho
ma numbers, equal propeller slip in full size
and on model scale and geometrical similarity)
are neglected.. The kinematic condition of- equal propeller slip and equal number of Thoma (equal ratio of static and dynamic, pressure) is howt.ver, always fulfilled. This is realized by adapting .the static pressure in the tunnel after having chOsen
4
Fig. 1 - Wake field of a single-screw ship.
pressure is reduced above the fi-ee water surface
in the dome fitted at the tunnel top part. The
revolutions of the model screm, are adjusted to
obtain the same slip as the full size propeller. The axial components of the velocity field as
measured behind the ship model concerned can be simulated. in the tunnel by means of a flow regulator. Fig. / -gives an impression about the non-uniformity of the velocity field behind a single screw ship. This figure shows lines of
equal wake fraction tj)=lVa/Vs (Vs= speed
of ship and Va=local axial speed).
The radial and tangential components were
neglected up to now, as it was assumed that
their influence would be of a second order.
Recent experiments, however, have indicated that
this assumption is not correct. This is clearly
shown by the example' presented in figure 12. Therefore it is intended to modify the tunnel
in the future sothat three-dimensional fields can
be simulated.
Tlie effect of the non-uniform flow in axial direction on the cavitation phenomena appears from a comparison between the figures 3 and 4, in which the same propeller model is shown at
'equal loading in a uniform and in a non-uniform
flOw respectively.
A typical damage due to caiiitation-erosion is
shown in figure 5, a picture of a full size prop-eller, on whichthe trailing edge was bent to the face at about 0.8 of the propeller radius.
b. Measurement of propeller excited vibratory
forces in the propeller shaft.
^10° 240° 270° 300°
Fig. 2 - Axial and
tan-gential velocity
com-ponents of a single-screw ship.
Fig. 3 - Propeller model in uniform flow.
Fig. 5 - Cavitation-erosion on trailing edge of full
scale propeller.
defined as the product of the number of
revolutions and that of blades.
Vibrations excited by cavitation and local blade vibrations. These are in general of a
relatively high frequency.
The hydrodynamic forces as mentioned under
item 2 are measured on scale models in the
Netherlands Ship Model Basin.
Variations in the lift of the blade elements
occur during a propeller revolution due to the non-uniformity of the velocity field behind the
ship at
the location of the propeller. Thesevariations produce thrust-, torque- and bending
moment variations in the shaft. The last
mentio-ned variations are directly caused by the eccen-tricity of the propeller thrust and the occurrence of lateral forces.
The complete scheme of forces is shown in
principle in figure 6. The dynamometers latterly developed in the USA, Germany and the
Nether-lands for recording thrust- and torque variations
per revolution of the propeller are based on
inductive-, capacitive as well as on strain gauge recorders.
When studying the technical problems for
recording bending moment in the shaft, all these principle appeared to fail.
The head of our instrumentation department, Mr. Wareldsma, succeeded in applying the Sampl-ing measurSampl-ing principle for these dynamic mea-surements. The result was a six-component dynamometer with a very high own frequency
(1000 Hz) which provides the possibility to
determine the phase differences between the
harmonic components and the amplitudes of the measured signals.
Some results of measurements with this bal-ance are presented in the figures 7 and 8. The
first one gives the results for a four-bladed
prop-Fig. 4 - Propeller model in non-uniform flow at the same loading.
In general the propeller excited vibrations can
be divided into three groups:
Vibrations due to unbalance with a frequency
equal to the shaft rpm.
Vibrations generated' by the hydrodynamic
forces of the propeller in the non-uniform flow behind the ship, in particular behind single screw ships. The frequency of these
vibrations corresponds to the blade frequency
6 1
r
...,
/ i : -... ----GENERATOR C1 - 4.5 trI a o 0° 90. rx ; --5/ lAREVOLUTiON OF PROPELLER < L-4-10I
VERTICAL THRUST ECCENTRICITYTHRUST VARIATIONS Is +'10 TOR UE VARIATIONS 0 15; CC 90° CC 1.41
VERTICAL TRANSVERSE FORCE
HORIZONTAL THRUST ECCENTRICITY I I I \ I 0.7R 90°
71,PLANE OF TRANSVERSE FORCES
Fig. 6- Six components acting on a screw propeller.
eller. It appears that the torque and thrust smaller thrust and torelue variations, but a larger
variations are large, the bending moment variat- lateral force.
ions small. The five-bladed propeller (fig. 8) These results are often confirmed in full size.
produces for the same ship model considerably The exchange of four-bladed for five-bladed
180° VECTOR DIAGRAM OF THRUST ECCENTRICITY 90 o° o° VECTOR DIAGRAM OF TRANSVERSE FORCES HORIZONTAL TRANSVERSE FORCE -10 0 4-10 0 +5 +10
WOF FROPELLEF RADIUS 0100F AVERAGE THRUST
PLANE OF- TRANSVERSE FORCES
TORQUE
5 THRUST
TRANSVERSE FORCE
THRUST ECCENTRICITY,
Fig. 7 - Dynamic phenomena
on a four-bladed propeller
Fig. 8 Dynamic phenomena on
a five-bladed propeller behind the
same model. +5 VERTICAL TRANSVERSE FORCE
-5 V5REVOLUTION OF PROPELLER +5 ?' 2° 72° TORQUE VARIATIONS 72° 5
propellers in order to avoid large thrust- and torque variations has led to the disappointing
result that often difficulties were encountered in
the propeller shaft bearing due to the large
bending moments.
3. NEW AFTERBODY CONSTRUCTIONS. a. Combination of a Hogner-form and a nozzle.
Systematic experiments have been carried out latterly at the NSMB with some afterbody forms.
The quality of the propulsion as well as the
cavitation properties and the propeller excited
forces have been examined.
One of the solutions, favourable with a view to cavitation and propeller excited force
variat-ions, was the already formerly introduced Hogner cigar-shaped afterbody, which was later on
app-lied by A. G. Weser. Such a form is, however,
inferior with regard to the propulsive quality. The required engine power is about 3% more
than that for the comparable ship with normal
moderately U-shaped afterbody sections.
A considerable improvement was obtained by the application of such a Hogner form combined
with a nozzle. The nozzle, a German invention of Kort, consisting of a profiled ring around the screw (fig. 9), has the property that it reduces
the propeller loading by the stimulation of a
1. VECTOR DIAGRAM OF THRUST ECCENTRICITY 2° 72° HORIZONTAL THRUST ECCENTRICITY VECTOR DIAGRAM . OF TRANSVERSE FORCES 10 0 + 10 0 t +10
Vo OF PROPELLER RADIUS lo OF AVERAGE THRUST
HORIZONTAL TRANSVERSE
FORCE
circulation around the ring.
Moreover the tip vortexes of the propeller, which come into existence by flow round the
blade tips, are suppressed. These phenomena lead
to an increase of the efficiency.
Of course the ring has a certain resistance,
which increases with increasing speed of advance.
Depending on the fact whether the first men-tioned influences of the last menmen-tioned resist-ance dominate, a reduction or an increase of the engine power will occur. This includes that
im-provements may occur especially at high propel-ler loadings.
The nozzle has now been developed so far
that its application to seagoing cargo liners and tankers may be of advantage from a
hydrody-namic point of view, especially if combined with
the Hogner afterbody form.
Savings in engine power of about 5% can
already be obtained. Moreover the ring
homo-genizes the flow at the location of the screw. An additional advantage is that the ring takes
a part of the propeller thrust, by which the
cavitation danger is reduced and the absoluteforce variations are decreased.
A practical difficulty is the connection of
the ring.
The designer of the construction wishes to root a part of the ring in the ship (fig. 10). Of
+20
VERTICAL THRUST ECCENTRICITY
5
+5
THRUST VARIATIONS
8
Fig. 9 - Screw with nozzle.
course this reduces the effect of the nozzle (loss in circulation and reduced homogeneity of the
flow). Designers are looking for a suitable
solut-ion of this pure constructsolut-ional problem.
Nozzles are already applied to twin screw
ships. They are arranged completely free from the hull. This stern arrangement was made
among others to a Dutch built 6000 hp twin screw tug for the Suez Canal Authority. The
°
figures 11 through 14 show the nozzles and
nozzles arrangement for the model and the full-scale vessel.
Fig. 11 - Nozzle arrangement on model of Suez
Canal tug.
b. Large hub to diameter ratio propeller with
programmed blade control oHaselton»
pro-peller.
A new set up of an already old principle on
the propulsion of submarines was introduced last
year by Cdr. F. R. Haselton of the U.S. Navy.
The propeller consists in principle of a disk,
rotating around a horizontal exis. On this disk a number of blades are fitted, the pitch of same can be controlled continually. The Netherlands
Ship Model Basin, which examines this propeller
to model scale under contract with the Office of Naval Research (0.N.R.) of the U.S. Navy, ma-nufactured in the first instance a 3 m model of
the submarine, equipped with one propeller (figures 16 and 17).
The construction was made in such a way
that the pitch of the blades could be controlled simultaneously over the circumference as well as in a cyclic manner (figure 18).
Fig. 10 Single-screw ship model with ccHognero afterbody and nozzle.
--/
7
-Fig. 12 - Nozzles for Suez Canal tug under
cons-truction.
The propeller was also shrouded. The
per-formance of this propeller could be determined by means of a six component balance, while for
simplicity reasons the model was attached to
the towing carriage.
The actual submarine should be equipped
Fig. 13 - Hull section with nozzles fitted for Suez Canal tug.
with two similar propellers, a bow and a stern propeller. The advantages of this system are
clear. Manoeuvering can be realized by variation
of the magnitude and direction of the propeller thrust, also at zero headway, which is very
im-portant indeed.
Such a propeller can absorb very large power
due to its dimensions, anyhow a larger power than that of a normal propeller, the power
ab-sorption of which is restricted to about 60,000 hp
due to its dimensions and cavitation danger. Cavitation and vibration danger of a «Hasel-tom> propeller are restricted by the favourable arrangement in an almost homogeneous flow. The stern propeller is favourably located, com-pletely in the boundary layer of the submarine. The bow propeller which is contra-rotating to
the stern one will increase the ship resistance
by its slip stream. Novertheless this arrangement is expected to have a good efficiency. At present
the testing program is being continued with a submarine model, having a Icngth of 7 m and equipped with two similar propellers.
Ii was wondered wether this new propeller could possibly be applied to single screw
mer-chant ships.
It was tried to develop an afterbody form
to which this propeller could be fitted (fig. 13).
The advantages which could possibly be obtained
with such a propeller are here obvious as well, because the direction of the propeller thrust can
be controlled here also by cyclic pitch variation.
Manoeuvering can be realized also with zero
headway sothat the rudder can be omitted.
The location of the propeller is extremely
a
=SOP---;
_ , r
10
'
'Nek,
Fig. 15 Single-screw arrangement with complete
nozzle.
favourable. It works completely in the ship's
boundary layer which may give rise to high
hull-efficiencies.
Unlimited powers can be absorbed with less
or no cavitation and vibration troubles. The load of the propeller shaft can be adapted to the local conditions. Guide vanes can be applied
success-fully.
It appears from preliminary tests that the hull efficiency of the ship, which amounts to
about 1.2 for normal single screw ships is about 2.0 or more. It is true, that the efficiency of the
Fig. 16 - Front part of sub
marine model fixed to a six
component balance.
propeller itself was poor, but the propeller was not adapted to the model concerned.
The resistance of the ship with cigar-shaped
afterbody was equal to that of the corresponding
optimum conventional ship. Much systematic
research should be carried out as no data are available on the optimum propeller diameter
in connection with the number of r.p.m., the boss
diameter ratio, the shape of the shroud, the
Fig. 14 - Nozzle arrangement on
Suez Canal tug.
Fig. 17 - Shrouded bow propeller with programmed blade control.
Fig. 18 - Details of construction for blade control. shape of the blade sections, the number of blades
etc. The research is strongly developed and the results will be published as soon as possible.
c. Asymmetric afterbody.
As soon as the propeller starts rotating
be-hind a single screw ship, the flow distribution becomes asymmetric due to the tangential and radial speed components. This causes among
others lateral forces and eccentricity of the
Fig. 19 - Merchant ship model with large hub to
diameter ratio propeller and nozzle.
propeller thrust, which may lead to
consider-able bending moments. Research is being made
with a model having an eccentric shaft line
location, retaining whether or not the symmetric
hull form, in order to reduce the bending moments.
When writing this paper the results of the first tests are not yet known to the author but
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