See note inside cover
Lab.
y.
Scheepsbouwkunde
Teciinkche Hogeschool
Dell LNATIONAL PHYSICAL
LABORATORY
SHIP DIVISION
DEVELOPMENT OF EXPERIMENTAL TECHNIQUES
FOR SHIP MODEL WORK
by
A. Silverleaf
This Report is a reprint of a paper presented during
the Symposium held in Hamburg during September 1964,
to celebrate the 50th anniversary of the
Hamburgis che Schiffbauversuchsanstalt.
A Station of the
Ministry of Techno]ogy
SHIP REP, 73
Crown Copyright Reserved
Extracts from this report may be reproduced
provided the source is acknowleclge&
Approved on behalf of Director, NPL by
Mr. A. Silverleaf, Superintendent of Ship Division
Development of Experimental Techniques for Ship Model Work
Introduction
Remarkable changes have occurred in ship model laboratories
during the past 50 years, and these have been particularly rapid in the last 15 years. These changes are due primarily to the considerable increase in financial support for ship model
work because of the growing realisation of the wide and
valuable role that experiments with models can play in ship design. Two important general scientific advances have also had a major influence on the development of techniques for experiments with ship models; the wide-spread use of
elec-troflic equipment has made it possible to carry out many new
kinds of measurement, while the recent rapid adoption of high-speed computer methods has, among other effects, en-couraged the construction of elaborate facilities for compli-cated experiments which require computers for reducing and
analysing the great amount of data which they produce.
The most striking trend in recent developments in
tc(h-nique is the emphasis on the closer simulation in the
labora-tory of full-scale conditions encountered at sea. This stress on
environmental similitude is mainly evident in the design of
large experiment facilities such as seakeeping basins in which
irregular cross seas can be generated, water tunnels with
re-sorption circuits, flow channels for studying ship performance in river and tidal currents, and in the use of fully-instrumented, remote-controlled ship models for seakeeping and manuvring experiments. Coupled with this is the development of means of
making more realistic measurements with more direct
rele-vance to ship design; many such measurements are necessarily
of unsteady or fluctuating forces and motions, and depend
strongly on the use of sensitive, reliable and accurate electronic
equipment. Examples of this approach are measurements of motions and propulsion forces with ship models in irregular
waves, unsteady propeller force measurements in non-uniform
inflows in water tunnel and towing tank experiments, and
techniques for the study of unsteady cavitation phenomena on foils and other bodies.
There have also been significant advances in experiment
methods for basic studies in ship hydrodynamics. Knowledge of ship resistance has recently been extended by experiments in which individual drag components have been independently
determined from measurements of surface wave patterns and
detail wake surveys. Similarly, measurements with
multi-com-ponent force balances have provided much essential data on the forces and moments on surface and sub-surface bodies important in the development of new types of marine craft.
Forced motion experiments with ship models have also aided
the understanding of manuvring and seakeeping qualities; here recent techniques include the construction of large
manuvring basins with rotating arms, and the use of
electro-mechanical mechanisms for exciting either vertical or
horizon-tal motions on models in towing tank experiments. Other ex-amples of recent model techniques for basic studies are gas
ejection for simulating fully-cavitating or ventilated conditions for hydrofoils or propellers, and high-speed photography and
acoustic measurements for examining cavitation phenomena
generally
-A. Silverleaf
(Ship Division, National Physical Laboratory)
These developments in experiment techniques have already had marked effects on the design of conventional displacement ships, and are having a major influence in solving some of the
outstanding hydrodvnamic problems in the design of
uncon-ventional high-speed marine craft, including submarines,
foil-craft and air cushion vehicles. Many ship model laboratories throughout the world have made significant contributions to the developments which have occurred since the HSVA was
founded fifty years ago, and have freely shared their ideas and exchanged information on improved equipment and better
ex-periment methods. A thorough review of these changes with an adequate bibliography would be unacceptably long;
con-sequently the references in the text to the writer's own
labora-tory and to certain others are intended only as illustrations
drawn largely from personal experience; it would be possible
to draw equally appropriate examples from many other
sour-ces.
Environmental Conditions
Ship designers have made practical use of experiments with
scale models for many years. Two centuries ago quite elabo-rate attempts were being made to determine the resistance of
a ship model by pulling it through an artificial outdoor pond,
and such simple experiment conditions are still used with
success. However, it is now almost a hundred years since
Wil-liam Froude persuaded the British Admiralty to grant him £ 2,000 to build the first indoor ship model tank and to carry Out ifl it all the experiments necessary to solve 'for ever' the
problems of ship resistance. Although the simple basin which
was excavated in a field beside Froude's home was soon dis-mantled and abandoned, it marked the real beginning of the ship model laboratory of today. The first model tanks could
provide only artificial, ideal conditionscalm water and a
towed, partly-constrained hull modelbut the real conditions
in which ships operate should also he closely simulated in the
laboratory, and there has been a continuous attenipt to meet this need. The introduction of self-propelled models, the use of simple wavemakers, and the early development of water
Fig. i Large towing tank and high speed carriage at NPL
Fig. 2 seakeeping basin at NSMB; generation of oblique regular waves
tunnels to reproduce cavitation phenomena, significantly im-proved the simulation of realistic environmental conditions, hut until fairly recently such progress was hampered by lack of funds as well as inadequate instrument and equipment techniques. These difficulties have almost disappeared today in many maritime countries, in some cases because of naval
requirements, in others because of the growing needs of owners
and builders of merchant ships. The vastly increased
resour-ces now devoted to many ship model laboratories are strikingly
evident in their buildings, in the cize of their major experi-ment facilities (Fig. 1), in the complexity of the expensive equipment they use, and in the far greater number of staff
they employ. The outdoor pond has become a miniature indoor ocean.
Simulation of Sea Conditions
The accurate simulation of sea waves is essential to the use of models in studying the behaviour of ships in realistic service conditions. Until recently all model experiments in waves were confined to head and following seas, since these were all that were possible in conventional long, narrow towing tanks with
a wavemaker at one end. Such experiments were still further restricted to regular waves of different height and length by
the limited operating characteristics of the wavemaker, usually
either a mechanically operated plunger or hinged flap, and
ignorance about the nature of sea waves. During the past few years knowledge of wind and wave conditions at sea has been greatly increased by the oceanographer and meteorologist, who
have shown that even confused seas can be described in
mathematical terms by energy spectra. This has enabled sea
wave conditions to be simulated realistically in the laboratory by generating trains of irregular waves having closely defined
distributions of wave height with frequency. Such irregular
wave systems can be generated by controlling the mechanism
which actuates the motion of the wavemaker: at N.P.L.. for
instance, any type of discrete wave spectrum can be simulated by a harmonic synthesiser which combines simultaneously up to 16 sine waves of independent amplitudes and phases but of
fixed relative frequencies. The synthesiser output controls a
servo mechanism which in turn Controls the hydraulic activa-tors of a plunger-type wavemaker.
While experiments in such irregular head or following seas provide much more realistic information than those in regular.
uniform waves, these are not sufficient to determine the full
seakeeping characteristics of a ship form. Model experiments are needed at other headings and in confused seas, and durin
the past decade several special seakeeping tanks or basins have been built to enable ship models to be run under these conditions. These seakeeping basins are probably the most dramatic example of the development of techniques for ship model experiments; in plan form they are generally either broad rectangles or square, though other shapes, such as a sector of a circle, have also been employed, and the largest yet built (at D.T.M.B.) has an uninterrupted water surface more than 120 ni X 60 m (400 ft X 200 ft). Wavemakers are
fitted along two or more adjacent sides; when these are
arranged as a series of short units with a co-ordinated actuat-ing system controllactuat-ing amplitudes and phases of the separate
elements, it is possible to generate regulare waves of set height
and length travelling at almost any desired direction relative
to the face of the "serpentine'S wavemaker. Thefirstseakeeping
basin of this kind was completed in 1956 at the N.S.M.B. (Fig. 2); in this the wavemaker elements are hinged flaps operated and set mechanically, while in the later seakeeping tank at A.E.W. hydraulically operated plunger wavemakers
are fitted, and at D.T.M.B. the wavemakers are pneumatically
actuated. It is possible to generate irregular wave systems in such sekeeping tanks, but if these are to be strictly uni-direc-tional or long crested their direction of travel must be normal to the face of wavemaker. Some seakeeping basins have no
overhead carriage or other equipment to guide the model, and
are thus suitable only for independent remote-controlled
mo-dels. Other tanks have an overhead track, either fixed in
direc-tion or capable of adjustment relative to the wavemakers. Clearly the completely free model more closely resembles a ship at sea, but, as discussed later, an overhead guidance
sy-stem has some advantages to balance the additional complexity and cost which it introduces.
Model Response in Calm Water:
During the past few years there has been a marked growth of interest in model studies to determine the manuvring qua-lities of ships and submarines, and elaborate faciqua-lities have been constructed to provide a suitable environment for these
experiments. Such manuvring tanks may be no more than
un-restricted, protected expanses of calm water, similar to
sea-keeping basins, in which force and motion measurements can be made on completely free or partially guided models.
Alter-natively, they may be large circular or square tanks with a central pillar carrying a horizontal, rotating beam or arm to which the model is rigidly attached so that the hydrodynamic
forces exerted on it can be measured as it follows a constrained
circular path. Impressive examples of such facilities are the
circular manuvring basin in Paris for free models and the
more recent one at the Admiralty Experiment Works, Haslar, which also serves as a seakeeping basin (Fig. 3): at A.E.W. the overall water surface is about 120 m X 60m (400 ft X 200 ft) with a rotating arm 29 m (95 ft) radius towards one
end and wavemakers along two sides at the other end.
Flow Channels and Tanks
In spite of their obvious advantages, conventional towing
tanks have some limitations for experiments with ship models: there is always a limit on the time available for measurements
even in the largest still water tanks; it is difficult to simulate shallow water conditions effectively except by lowering the water level, and this is frequently not readily possible; it is often desirable to reproduce conditions due to a current in a river or close to a coast. To overcome these restrictions a
number of large flow channels and tanks have been constructed
corn-pared with the larger towing tanks, one or two are extremely
complex and expensive facilities.
The principle of a circulating water channel is basically simple; water is pumped continuously through a free-surface working section which forms part of a closed ioop in either the vertical or horizontal plane. However, it is exceedingly difficult to obtain a steady and uniform flow through the
work-ing section, so that the resistance measured on a floatwork-ing
body in a flow channel may not agree closely with that of the same body towed in a still water tank. Another approach is to
substitute a working section with a free surface for that nor-mally forming part of a water tunnel; although this also does
not give completely satisfactory flow conditions, it enables the
pressure over the free surface to be varied. A recent attempt to achieve a high quality flow in a circulating water channel has been made at NPL, where detail studies with a model preceded the construction of the large channel due for com-pletion next year; the circuit has several unusual features, including a weir at the exit from the working section to
iso-late it from any downstream flow disturbances.
Several tanks have been designed specially for experiments in shallow water, and some have facilities for simulating river
currents by inducing a flow along their length. Here the
pro-blem of flow steadiness and uniformity is not so acute, partly
because the speed of the current is low relative to the depth
of water, and partly because the slower boundary layers close
to the sides and bottom of the tank also occur in a flowing river or canal. The most elaborate flow tank yet designed is that nearing completion at NSMB; this has an unobstructed
water surface 60 m X 40m (200 ft X 135 ft), with depth
variable up to 1.2 m (4 ft) and facilities for generating waves
as well as currents.
Water Tunnels for Propeller Experiments:
The first large water tunnel to provide suitable conditions for studying cavitation on propeller models was constructed
/ by Parsons over 50 years ago, but recently two significant
developments have considerably improved the way in which a water tunnel can simulate the environment in which ship propellers, hydrofoils or torpedoes operate. The addition of a
resorption device (generally termed a resorber) enables experi.
ments to be carried out in water containing as much air or
Fig. 3 Seakeeping and manoeuvring tank at AEW
other entrained or dissolved gas as does sea water. The first resorber, designed for a relatively small, high speed tunnel
for fixed bodies at Caltech, consisted of a deep loop extension to a conventional Circuit, and others similar in principle have
since been incorporated in larger tunnels in Britain and the United States (Fig. 4). Alternative forms, based more on the
principle of an air separator, have recently been fitted to smal-1er tunnels or to circulating water channels with a controlled. pressure working section.
Fig. 4 Model of large water tunnel with resorber at NPL Schiffstechnjk Bd. 12-1965Heft 63
The second recent major development in water tunnels has been the closer simulation of the true flow conditions in which
the test body operates. The inflow to a ship propeller is far from uniform, and several devices have been designed to re-produce in a tunnel the irregular flow conditions at the stern
of a ship; these devices include sets of screens or gauzes, dis-torted models of part of the ship hull, and a honeycomb con-taining a grid of valves to act as a flow regulator (Fig. 5). Sudi
flow control devices are inserted in the tunnel immediately
upstream of the model in the working section and this
delibe-rate disturbance also encourages the installation of the shaft carrying the propeller model in its correct position upstream
of the model, rather than in the conventional downstreanì
posi-tion intended to give a clear, uninterrupted inflow. Experi-ments in water tunnels with propeller models in these more realistic, irregular flow conditions rather than in idealised uniform inflows reflect the emphasis on simulating true
en-vironmental conditions.
Realistic Measurements
Closer simulation of the true environment for ship model
experiments must be accompanied by realistic measurements of the environment itself and of model performance if full
ad-vantage is to be taken of the capabilities of the elaborate and expensive facilities now found in many ship model labora-tories. Considerable progress has been made during the past
few years in developing methods of measuring quantities which
have more direct relevance to ship design than the restricted
information which was all that could be obtained before
elec-tronic instruments were available. Mechanical and electrical
measurement devices are still important, and have been greatly
improved in quality and extended in range, but the
introduc-tion of electronic methods has hadand is having a
domi-nant influence on the work which can be carried out with ship
models. This is particularly evident in measurements of un-steady or fluctuating forces and motions, which are of
Li-creasing importance, but in many cases the measurement of a nominally steady quantity is facilitated by the use of electronic
equipment, and may even be impossible without it. These
general points are illustrated here by brief descriptions of
recent typical developments in measuring methods and
appa-ratus; these and many other similar developments are
des-cribed in detail in numerous papers and surveys.
Fig. 5 Flow regulator in water tunnel at NSMB
Ship Models in Waves:
In model experiments in waves in either towing tanks or
seakeeping basins it is essential to measure the wave systems
themselves, and to do so in a manner which determines the precise conditions encountered by the model throughout the experiment run. For several years the standard tool for this purpose has been the wave height probe in which a sensing element is partly immersed in the water so that thi wave
mo-tion affects its electrical resistance or capacitance, while attempts have also been made to develop wave-sensing
devices in which there is no physical contact with lhe water, sudi as the sonar probe from DTMB recently evaluated in several laboratories. In a tank or seakeeping basin the wave
system often varies from point to point; a single fixed probe is
then inadequate and a grid of probes can be used, although
this is expensive and requires an elaborate method of analysis
to derive the conditions at any specified point at a particular moment. Alternatively, the wave probe can travel with the model, provided that it is linked in some way io a carriage
or beam which provides a continuous fixed datum. However,
the wave form recorded by a probe travelling with a model
moving at steady speed through an irregular though repetitive wave system is non-periodic, and a very large number of
mea-surements of the height of the water surface must be taken at short intervals to define accurately the components of the
wave system. Also a travelling wave height probe which
pier-ces the free surface can only be used at speeds below that at
whidi ventilation affects its behaviour.
It is now possible to make experiments in eithor calm water
or waves with ship models which are completely free of any constraint and are guided and controlled entirely by a radio
or similar control system operated from a shore station
(Fig. 6) ; if required, this control station can be mounted on a travelling carriage to maintain reasonably close contact with the model. Such a control system consists of a transmitter and
aerial ashore with a transistorised receiver and aerial aboard
the model; the output from the receiver actuates
battery-driven motors which control the model propellers and rudder.
The model track can be determined by a sonar position plot-ter; this consists of a radio link between shore and model to
actuate an ultrasonic transmitter aboard the model which
sends a pulsemodulated signal through the water to several shore receivers. A completely free model of this kind may also carry gyros to measure its continuously, with sensitive galvanometer film recorders coupled to their outputs, though it is possible to transmit this information to recorders ashore
by a telemeter link (Fig. 7).
In experiments in seakeeping basins the model course may be kept unchanged and the wave direction varied, or the waves
kept fixed in direction and the model course altered. Both techniques have been adopted for cross sea experiments, and both raise similar problems as to the extent and method of guiding the model. In cross seas a ship yaws at a frequency equal to the wave encounter frequency and also has a mean angle of yaw relative to the track followed by its centre of
gravity. A completely free model can be steered to maintain a
set heading as a ship does, even if this requires a
remote-controlled automatic steering device, which may be expensive and difficult to accommodate in a small model because of the
disturbing frequencies it will encounter. On the other hand,
a model guided to some extent by an overhead travelling
car-riage has to move along a pre-set track but its heading is not fixed and it will take up that corresponding to its mean yaw
for the prevailing wave conditions. Because the datum provided
by the travelling carriage is available for the measurement of motions, the equipment carried in the model can be reduced
I- i. 7 AEW instrumented free model
BRIDGE CONTROL UNIT
YAW SUPPLY RECTIFIER
YAW AND YAW VELOCITY GYRO UNIT MAIN MOTOR CONTROL UNIT MAIN GEARBOX
RUDDER CONTROL
GEARBOX AND
QUADRANT
Unsteady Propeller Forces:
For a ship model in waves the propulsion forces vary
con-tinuously about the mean values for equivalent steady calm
water conditions, while unsteady hydrodynamic forces exerted
on the blades of a propeller as it rotates in the non-uniform flow field at the stern of a ship are a major factor in exciting undesirable vibration in the propeller shaft or the ship struc-ture. These- unsteady propeller forces can be assessed from
measurements on models of the fluctuating forces and moments
induced in the shafting, and there has been considerable
activity recently in developing dynamometers of this kind for
use either with ship models in towing tanks or in similar ex-periments in water tunnels. The direct dynamic calibration of such unsteady force dynamometers is difficult; although
cali-bration rigs have been designed and built, their accuracy and reliability are still rather doubtful. The unsteady forces are
generally measured by inductance gauges or resistance strain
gauges attached to the propeller drive shaft either within the propeller boss or close to it; a measuring head of this kind
which responds to all six components of force and moment has been developed at NSMB, and this is used with a complex re-cording system which isolates in turn each desired harmonic of each fluctuating quantity (Fig. 8). A different method has been adopEed at NPL to eliminate effects due to the elastic
proper-ties of the propulsion system and shafting; in this apparatus. initially confined to the determination of thrust and torque
fluctuations in calm water, the natural frequencies of axial and
torsional vibration are much higher than the highest fluctua-tion frequencies to be measured. To ensure that vibrafluctua-tion or distortion of the hull model does not affect the dynamic
be-haviour of the shaft system, in this approach the propeller, its
shaft, and the motor drive are mounted on a rigid frame iso-lated from the hull, while the propeller shaft carries a
fly-Fig. 6 Remote-controlled model in seakeeping tank at NPL
RADIO
AERIAL-ROTARY INVERTER (115v 400c/s 3)
2-5 Kc/ OSCILLATOR SUPPLY JUNCTION BOX
SHAFT REVOLUTIONS
MAIN BATTERIES
ROLL PITCH AND HEAVE GYRO UNIT
WATER LEVEL PROBES
MAIN MOTOR (24v 14 H.P.)
RUDDER CONTROL SERVOMOTOR AND GEARBOX
RADIO LINK
COMMAND UNIT TRANSISTORISED CLOCK -- RECORDING GALVANOMETER CONTROL JUNCTION BOX WATER LEVEL BRIDGES DOUBLE INTEGRATING UNIT AUTOMATIC STEERING CONTROL UNIT
wheel and is belt-driven from the motor to avoid mechanical
vibration effects from a gear drive.
Propeller Blade Deflection:
The deflection of the blades of a propeller when it operates under load may be sufficient to affect its effective pitch,
par-ticularly for highly-loaded screws with wide blades or fully-cavitating propellers having blade sections with very thin leading edges. Some attempts have been made to measure
local blade strains on ship screws at sea and on model
propel-lers in water tunnels by strain gauges attached to the blade
surfaces, and this technique is now well established. An alter. native approach, also used successfully at N.P.L., depends on precise optical methods of observation and measurement
Fig. 8 NSMB dynamometer for measurement 01 fluctuating propeller forces
Fig. 9 Measurement of propeller blade deflection at NPL
(Fig. 9); the deflections of a propeller blade operating in a
water tunnel are determined at a number of points over its
sur-face by sighting their axial positions with an alignment telescope, first with the propeller at rest and then with it
rotating. Each point under observation is illuminated in turn
by a precisely controlled stroboscopic Hash so that ils angular
position remains unchanged. The alignment telescope is set normal to the propeller axis and mounted on a lathe saddle
which can be moved parallel to the axis on a lead screw; alter-natively, if the axial movement of the point under observation
is small, this can be determined using the internal prism
sy-stem of the telescope without moving its body. The observation
points are defined by photographically reproduced marks on gelatin film stuck to the blade surface. A second telescope is used to observe the position of a reference datum on the pro-peller boss. After initial difficulties had been overcome, the
accuracy of measurement of blade deflection was estimated at ± 0.05 mm (± 0.002 in.).
If strain measurements on model propellers are to provide
realistic data from which to predict full scale stresses, then the deformation under load of the model must be similar to that of
the ship screw. This condition of hydroelastic similarity may
be closely simulated by making the model from a more flexible
material, such as an epoxy resin with a suitable filler to ad-just the elastic moduli in bending and torsion as required.
In-deed, hydroelastic similarity is an essential feature of growing importance in model experiments to investigate deflection and flutter characteristics of many bodies which move at high speed through water; these now include hydrofoil units for foilcraf t and stabiliser fins as well as propeller blades.
Flow Observation and Measurement:
The observation and measurement of real flow characte-ristics are important in many experiments with ship models,and numerous techniques have been developed either for
gene-ral use in a wide range of experiments or for particular
pro-blems. Methods recently devised to visualise local and general flow paths in water include improved oil and paint techniques
to define flow directions on the surface of hull models, and
dye ejection methods to demonstrate the character of the
boun-dary layer flow on the blade of a rotating model propeller. Studies of flow separation at the stern of full form hulls have been made using short threads attached to the model surface and by photographing the paths of clusters of small particles
having neutral buoyancy. However, there are still many design
problems for which adequate flow visualisation and
measure-ment methods are not yet available. For instance, it is often
suggested that bossing design would be improved by a reliable
and simple method of determining flow directions over the
whole of the bossing with the propeller operating.
Observation of any flow cavitation is clearly important in many design problems. For a fixed body, such as a rudder or
a hydrofoil, such observations are relatively simple. The
cavi-tating region is clearly visible, and the fine structure of the
flow is well defined by short-exposure photographs. For pro-peller models and other rotating bodies, short exposure photo-graphy is also a valuable tool (Fig. 10), while visual observa-tionS are made using stroboscopic illumination, and
consider-able improvements have been made in the capacity and re-solving quality of commercially available stroboscopes. The
television camera and video-tape recording have also been used successfully to observe cavitating flow past propellers. Knowledge of the pressures within and on the boundaries of
flow cavities is also of importance, and techniques have been developed for their measurement. The pressure on a fixed solid surface within a cavity can be measured fairly simply by
con-ventional surface pressure tappings and openendled pressure
leads to a manometer, provided precautions are taken to ensure
that the pressure leads are completely air-filled and free of liquid when the measurement is made. The pressure at any point in a cavitating flow can be measured by an air-blown
manometer which operates equally well in a water-filled region, a gaseous cavity or a region of mixed flow.
Basic Studies
Improvements and developments in experiment techniques have had a profound effect on investigations into fundamental aspects of ship hydrodynamics. Many significant changes were
directly stimulated by doubts about basic principles, as much as by the need to extend the range and scope of experiments.
Thus anxiety about scale effects on resistance and propulsion
experiments with small models was a major impetus to the construction of very large towing tanks to permit the use of
larger models, just as the need to know more about the
hydro-dynamics of high speed craft has been responsible for the development of novel techniques to study the wave making
of hovercraft and of equipment for making measurements very
rapidly. Here also electronic methods have had a major in-fluence, and these changes are illustrated here by brief
des-criptions of some typical developments.
Forces on Towed Bodies:
Measurements of the drag or resistance of a towed hull
model or other body at a series of steady speeds in calm water
are still among the most important in basic studies of ship
hydrodynamics. The accurate determination of the
resistance-speed curve for a towed body depends principally on three factors; the performance of the towing vehicle, the
measure-ment of speed, and the measuremeasure-ment of the towing force: con-siderable improvements have recently been made in all these.
The speed of a towing tank carriage is now commonly kept extremely uniform during the measuring run by an automatic
speed-control system and is accurately measured by counting
pulses triggered either by markers along the tank or by
divided disks rotating with the carriage wheels. However, these are related to the speed of the carriage over the ground, rather
than to the speed of the towed body through the water. Due
to thermal convection currents and residual components of the disturbance created by the preceding experiment, the water in
a towing tank is never really at rest. The wave system due to the model can be rapidly absorbed by side beaches or wave suppressors as well as end beaches, and these are now often
fitted in tanks in which precise measurements are to be made
The drift or current set up in a tank by the viscous wake of the model is probably the major uncertainty in determining speed, and a periodic motion representing the fundamental oscillation of the water in the tank may be superimposed on
this drift. Attempts to minimise these effects include anti-drift
curtains across the tank which are carefully lowered before each run. but an accurate method of measuring mean speed
through the water has yet to be devised and proved.
The most direct way to determine the resistance of a towed
body is to measure the horizontal force needed to maintain
it in motion at a set speed. Even the conventional mechanical resistance dvnamometer long used for such measurements has
been improved recently, and modern dynamometers of this type, incorporating a lever system with a spring and a dead-weight, are frequently semi-automatic in operation. An
alter-native approach is to use a balance without bearings or pivots,
in which the towing force is determined by measuring the deflection of a fairly stiff flexure forming part of the sup-porting structure. Several such dynamometers have recently
been developed, since they have many advantages, particularly
for non-buoyant bodies. In these flexure dynamometers the
whole force is taken on a stiff spring in the form of an elastic element working at a low strain level. This strain is measured by some form of electro-mechanical transducer, the output of which is amplified electronically and recorded either as an
ana-logous trace or in digital form, often on punched tape.
Although in principle such a dynamometer presents no serious
problems, in practice this is often not so, since difficulties
arise in accurate calibration of the system and its components. It is necessary to consider carefully the use of electrical
dyna-mometers for force measurements, since often a simpler
mechanical type is quite adequate. However, if the forces can-not be measured in any other way, then clearly a flexure
dyna-mometer should be used; an example of this is the series of
multicomponent balances developed at N.P.L. to measure the six forces and moments on restrained, towed bodies.
Several ways of inferring the hydrodynamic forces on a towed body from indirect measurements have recently been developed and improved. Thus the lift and drag of a hydro-foil can he derived from measurements of the local pressure at a number of positions on its surface. In this technique as
originally used a series of open-ended flush pressure tappings
in the surface of the foil were connected to a simple
multi-tube manometer. For extensive pressure distribution measure-ments on foils towed at high speeds this method is extremely laborious, but the development of a hydrostatic scanning valve
enabled a large number of pressure to be measured during a
short duration experiment run using only a few electrical
transducers to convert all the pressures into a form suitable
for direct digital recording.
Another indirect method of determining the wave resistance of a towed body is to deduce it from measurements of the sur-face wave pattern which it creates. This technique has recently been developed and used with considerable success; accurate measurement of the surface wave disturbance is essential, and this has been achieved with the same type of wave height probes
used for measuring waves in studies of seakeeping qualities. A further measure of the resistance of a towed body can be derived from measurements of the total and static heads at a number of points in a transverse plane downstream of the
body. This technique has also been improved recently, although the physical significance of this wake traverse resistance is not
generally agreed. All these developments in technique have
Fig. 10 Fully-cavitating propeller in water tunnel at NPL
enabled considerable progress to be made in studies of thc
components of resistance.
High Speed Marine Craft:
The very great increase in active interest in high speed marine craft of all types, particularly hydrofoil ships and
hovercraft, has been vividly reflected in the development of
model experiment methods. The designer's approach to these
craft is essentially that of the aircraft designer rather than of the traditional naval architect, largely because the problems involved have many resemblances to those encountered in developing high performance aircraft. Indeed, many aircraft manufacturing firms are now actively engaged in the design
and construction of hovercraft and foilcraft. and their attitude
to model experiments is having a striking effect on the work of many ship model laboratories. In addition to experiments relating to specific proposed craft, there has been a marked
increase in the demand for basic hydrodynamic data as a
back-ground to design, and this has stimulated a wide range of
novel model techniques.
These new methods are, of course, mainly attempts to over-come some of the difficulties inherent in experiments in high speed flows. Thus experiments at speeds of 10 m/s and above
in even the largest of existing towing tanks are severely
re-stricted in time, and high speed measuring and recording tech-niques, often initially developed for other purposes, are now an accepted part of many experiments with models of
conventio-nal and unconventioconventio-nal marine craft. The adaptation of these measuring techniques to the special conditions of a towing
tank sometimes presents difficulties, but these can almost al-ways be overcome, and these advanced instrument technique are now applied to a wide range of problems.
Basic investigations into ventilated and cavity flows, of par-ticular importance in relation to high-speed craft, are now far
more numerous than even a few years ago, with a marked trend to move from studies of steady phenomena to more
detailed investigations of fluctuating unsteady flows. One
recent innovation in cavity flow experiment techniques is that of forced cavitation, in which air or some other gas is
delibe-rately introduced into a region in which a cavity will form naturally at a low flow pressure in order to stimulate its
for-mation at higher flow pressures; in such cases it is obviously
necessary to measure the pressure within the cavity to define
the effective cavitation number, and it is often also necessary
to measure the rate at which air is supplied to maintain the
cavity. There is also a growing demand for knowledge of the hydrodynamic effects of moving pressure disturbances of the
kind created by an air cushion; this is a direct consequence - 115 - Schiffstechnik Bd. 12-1965Heft 63
of the recent intensive development of air cushion vehicles of
many types, particularly hovercraft, in which model experi-ments have played a significant role. For such investigations many of the models are extremely complicated, with self-contained power units for lift and propulsion, and are very
different from the simple models of displacement ships almost universal only a few years ago.
Coniptiter Applications
It is difficult to imagine a modern ship model laboratory
without ready access to a high speed computer; this is perhaps the clearest indication of the effect whid) computers have had on ship model experiment techniques. Many calculations which
were possible in principle but far too laborious to be carried out in practice are now taken for granted as routine opera-tions; many experiments which are now an accepted part of ship model work were previously almost impossible: many relatively straightforward calculations are now carried out
more quickly, economically and accurately. These
consequen-ces of the adoption of computer methods are not confined to
ship model experiments, and it is possible to find more striking
examples of the influence of computers in other branches of applied science; however, the ultimate effects in ship
hydro-dynamics may well be far reaching.
Design Calculations:
Many of the routine calculations carried out in a ship
design office must also be made in the laboratory for scale models. Thus hydrostatic calculations, including stability
characteristics, for ship models are frequently made on
com-puters, with special devices for transferring body plan offsets to punched tape. Model laboratories have taken a leading part
in developing methods of defining ship hull forms in mathe-matical terms, and these calculations, typical of those too
laborious for manual computation, depend entirely on the use
of computers. Most propeller design methods which take
account of the flow in any detail also demand computers, and
here again ship model laboratories have taken a leading part in their development to such an extent that such calculations are now regarded as routine in many laboratories.
Calcula-tions of the flow around hydrofoils and other bodies are under
intensive development, and related basic research linking theory and experiment is now making appreciable progress
largely because computers enable these flow characteristics to be calculated.
Data Analysis:
The processing and analysis of data from ship model ex-periments naturally provides many opportunities for useful
applications of computers. Several independent attempts have
been made to process the data obtained from resistance and
propulsion experiments with models in calm water, since here a relatively small amount of information is subjected to lengthy
and involved manipulation, particularly if the data is to be
faired by machine and converted to ship values by more than
one extrapolation procedure. Indeed, the computer program written at N.P.L. to analyse and fair the raw data obtained
directly from such model experiments is exceedingly lengthy and complex. and involved the development of novel
smooth-ing and cross fairsmooth-ing procedures suitable for resistance cur-ves for all types of cur-vessel and for accurate interpolation in propulsion experiments carried out at varying speeds and
loadings. Computer programs of this kind have also been
pre-pared to analyse the data from propeller experiments in water
tunnels. Such programs have done more than improve the speed and accuracy with which final results can be obtained; in many cases they have indicated changes in detail
experi-ment procedure which would improve the consistency of ship
performance estimates, and have thus affected the planning
of model experiments.
The analysis of data from experiments in which rapidly
varying quantities are measured, such as experiments to study propeller vibration effects, is very much dependent on high speed computers. Any attempt to resolve complex records of
fluctuating forces or motions into their harmonic components is far too tedious for manual calculation of all but the smallest
sample, and computer analysis is now almost standard
prac-tice.
An important application of computer methods to ship
model work is the use of statistical analysis techniques to
cor-relate the information available in a large body of previously
unrelated experiment data. Regression analysis applied to the resistance coefficients for an extensive sample of trawler
models, for instance, showed that the resistance of any one model could be calculated with practical accuracy from the values of six principal hull form parameters clearly defined
by its geometry. Further, the computer analysis indicated that lower resistance coefficients would be found for forms having
different combinations of these parameters from any in the sample; subsequent experiments confirmed this prediction,
and this approach to the planning of experiments with metho-dical series of hull forms is likely to have a growing influence.
Experiments in Waves:
The development of techniques for seakeeping experiments
with ship models depends strongly on the use of high-speed computers. Indeed, it is probable that the elaborate and
ex-pensive seakeepirig basins built recently would not have been
constructed if computers were not available for the
calcula-tions essential to model experiments in complex wave systems. The analysis of motion records, the computation of significant wave heights, the calculation of motion statistics, of response
spectra and cumulative motion distribution diagramsall
these require computer programs if they are to be carried out utilising sufficient input information to give realistic results
in reasonable time. Comprehensive experiments with a single
model in waves may produce very long lengths of punched
tape; without computer analysis this cannot be handled.
Computers also play an important part in formulating ex-periment conditions. Thus the settings of the harmonic syn-thesiser controlling the wavemaker at NFL are calculated by
a standard program to give any required wave spectrum
for any scale of model, and similar programs are used to derive
motion responses at oblique headings from experiments
car-ried out in head seas.
Model Manufacture:
Propeller models are now made by processes far more pre.
cise than those previously used, and these processes are readily
adaptable in principle to machine tools controlled by
com-puter programs. At NSMB propellers have recently been
completely cut from a simple casting blank by such a com-puter-controlled profiling machine, and it is likely that hull
the manufacturing process to a method of defining the hull
form in mathematical terms.
General Comments
The range of work carried out in a well-equipped ship
model laboratory today is far greater than even a decade ago,
and is still increasing rapidly. The practical value of model experiments for many purposes is now widely recognised. and the problems facing the experimenter are quite different
from those a generation ago. The standard of accuracy to aim
at, the variety of types of equipment available, the growing number of laboratories engaged in developing techniquesall these pose questions which, although not new to the
experi-menter, may demand different answers from those which were
obvious and adequate not so long ago. Today the model ex-perimenter must frequently remind himself of some general principles which have not changed and which can still guide
his decisions.
First, the accuracy required in any experiment must be
realistically assessed. The natural tendency is to strive for the highest accuracy possible; sometimes this is unnecessary, and sometimes a false impression of the accuracy achieved is based
on an ability to ignore extraneous errors which are too
awkward to control. Second, the more simply an experiment can be carried out, the better; complication is not a virtue, only a necessary evil. Third, experiment techniques should
be designed to give the best overall performance; in complex experiments it is easy to overlook an unavoidable weakness in one stage which can adversely affect the whole sequence of operations. Thus, for instance, the choice between a novel
automatic data recording system and a simple but well-proved
dial indicator may not be the obvious one. Fourth, in com-paring techniques used in different laboratories there is a natural temptation to believe that what is most convenient for
oneself is the best for everyone; there is generally more than
one good solution to every problem of experiment technique.
Propellertheorie, Hydrodynamische Probleme von Prof. Dr.-Ing. W. H. Isay
Springer-Verlag BerlinGöttingenHeidelberg 1964
247 Seiten, Format 16 X 24, Preis 49,50 DM
Wie alle Gebiete der Naturwissenschaft und Technik so hat auch die Propellertheorie mit ihren Anwendungen in den ver-gangenen Jahren eine schnelle Entwicklung genommen. Heute besteht auf jedem Teilgebiet der Wissenschaft sowohl für die
interessierten Forscher als auch für Anfänger ein dringendes Bedürfnis nach Büchern oder Abhandlungen, in denen eine
zusammenfassende Darstellung des gegenwärtigen Standes der Forschung und ihrer wichtigsten Ergebnisse gegeben wird. Ein solches Bedürfnis liegt vor allem deshalb vor, da heute in
An-betracht der großen Zahl der in der Fachliteratur weit
ver-streuten Veröffentlichungen leicht der 1berblick verloren geht.
Um hier Abhilfe zu schaffen, gibt es mehrere Wege: Einer besteht darin, wichtige Einzelarbeiten, die untereinander
ge-nügend Berührungspunkte haben, in einem Band zusammen-zufassen, der dann für Spezialisten und interessierte Studenten geeignet ist. Eine andere Möglichkeit ist, daß ein Fachmann in einer Monographie eine Einführung in das betreffende Gebiet
und eine tlbersicht über den gegenwärtigen Stand der
Er-kenntnis gibt. Schließlich ist es angebracht, ein Buch für einen allgemeineren Leserkreis zu schreiben, sobald ein Fachgebiet eine bis zu einem gewissen Grade endgültige und
abgeschlos-sene Entwicklung genommen hat.
Das vorliegende Buch scheint nicht genau in eine der drei eben genannten Kategorien zu gehören; es stellt vielmehr
eher eine Mischung von ihnen dar. Dieses ist erklärlich, denn viele wichtige Probleme der Schiffspropeller sind heute noch in der Entwicklung, und es besteht zum Teil noch keine endgül-tige Klarheit über die wirksamsten Methoden zu ihrer erfolg-reichen Behandlung. Unter diesen Umständen muß es für den
Verfasser eine erhebliche Arbeit gewesen sein, das Buch zu
schreiben.
Vom Leser des Buches werden schon gewisse
Grundkennt-nisse der Materie vorausgesetzt. In diesem Zusammenhang wäre ein kurzer Rückblick auf klassische Arbeiten und ihre Grundlagen von Wert gewesen. Es wird dann leichter ver-ständlich, daß eine Traglinientheorie mit der Voraussetzung mäßiger Belastung für Luftschrauben mit ihren schmalen
Blättern mehr geeignet ist als für Schiffspropeller. Vergleiche mit Experimenten in den entsprechenden Fällen würden diesen Punkt noch weiter aufklären.
Kapitel I beginnt mit der Entwicklung des Formelsystems
der Traglinientheorie für einen Propeller in instationärer Strömung; dieses wird vom Verfasser als Grundlage der
Theorie des Schraubenpropellers bezeichnet. Die freien Wirbel
werden dabei auf regulären Schraubenflächen angeordnet, entsprechend der Bedingung optimaler Belastung der
Pro-pellerflügel. Für einen Anfänger dürfte es immerhin schwierig sein, die genaue Bedeutung dieser und anderer Annahmen zu
erfassen. Nach diesem Anfang wird die stationäre Propeller-strömung behandelt. Die verschiedenen Induktionsfaktoren werden eingeführt und diskutiert. Ohne auf Einzelheiten ein-zugehen, wird auch die klassische Arbeit von Goldstein
er-wähnt. Mit Hilfe der Weissingerschen erweiterten
Traglinien-theorie wird aus der Randbedingung am Propellerflügel eine Integralgleichung für die Flügelzirkulation abgeleitet.
Ver-schiedene andere Rechenverfahren werden beschrieben, jedoch
ohne eingehendere Erläuterung. Im letzten Kapitel der sta-tionären Theorie wird eine kurze Ubersicht über die Trag-flächentheorie mit einem Verzeichnis der Literatur gegeben. Der nächste Abschnitt ist der instationären Propellertheorie gewidmet. Zunächst wird die instationäre Strömung ent-sprechend einem ungleichförmigen aber stetigen (von Lerbs
angegebenen) Nachstromfeld behandelt. Danach wird der all-gemeine Fall einer zeitabhängigen Schwingungsbewegung untersucht und schließlich die instationäre Tragflächentheorie
in Anlehnung an die Arbeiten von Hanaoka diskutiert. Im
vierten Abschnitt von Kapitel I werden Gegenlaufpropeller
be-Bücherschau
handelt; jedoch ist dabei der interessante Vergleich zwischen Gegenlaufpropellern und Einzel- oder Doppelschrauben nicht erwähnt, der kürzlich von van Manen durchgeführt wurde.
Kapitel II ist den Ringflügeln und Düsenpropellern
gewid-met. Zunächst wird die Weissingersche Theorie der Ringflügel dargestellt. Danach wird das Problem des Düsenpropellers
be-handelt, und zwar mit Hilfe der Traglinientheorie für den Propeller und der Tragflächentheorie für die Düse. Für die Auflösung der dabei auftretenden Integralgleichungen wird die Zirkulation bezüglich der Winkelkoordinate durch ein Fourierpolynom approximiert. Den Abschluß des Kapitels
bildet die einfache Näherungstheorie für Düsenschraubcn von Dickmann und Weissinger, die im wesentlichen auf Impulsbe-trachtungen bzw. auf der Strahltheorie basiert.
In Kapitel III wird die Wechselwirkung zwischen Schiff und
Propeller diskutiert. Das Druckfeld eines Propellers wird be-rechnet in Anlehnung an die Arbeiten von Breslin und seiner Mitarbeiter. Die Behandlung der Wechselwirkung basiert im wesentlichen auf der Theorie von Dickmann; der Propeller wird dabei durch eine Senkenscheibe ersetzt. Die gegebene
Darstellung ist mehr mathematisch ausgerichtet, und die physikalische Erklärung von Weinblum (die der Besprecher besonders einleuchtend und interessant findet) wird nicht
er-wähnt.
Das Kapitel IV über Voith-Schneider-Propeller enthält im
wesentlichen eigene Arbeiten des Verfassers, der diesem Ge-biet offenbar einen bedeutenden Teil seiner wissenschaftlichen Arbeit gewidmet hat. Immerhin gibt es im Zusammenhang mit dieser Propellerströmung noch einige strittige Effekte, so daß
auch eine Diskussion der von anderer Seite entwickelten
Ge-danken und Ansätze interessant und wertvoll wäre.
Das letzte Kapitel behandelt die Theorie der Unterwasser-tragflügel und den Einfluß der freien Wasseroberfläche auf untergetauchte Propeller. Die Beifügung eines Abschnittes mit einer eingehenden Untersuchung der Strömung um Was-sertragflügel (auf diesem Gebiet hat der Autor auch selbst
gearbeitet) gibt dem Buch ein besonderes Gepräge. (Das
letztere Gebiet allein könnte natürlich auch in einer anderen Monographie behandelt werden). Auf sehr interessante Pro-bleme führt die Behandlung untergetauchter Propeller nahe
der Wasseroberfläche.
Abschließend erscheinen dem Besprecher noch einige Punkte erwähnenswert:
Ein gegenwärtig im Mittelpunkt des Interesses stehendes
Gebiet ist die Tragflächentheorie für Propeller. Im stationären Fall ist diese Theorie bereits ziemlich weit entwickelt, und der
Wert des Buches hätte sicher gewonnen, wenn dieses Gebiet eingehender behandelt worden wäre. Daneben erscheint eine Diskussion der Einzelheiten der verschiedenen numerischen Lösungsmethoden wichtig, sowie der Möglichkeiten für ihre Verbesserung. Offenbar um den Umfang des Buches zu be-schränken, wurden die Probleme der superkavitierenden" und ventilierten" Propeller nicht behandelt; dennoch spielen diese zweifellos eine wichtige Rolle. Schließlich enthält das theoretische Buch keinerlei eingehende Diskussion
experi-menteller Untersuchungen und Ergebnisse. Im allgemeinen ist
auch dem Vergleich zwischen Theorie und Experiment nicht die ihm bei der Beurteilung theoretischer Resultate
zukom-mende Bedeutung beigemessen worden.
Es ist zu hoffen, daß neue und weitere zu erwartende Er-gebnisse auf dem Gebiet der Propellerströmung den Autor
zu gegebener Zeit veranlassen werden, eine neue und in ihrem
Umfang erweiterte Auflage des Buches vorzubereiten, in der auch die eben erwähnten Probleme gebührend berücksichtigt
werden.
Nichtsdestoweniger ist das vorliegende Buch zur Zeit offen-bar (abgesehen von einem russischen Buch) das einzige Werk
über die moderne Theorie der Schiffspropeller. Es dürfte ein
nützlicher Bestandteil der Bibliothek der an diesen
Pro-blemen interessierten Personen sein.
