THE NETHERLANDS
SHIP MODEL BASIN
BY
Prof. Dr. ¡r. W. P. A. VAN LAMMEREN
SUPERINTENDENTPublication No. 110 of the Netherlands Ship Model Basin, Wageningen, Holland*
* This publication is composed of lectures and articles by L. Troost, W. P. A. van
CONTENTS I. Plan, organization and management
Introduction
Form of organization Exploitation
Survey of the development of the N.S.M.B.
II. Equipment and working-methods
A. The building
B. The manufacture of the ship models
C. The manufacture of the screw propeller models D. The model basin with the towingcarriage
The basin
Towing-carriage and rails
E. The cavitation tunnel III. Measuring Technique
A. Review 01 lests and measurements 1. Tests carried out in the basin
Resistance tests Propulsion tests
Open water propeller tests Steering tests
Rolling tests Wake tests
2. Tests in the cavitation tunnel With screws
With profiles
3. Trial trip measurements
4. Measurements taken on the 72 ft. long standard model boat of the
N.S.M.B.
B. Measuring Equipment
1. Equipment used for tests in the basin Resistance dynamometer
Inboard dynamometer (Gebers system) Inboard dynamometer (Gutsche system) Inboard dynamometer (Kempf system) Apparatus for open water propeller tests
Calibrating apparatus for dynamometers
Centrifugal dynamometer
Apparatus for submarine tests Apparatus used in steering tests
Apparatus for rolling tests
1. Apparatus for wake tests and the measurement of the rotation component of the water in the screw disc
2. Equipment used in the cavitation tunnel Thrust- and torque-measuring apparatus Stroboscopic lighting apparatus
Apparatus for measuring speed and pressure
Apparatus for measuring pressure-distribution, lift and drag of
profiles
3. Equipment used on trial trips Log apparatus (Kempf method)
Maihak torsionmeter Anemometer
Amsler towing-force dynamometer
4. Equipment used on the standard model-boat Resistance dynamometer
Propeller dynamometer Speed-measuring apparatus
I.
Plan, organization and management
Introduction.
The Central National Research Council of the Netherlands, whichcame into being in 1932, now controls about 50 laboratories, institutions and experimental
groups with about 2400 employees including approximately 400 university-trained staff members.
An institution within the framework of this large organization is the Nether-lands Ship Model Basin Foundation (Stichting NederNether-landsch Scheepsbouw-kundig Proefstation; N.S.M.B.) at Wageningen. In contrast toa large majority
of the institutions above referred to, which are under the direct control of the Central National Council (T.N.O.), this is an independent foundation, which however, collaborates closely with the Central Council and,
as far as the
financial contribution from the government is concerned, falls under thead-ministration of the National Industrial Research Council. This situationarose
from the fact that the N.S.M.B. had come into existence as early as 1929, hence before the Act relating to Applied Scientific Research had been passed.
When drafting the statutes, however, the founders, viz, the State and four Dutch Shipping Companies, took fully into account the terms of the Act then in course of preparation. In a certain sense the N.S.M.B. may therefore be considered as the pioneer in the practical development of industrial organi-zation, and its experience has been of inestimable importance.
Form o/ organ;ization
The deed by which the Netherlands Ship Model Basin was founded dates
from 28th June, 1929, and records as its founders: the State of the Netherlands,
and a. the Steam Navigation Co., ,,Nederland" Ltd., b. the Royal Rotterdam Lloyd Ltd., c. the Royal Packet Navigation Co. Ltd., and d. the Netherlands Indies Tank-Steamer Co. Ltd. A Management was created consisting of a Board of Directors of five members, assisted by an Advisory Committee. One of the members (the Vice-Chairman) shall represent the appropriate minister,
at present the Minister of Transport and Energy; he has the right to suspend a resolution of the Board of Directors pending the decision of the Minister. The appointment and dismissal of the remaining members, or any increase in their number, is in the hands of the Board of Directors after consulting the founder shipping companies and the Advisory Committee, such resolutions
requiring the sanction of the Minister. So far the practice has been that two of
the members shall represent the founder companies and the two other members,
the Department of Naval Architecture of the University of Technology at Delft and the Dutch Royal Institute of Engineers respectively.
The meetings of the Board of Directors are attended by the Superintendent and by the Chairman and Secretary of the Industrial Research Council.
The Advisory Committee consists of a varying number (at present about 30) of members who represent some five ministerial departments, the founder
6 PLAN, ORGANIZATION AND MANAGEMENT
companies and the contributing shipping companies, shipbuilding yards and engineering works. At the recommendation of the Board of Directors, the Committee shall appoint a Sub-Committee for Scientific Research.
The general management of the experimental station of the Foundation is entrusted to the Superintendent who, having been appointed by the Board of Directors, is reponsible to them.
The financial resources of the Foundation consist of the Foundation Fund, of payments received for investigations made and advice given by the Foun-dation, and of contributions from the Research Fund.
On the basis of fifty-fifty, guaranteed subsidies of 90,000 guilders ( 9,000)
per annum, for a period of 10 years, are givenby various industries and by the State with a view to creating a Research Fund. It has also been possible up to now to supplement this sum by a contribution of 10,000 guilders ( 1,000) yearly from the Shipbuilding Trade Group and by a similar amount from the State, granted on the fifty-fifty basis. Any deficits will in future be met by the Association of Supporters jointly with the State.
c. Exloitcition.
The object of the experimental station is to promote efficient ship designing
by means of scientific research in the field of hydrodynamic problems connected
with it. This end is pursued chiefly by carrying out and analyzing experiments
on a reduced scale model with a view to ascertaining the most economical shape
of ships' hulls and their propellers.
In this last sentence the important advisory task of the Foundation is indi-cated. Although the theory of propulsion has made great advances during the last two decades, empiric investigations on small scale models is required for the rapid and concrete answering of questions from industrial quarters. Scale experiments permit reliable forecast of service results of ships and the choice of the most economical shape of hulls and propellers. In practicallyall
designs of ships made by naval experts, use is made, even in the earlier stages, of published results of systematic research in this field. To-day scarcely any
ship of any importance is built without model tests having been made previously.
It is of importance not only for naval architecture and for the owner
that a
ship shall have the greatest possible speed for a certain deadweight and engine power, or that for a certain speed the least possible power and fuel will be consumed, but this is also of value for national economy in general.The main source of revenue of the laboratory is found in the fees paid by
principals for investigations carried out to their orders. The expense of scientific
research, the results of which arc published in technical journals at home and abroad, are chiefly met by contributions from the Research Fund. The results of a commission by a principal are, on the other hand, his property,unless he should decide otherwise. The strictest secrecy is naturally observed withregard to such results.
PLAN, ORGANIZATION AND MANAGEMENT 7
ship's hull, the lines plan, is submitted to the Ship Model Basin. From these drawings a ship model is made of paraffin wax in the laboratory workshops equipped for this work. For sea-going vessels this model will as a rule have a length of 6to 7 metres (20 ft.-23 ft.), hecausesmallermodeisleadtovariation in the flow-pattern and thus produce unreliable results. Moreover, the model must be large enough to contain the propelling machinery and various mea-suring appliances required for normal tests with self-propelled models.
At first the model is examined while travelling in a large basin filled with water, the model basin, by towing it in a straight line throughout the length of the tank, at the required displacement and draught reduced to scale. During
this operation the resistance encountered in the water is measured and recorded
for a number of speeds increasing at regular intervals, the so-called resistance tests". By converting these results the effective or tow-rope horsepower (E.H.P.) required for this ship is determined. Further, the expert conducting the experiment can, by observing the wave pattern, receive an impression of
the possible defects in the shape of the ship's hull.
For the self-propulsion tests which then follow, the model is fitted with a built-in electric motor, driving the propeller organs (screws, paddles etc.) by means of an inboard dynamometer. This apparatus is capable of recording the torque on the propeller shaft and the thrust delivered by the propeller model. The number of revs./min. is also recorded. By conversion of these data the shaft horse power (S.H.P.) is calculated. The quotient E.H.P./S.H.P. will give the quasi propulsive coefficient (Q.P.C.).
The results of E.H.P. and S.H.P. as obtained from the model tests are then converted into non-dimensional coefficients and plotted in statistical diagrams. By comparing these non-dimensional figures with the results of models that have been tested formerly, an opinion can be formed as to whether the results obtained are satisfactory or whether they are capable of improvement. If necessary the ship-owners are advised to modify the original design. In such a
case the new design is usually made by the staff of the Ship Mode] Basin in close
co-operation with the client. Alterations can be made very easily to models which are constructed of paraffin wax. The modified model is re-tested until satisfactory results are obtained.
By testing the propeller model separately (in open water) and by comparing the characteristics thus obtained with those produced in combination with the ship model, it is possible to analyse all the factors affecting propulsion and to take steps to improve them. Many other factors may have to be investigated, according to the nature of the object, such as flow measurements, tests in shal-low water, towing tests (for tugs), steering and turning tests, rolling tests etc. On the conclusion of the tests the principal receives a report containing all
the necessary data and plans required for the hull form and her propellers.From
these details the trial trip speed and the service speed which may be expected,
can be determined with a high degree of accuracy.
The time required for the tests, including the manufacture of the models, is
8 PLAN, ORGANIZATION AND MANAGEMENT
normally 2-3 weeks, but in urgent cases it can be reduced to 8-10 days. Owing to the post-war excess of orders one must now, however, reckon on a delivery period of G-8 weeks. The costs of such tests naturally depend on the extent of the programme, but as a rule amount to a few thousand guilders
(a few hundred pounds). For large pro] ects these costs are of minor significance.
For smaller objects such as pleasure and fishing vessels, the programme must sometimes be reduced in order to lower the costs.
All test results are analysed and reduced to statistics. These are compared with each other and with the Model Basin's own research work. Great value is attached to comparative analyses of trial-trip and service results of ships of
which the models have been previously tested. The Ship Model Basin has become
a centre of knowledge and experience in this particular field. This experience
is expressed in the advice supplied to principals and in articles published in books and technical journals.
d. Survey of the development of the N.S.M.B.
During the twenty years of its existence, it has been proved beyond doubt that the Netherlands Ship Model Basin fills a great need. Its staff, which on the opening day in 1932 comprised some ten persons, had within twelve months been doubled, and in 1940 was increased to 42. The staff now consists of 78
persons. As far back as the pre-war years, the N.S.M.B. had gained a certain reputation abroad by means of its methods and publications, so that about 40% of the paid orders came from other countries. In normal years it was consequently entirely self-supporting. During the war period the Institute experienced great difficulties, particularly owing to the loss of contact with its foreign connections. This culminated in the forced evacuation in October 1944 and the subsequent mass looting by the German occupier, most of the important machines being removed to Germany. Some of these machines were
traced and recovered after the liberation, but the chaos was so complete that the
exploitation of the Institute could not be resumed before March 1946, and then only partially. Within a short time it appeared that the extent to which both home and foreign shipping and industry called for the assistance of the N.S.M.B. was so great that the demand for advice could only be met by the
strenuous efforts of an increased staff. More than half the commissions received
originate, directly or indirectly, abroad.
In the period preceding the war, important extensions were made, first to the offices, subsequently to the laboratory, and later still by the addition of the cavitation tunnel. The cost of the latter, about 250,000 guilders ( 25,000), was
again met by the State, by industry and by the Institute itself. After the war,
in 1951, an extension to the towing-tank of 92 m (302 ft.) was completed, while
additions were also made to the drawing-offices and workshops. The costs of the former, amounting to about 850,000 guilders (t 85,000), were born by the Royal Dutch Navy and the Institute itself, and of the latter - amounting to
PLAN, ORGANIZATION AND MANAGEMENT 9 subsequent additions proving necessary was taken into account when the Model
Basin was first built in 1930-1932.
In connection with the limited funds then available, it was decided in 1932
to restrict the length to 160 m (525 ft.). The width and depth however were given
dimensions 105 m (34 ft. 6 in.) and 55 m (18 ft.) respectively in order to be in suitable proportions to an eventual increased length, the necessity for which could then already be foreseen. Gradually ships' speeds increased to such an extent that the necessity of lengthening the tank became more and more obvious, if the Model Basin was to maintain its prominent place in the years to come. For, judging by the amount of business done at present, it may be
con-sidered as one of the largest in the world. Between 100 and 120 ship-models are
made every year and in general it may be said that test-programmes are be-coming far more extensive than was formerly the case.
The increase of business since the war is partly due to advances in scientific research. Shortly after the war a special department for scientific research was set up, financed by the Research Fund. A number of important research-programmes of current interest are now being carried out in this department. The war years provided the staff of the Model Basin with the opportunity of writing a comprehensive text-book on "Resistance, Propulsion and Steering
of Ships", editor-in-chief Prof. Dr. ir W. P. A. VAN LAMMEREN, in which a great
deal of the Model Basin's test-result material is published. An English edition appeared after the war, the Dutch edition having been sold out. Thanks to its extensive circulation abroad this book has done much to make the N.S.M.B. known in other countries. Apart from this, up till now 115 publications by members of the staff have appeared in technical journals at home and abroad.
Contact with scientific circles abroad is maintained and strengthened in
yet another way. Shortly after the opening of the Model Basin in 1932, its Board
of Directors took the initiative in convening a first international congress of tank superintendents. Owing to the unqualified success of this congress, it was
followed before 1940 by similar meetings in London, Paris and Berlin. Plans to
meet in Rome were however frustrated by the outbreak of war, and it was not until 1948 that the first post-war meeting was held in London, followed by a conference at Washington in 1951. International co-operation is now being brought into play in the approach to important scientific problems.
II.
Equipment and Working-methods
A. The Building
Fig. i shows the outside of the building (south side).
In fig. 2 a general plan is given of the whole building as it appears after the completion of the cavitation tunnel and the extension to the towing-tank. The most striking feature is of course the tank itself, 827 ft. long, 34 ft. 6 in. wide and 18 ft. deep.
In order to keep the building costs as low as possible, it was decided at the outset to build the tank on and not in the ground. It was also essential that this latter should be of a sandy nature. In this way a great deal of expense was saved which would otherwise have been incurred owing to the necessity for driving piles in soft soil, digging costs etc.
After a long search the Board of Directors received an offer from the Cor-poration of Wageningen to place a free site at their disposal. After due con-sideration this offer was gratefully accepted, as a result of which the Model Basin made its appearance at Wageningen.
The site in question was covered with a layer of clay, about i m (3 ft.) thick, which had first to be removed in order that the foundations of the tank could be laid directly on the sand.
The simplest method of construction was found to be a thin-walled building-supported by outside concrete buttresses reaching up to the roof,there con-nected by steel I-girders, which latter at the same time serve to support the roof. These girders take the tension caused by the water-pressure on the side walls of the basin when filled (see section AB in fig. 2).
The tank is insulated on the outside by banked-up earth in order to protect the water from changes in temperature (see section AB in fig. 2).
2d Floor 01 lnsPrumen Room B 12 13 14 15 16 2 3 28 27 a 4 5
!a.
....U.U.U.U.UUUUSUUUUUUUUUUUUUU u...
U
4e 480 ModeL Basin 1stFloor
Model Basin
SecHon AB
Fig. 2. GeneraI plan of the Netherlands Ship Model Basin.
¶ Eli
Il
45 Ground Floor û 11EQUIPMENT AND WORKING-METHODS
Il
As a result of this construction, the working-floor of the tank lies above ground level, at the same height as the first floor of the building, and it was deemed advisable to build the work-shops at this level also. A result of this was, however, that offices, drawing-offices, store-rooms etc. had to be situated under the work-shops, which imposed certain conditions on the equipment for deadening the sound of the machines. This disadvantage, however, was more
than compensated by the advantage of having all work-shops on the same level
as the top of the tank. The main hall containing the basin lies east to west with lighting from the north only, so that the water is not subjected directly to the Sun's rays. This is done in order to prevent all kinds of undesirable growth.
The main hail was built wide enough to allow of a passage about 3 ft. wide being left on either side of the basin; the height of the hall, about 20 ft, was determined by the length of wires required for hanging models which might be overloaded (these are models the weight of which is greater than the water-displacement). The workshop end is higher to enable measuring apparatus etc. to be lifted from the towing-carriage. The sides consist of hollow brick walls built in between the concrete buttresses, which latter lie at regular distances of 13 ft. from each other.
In order to prevent mist and precipitation it was necessary to insulate the roof. For this purpose a double layer of terra-cotta plates with
insulation-canals was employed. The top layer was covered with rubberoid on the outside.
As these precautions were not found satisfactory in the long run, heating-apparatus was later installed against the inside of the roof. By means of
ther-mostats placed in the water of the basin, in the hail and also in the open air, the heating of the hall and more or less of the water itself is regulated in such a way
that the formation of mist and precipitation is automatically excluded. The terra-cotta plates proved unsatisfactory, and later on, during the extension to the tank, ordinary concrete plates were used as a roof-covering. A number of expansion joints were made in walls and roof.
The workshops are provided with a ceiling consisting of Masonite plates under the roof-girders, also for purposes of temperature insulation. In the ship-model workshop and in the instrument-room there is a concrete floor; but, on account of the offices underneath, the carpenter's shop and the instrument-maker's shop had to be soundproof and have therefore been equipped with
floors made of wooden blocks on a layer of sand on concrete. There the machines
have been firmly fixed to a special layer of felt, which in its turn is fixed to the concrete foundations. Foundation bolts were not used here.
The cavitation-tunnel building has been erected on the south side against the existing buildings; it was executed entirely in a steel skeleton form.
On the first floor of the building are to be found (fig. 2) the ship-model workshop (1) and an alcove for the paraffin-ovens (2). This room, which measures about 39 m x 13 m (128 ft. x 43 ft.), can be reached by means of two staircases leading out of the offices and a broad outside staircase from the
Fig. 3. Ship model workshop - south itI.
/!.,. "1H11.
111111S jj
Fig. 5. Preparation of the clay mould.
14 EQUIPMENT AND WORKING-METHODS
At the front of the building are situated the carpenter's shop (8), the in-strument-room (9) above which are two small spaces for varnishing and painting (10, lOa), the instrument-maker's shop (11) with an office for the foreman (12), a room for checking up screw-models (13), a small hail for con-ferences and lectures (14), a work-room for principals (15) and a hall (16). At the back of the ship-model workshop there is an open space (18) in front of the basin, giving access to the model-harbours and the paths alongside the tank. The foreman's office is also situated here (19).
The workmen's cloak-room (20) and wash-room (21) can be reached from the porch (17). Behind these is the staircase of the cavitation-tunnel building
(22), lavatories (23) and canteen (24). These rooms - (20), (21), (22), (23) and (24) - form a group which, although situated in the cavitation-tunnel building, does in fact serve the whole concern; as (20) and (21) were already provided, though to smaller dimensions, and the measuring-floor of the cavitation-tunnel had to be laid on the second floor, the space available on the first floor permitted of these extensions being made very easily. In this part of the cavitation-tunnel building there are no floors except in the spaces mentioned above.
At the end of the model basin there is a cabin (25) from which the
towing-carriage is driven.
A second floor has been built in the cavitation-tunnel building only; here are the testing-room (55), staircase and work-rooms (56) and (57). This floor can be reached by the staircase or by a flight of iron steps with a platform in
the ship-model workshop (1).
On the ground-floor, underneath the ship-model workshop, are to be found
a store-room (26), coal-bunker (27), central-heating installation for coal- (28) or oil-combustion (28a), phototype room (29), dark-room (30), library (31),
drawing-office for order work (32) with a room for the chief draughtsman (33),
fire-proof strong-room (34), office for the clerical staff (35), engineer's office(36) reception room (37), Superintendent's office (38), drawing-office for research
work (39), office for Head of Research Department (40), passage (41), lavatories (42) and cloak-room (43).
In the cavitation-tunnel building are the engine-room (44), foundry (45), heating-installation for the cavitation-tunnel water (46), fire-proof strong-room (47) and the staircase.
Besides these there is a cellar-space (48) underneath the first portion of the basin which at that point has not reached the full depth of 5½m. (18 ft.) This space is divided into three separate cellars, one of which is used for storing bicycles (48a). The building containing the rooms (49), (50) and (51) is the switchboard- and transformer-house for the cavitation-tunnel machines.The space (52) at the end of the basin contains the machinery for the towing-tank, the transformers for which are accommodated in the spaces (53) and (54).
EQUIPMENT AND WORKING-METHODS 15 B. The Manufacture of the Ship-models
The ship-models are cast in paraffin-wax. Compared with wood, this material
has the great advantage of being easily malleable in all directions, while the tools used do not soon become blunt. Where modifications to the hull are necessary, paraffin-wax can easily be added or cut away. After use the wax
can be melted down again and employed for casting a fresh model. On the other
Fig. 7. Completed clay mould.
hand it has several disadvantages, such as the considerable shrinkagewhich takes place after casting and the possibility that in warm weather long narrow models may show a tendency to bend. But these difficulties can readily be overcome, so that the use of paraffin-wax is infinitely preferable to that of
wood. Wood is used very occasionally, as, for instance, in the case of very large
Fig. 8. Manufacture of the core.
Fig. 10. Casting a ship model
iS EQUIPMENT AND WORKING-METHODS
in various towing-tanks. Models made for steering-tests are occasionally made
of wood.
The material consists of white paraffin-wax with a melting-point of 60° to
62° C. To this about 3°/e bees-wax is added in order to render it less brittle.
The model is cast on the large side, milled from a water-line drawing, cut off
and scraped smooth. Casting alone does not give a sufficient guarantee of
the form being correct, and the model-surface is not sufficiently smooth without
subsequent finishing-off treatment.
To this end the ship-model workshop is fitted out with two melting-ovens for paraffin-wax (in space 2, fig. 2), a moulding pit filled with clay in which the models are cast (3), a washing-trough (4), a milling-machine (5), a
marking-off table (6), several finishing tables, an overhead crane, a weighing-machine for models and a transport-canal in which the models are launched and led to the basin.
In fig. 3 the southern half of this workshop can he seen, with to the right the clay pit, covered with tarpaulin, to the left the milling-machine and in the right foreground the marking-off table with a model on it. Fig. 4 gives the northern half, again with a part of the marking-off table in the foreground and the finishing tables next to it. In the background, the transport-canal, the model-weighing machine and the overhead crane are visible.
The model is cast in the moulding pit filled with modelling-clay of a fine homogeneous texture and free of sand. The distance between the sections used here is a good 1% greater than the actual distance, to allow for shrinkage when the model coagulates. For the same reason, the mould is also made a good 1 cm (0,4 in). larger than the finished model is required. In fig. 5 a mould which has already been used is being prepared for a new model. The clay is removed by means of a U-shaped wire, with which layers of clay can be cut off; clay can be added where necessary by throwing it against the sides. When the mould has been roughly prepared it is finished off by means of wooden templates (see fig.6) placed in the correct positions in the clay and later
re-moved so that the form remains in the clay. The above-mentioned U-shaped wires are then used for finishing off the mould until it is quite smooth (fig. 7). As can be seen clearly in this figure, a horizontal flange is cast on to the upper
side of the model in order to strengthen the side-walls and to facilitate the attaching of clamps and instruments.
In the meantime the core has been made in the carpenter's shop. Two longi-tudinal ribs are connected by screwed-on transverse battens to which core templates are fixed. These core templates are of course smaller than those used in preparing the clay mould, the difference being the thickness of the
model (4 to6 cm.). Thin longitudinal laths are then nailed to the templates and
the whole covered with canvas (fig. 8). On the upper side of the core the canvas is carried across the underside of the transverse battens horizontally,
resulting in the complete closing off of the space between mould and core which
EQUIPMENT AND WORKING-METHODS 19 firmly in the mould by means of iron braces which hook into a ridge in the sides
of the pit wall (fig. 10). An iron pipe is placed on the stem and at the stern a casting funnel made of clay. The zinc gutter from the paraffin-oven issues into this. Thus unavoidable shrinkage holes are not formed in the mode] itself but in the iron pipe and in the casting funnel. By keeping up the flow of paraffin-wax so that the iron pipe is constantly filled before the model is completely coagulated, the model is moreover cast under pressure.
In the above-mentioned alcove (2) (see fig. 2) are to be found two ovens, one
with a capacity of 700 litres (155 gallons) heated by gas, the other of 300 litres (66 gallons) heated by electricity. In neither case is the paraffin-wax heated directly, but is brought to melting-point through the medium of water, placed under and around it and then subjected to heat. The temperature at which casting takes place is about 730 C.
The upper side of the core is left open. When casting is begun, hot water is poured into the core to prevent the first paraffin-wax from coagulating too
rapidly, which might give rise to the formation of layers and to blistering. Later the hot water is gradually replaced by cold in order to accelerate the cooling-off
process. The weight of the water serves also to compensate the upward thrust on the core by the paraffin-wax.
As soon as the model has coagulated and before it has completely cooled off,
the core has to be broken to pieces and removed, as otherwise there is a chance that it may offer too great a resistance to the shrinkage of the model, as a result of which the latter may crack. Particularly in the case of submarines, it is not easy to determine the moment at which this must take place, as here the whole model is enclosed in clay. In this case a hole is made in the top of the model through which the core can be broken up and extracted with the aid of a long
hook.
A cast model of a submarine can be seen in fig. Il. The upper mould has partly been removed.
After removal of the core, the remaining water is drawn off (in the case of ordinary models), and the top of the sides of the model are planed smooth by a milling-machine running on rails over the clay-pit. In order to strengthen the model, port and starboard sides are connected by wooden clamps. Some very
large models require the insertion of paraffin-wax bulkheads. The time has now
come to loosen and remove the model from the clay, for, notwithstanding
shrinkage, a good deal of clay still sticks to it. This loosening is easily achieved by pouring water into a hole made in the clay close to the side of the model, and
by keeping up the flow for some time. As a result of the force of gravity and of capillary action, this water percolates between the clay and the paraffin-wax and at a given moment the buoyancy is greater than the adhesive force, so that the model floats upwards in the mould. It can then be taken out of the mould with the aid of the overhead crane which is equipped with a beam and slings. The average length of the models is 6 to 6¼ m. (20 ft. to 22 ft.), the weight of a cast but unfinished model is approximately 600 kg. (1300 lbs.) The
pre-Fig. 12. Milling-machine for ship models.
Fig. 14. Model during milling (the cutters were put out of action while the photograph was being taken).
22 EQUIPMENT AND WORKING-METHODS
paration of the clay mould takes roughly one day, during which time the core is also made. Casting, including 4 to 5 hours for cooling, also takes about one
day.
After being scrubbed down, the model is carried to the milling-machine (see figs. 3 and 12). With this machine water-lines are generally cut in the mode], giving it a ribbed appearance (figs. 13 and 14). The milled grooves are bounded by two planes which intersect at an acute angle; the intersecting-line of these two planes gives the correct shape to which the model must subse-quently be finished off by hand. Besides water-lines, it is also possible to cut
transverse sections with the milling-machine, but this is only done in exceptional
cases. The machine consists of the following parts:
a. a fixed frame (see figs. 3 and 12) on to which are fixed two vertical
milling-spindles, sliding symmetrically in relation to the centre-line plane; they can also be adjusted in the vertical direction.
L. a deep carriage travelling on rails under the frame (see figs. 3 and 12) in
which the model to be cut is placed keel upwards.
c, a table on raised rails (see fig. 12) on which the water-line drawing is placed. This table follows the backward and forward movement of the carriage. The two milling-spindles have an exterior square thread (fig. 14) and are
moved up and down by means of revolving nuts on the frame (fig. 12). In order to avoid wear and tear of the thread as far as possible, the weight of the spindles
with electric motors and cutters is compensated by counter-weights hanging on steel wires; the method of fixing these wires to the spindles may be seen on fig. 14. The spindles are driven by vertical electric motors, one attached to each spindle.
The cutters themselves each consist of a holder and two diamond-shaped
chisels (fig. 14). These chisels are adjusted anew for each model, by means of a
special apparatus, in such a way that they stand at the same height and at a constant distance from each other, as a result of which they describe exactly similar circles when revolving. For planing off the sides of a model, the above-mentioned diamond-shaped cutters are replaced by rectangular ones.
As it is impracticable in the drawing-office to make the water-line
milling-plans as long as the models (about 61/ m.), these milling-plans are usually foreshortened
longitudinally to a scale of 1 : 2 or i : 2.4. The breadth-scale is kept at 1 : 1.
For this reason the carriage and the table, which are driven by spindles, must be coupled together with corresponding gearing.
The water-line plans are drawn on marble slabs or on Kodatrace and contain water-lines at mm. distances from the base-line to the under side of the bilge, at 1 cm. (0.4 in.) apart to the upper side of the bilge and 2 cm. (0.8 in.) apart
between upper side of bilge to upper side of model (see figs. 13 and 14).
In fig. 15 a top view is given of the control-station of the milling-machine. In front to the extreme right are the pushbuttons for both milling-motors and the direction-switch for carriage and table. To the extreme left is the sliding resistance for regulating the speed of both.
Fig. 16. Celluloid ellipse on the milling-plan.
Fig. 18. Finishing off models of a sailing yacht and of an inland waterways vessel.
Fig. 19. Model of a ship with three propeller tunnels for shallow water.
EQUIPMENT AND WORKING-METHODS 25
By turning a hand-wheel (middle right), both spindles can be brought further
away from or nearer to the centre-line plane of the model. The frame, which
can be seen on the photo and to which a half-ellipse of celluloid is attached, can
he brought nearer to or further from the centre-line of the water-line plan by
turning the hand-wheel (centre right). This motion is followed by the two mill-ing-spindles.
By turning the hand-wheel with his right hand and regulating the speed of the table with his left, the model-maker keeps the celluloid ellipse in constant contact with the water-line to be cut (see detail fig. 16). The milling-spindles follow this movement, by which means the required water-line is cut in the model. The reason that the waterline on the milling-plan must be followed by the circumference of an ellipse is to be found in the fact that the water-line plan has been foreshortened in the longitudinal but not in the transverse
direc-tion. For while cutting is in progress, the extreme points of the revolving cutters
describe a circle which constantly touches the water-line being cut. If the cutting-plan were drawn as large as the model, a celluloid circle the same magnitude as the one described by the cutters would be required. The plan has however been foreshortened longitudinally to a certain scale, so that the celluloid circle has to be foreshortened to the same scale, the result of which is an ellipse.
To the right of fig. 13 the beam can be seen with the canvas slings in which the newly-milled model can be slung for transport to a finishing-table. The superfluous paraffin-wax is first roughly scraped away, then planed off and lastly the model is finished off smoothly with steel scrapers and polished with wool; any open pores are filled up with plastoline. The exact form of stem and stern are finally given with the aid of wooden templates.
Bossings are always cast on later and adjusted to fit by means of zinc trans-verse templates; bilge keels and rudders are made of wood, shaft brackets of brass. Shafts are adjusted on the marking-off table (fig. 3) which is provided
with an electrically-driven measuring-frame for checking up the models. Before
the model is launched, the load-line is also marked off thinly to facilitate correct trimming later on.
In figs. 17 and 18 four different types of ships can be seen during the
finishing-off process. Fig. 19 shows a finished model of a ship with three propeller tunnels for shallow water.
Hanging in the canvas slings from the beam attached to the overhead crane,
the model is then weighed and launched. Great care is necessary here as sudden
cooling may cause cracking. If the model is not required immediately, it is kept under water, viz, just awash; by this means the temperature at the top and the bottom of the model is the same, thereby preventing the model from
warping (cat's back). When finished, a model 61. m. (22 ft.) long weighs about
450 kg. (1000 lbs).
Before testing can take place, models have to be kept under water for about 60 hours. At about the end of that time, the chemical reaction between
Fig. 20. View of the instrnment-rnaker's shop.
Fig. 22. Fat and plaster lumps.
Fig. 24. Milling-machìne for screw models.
Fig. 25. Mining-machine in operation showing supports for driving-surface (right) and back-surface (left).
the water and the paraffin-wax ceases. During this reaction the resistance of the model is greater than after it has taken place.
Altogether a period of at least 6 days is required for making a model, in-cluding the time it lies in the water.
C. The Manufacture of the Screw Propeller Models
Screw-models are made in the instrument-maker's shop, which is specially equipped for the purpose (fig. 20). As in the case of full-size propellers, the
models are cast in metal. Casting is done as follows: the helicoidal surfaces are
cut out in a lump of mutton fat by means of a vertical shaft, a generator-line cutter and a pitch triangle (see fig. 21). A steel plate cylinder is then placed round this mould and filled with plaster of Paris and water. When the plaster has hardened, the cylinder is removed and the plaster separated from the fat (fig. 22). On the helicoidal surfaces in the plaster, a number of arcs of a circle are described, in which small zinc templates of the blade sections are inserted at the required positions in relation to the generator-line. The spaces between these templates are filled up with clay until the whole of the screw
has been modelled in clay on the helicoidal surfaces. A layer of grease is spread
over the latter and the clay screw, the steel plate cylinder is now placed around the plaster of Paris mould and again filled with plaster and water, the result of which is a second lump of plaster (fig. 23). When clay and templates have been removed, the two lumps of plaster are dried in an electric oven for one night. They are then placed on top of each other and the joints are filled up with clay. The screw can now be cast.
The metal used for this purpose consists of an alloy of tin, antimony and bismuth. This alloy is easy to cast and to work on. Screws thus made are sufficiently strong for testing purposes and remain bright when in use. In the cavitation tunnel, where greater forces are brought into play, bronze screws have to be used.
The following operation consists of cutting the cast model screw on a special
milling-machine. The whole driving-surface is cut almost entirely according to a pitch-ruler built into the machine. In the back surface, grooves are cut according to a blade section cutting plan. The machine is shown in fig. 24, whilst in fig. 25 the piece of work in hand can be seen in detail with the two cutter-supports; the right-hand driving-surface support has been put out of action, while the left-hand support is in the act of cutting the back-surface. Screws with a diameter up to 50 cm. (20 in.) can be cut on this machine. Superfluous material is subsequently removed by means of a hand-cutter and files.
When the screw is nearing completion, it is checked up on a special
measuring-bench (fig. 26), designed and constructed in the instrument-maker's shop of the N.S.M.B. Both driving-surface and blade-thickness can be measured. A measuring-chart for each screw is prepared beforehand in the drawing-office for purposes of checking up at this bench. Besides the columns previously
Fig. 26. Measuring bench for screw models.
ki. 28. Workshop for casting and finishing bronze screws.
32 EQUIPMENT AND WORKING-METHODS
filled in, there are four open columns, one for each blade of the screw, in which the instrument-maker fills in his checking measurements, sothat any deviations
still shown by the screw in question can be seen at a glance.
A white metal screw-model can be made by one man in an average of 8 days.
A bronze model takes 4 to 6 weeks.
The screw-shafts used with self-propelling models aremade of stainless steel
and revolve in bearings made of synthetic resin with layers of fabric. Stuffing-boxes and oil lubrication are not used, there is only a slight water lubrication
to the extent that water leaks through the bearings.
Fig. 27 shows the stern of a submarine-chaser model complete with appen-dages and screw, ready for the propulsion tests.
Besides the instruments already mentioned, the instrument-maker's shop is equipped for general use with a series of lathes, drills, a planing-machine, a milling-machine, a grinding-machine, a nickelling bath, a metal sawing-machine etc.
Fronze screws are cast in a separate workshop and are finished off entirely by hand (fig. 28).
D. The Model Basin and Towing Carriage 1. The Basin
As the possibility existed of the model resistance being influenced by the presence of the side walls and bottom of the basin, advice was sought, when deciding ori the dimensions of the basin, from Prof. Dr. Ing. F. HORN (Ber-lin), the late Dr. G. S. BAKER (Teddington), and Prof. Dr. G. KEMPF (Ham-burg). The designer of the mechanical part of the Experimental Station, Dr. Ing. F. GEBERS of Vienna, proposed a width of 125 m. and a depth of 6.5
m. Dr. KEMPF and Dr. BAKER recommended as minimum dimensions a width
of 75 m. and a depth of 3.75 m., while Prof. HORN in a lengthy and well-sub-stantiated report advised transverse dimensions of lO5 m. and 55 m.
The basin was built according to the last-named dimensions, and, in
con-nection with the funds available at that time,
the initial length waslimi-ted to 160 m. (525 ft.) Later the length of the basin was increased to 252 m.
(827 ft.).
The length of the basin was determined by the highest speed which would have to be attained by the models. At the beginning of a run, an acceleration time is required to reach a certain constant speed, then follows the period during which the model runs at that constant speed and during which the measurements are taken, the run ending with a deceleration period during
which brakes are applied to bring the model to a standstill. The higher the speed
of the model, the longer will these periods become; the acceleration and de-celeration times can be curtailed as far as possible by mechanical means, but there is a certain minimum time during which the measurements can be taken,
Fig. 30. View of the basin.
34 EQUIPMENT AND WORKING-METHODS
irrespective of the speed of the model. With its present length, models of the fastest ships can be satisfactorily tested in the basin.
The transport canal, leading from the ship-model workshop into one of the two harbours for models, enters the basin on the west side. The harbours serve for temporary storage of models waiting to be tested. Between these harbours there are two piers and a trim-tank (fig. 29). The floors of the piers lie about 65 cm. (2 ft. 2 in.) lower than the water-surface so that a model can be observed under water through windows in the walls of the trim-tank. A model to be tested is fitted with the requisite ballast weights in the model workshop and then run into the trim-tank, where, by shifting the weights, it is adjusted to the required draught etched on the sides.
The bottom of the basin is here only 1.7 m. (5 ft. 7 in.) under the water-surface, sloping down gradually to the full depth outside the harbours. This presents no difficulties as far as tests are concerned, as the first part of the basin is covered only during the acceleration period. Underneath this raised part of the bottom are the three cellars which have already been mentioned. At the east end of the basin the full depth is maintained. A view of the tank ready filled is given in fig. 30.
For purposes of tests in shallow water, the basin has been fitted with a false bottom consisting of 25 pontoons. These pontoons, each S m. (26 ft. 3 in.) in length, are made of a steel framework covered with wood, and fit into each other. They are suspended on chains contained in vertical grooves in the walls
of the basin. The bottom is hoisted or lowered link by link (each being 30, 40 or
60 cm. long) by means of special winches. The last pontoon, at the end of the basin, is suspended obliquely just under the water-surface, in the manner of a ,,shore", to subdue the waves. In fig. 31 the false bottom can be seen hoisted entirely out of the water, while a model of the Wilhelmina Canal is being
constructed upon it.
The basin is filled by means of centrifugal pumps fromtwo wells 60 m. (200 ft.) deep. The water contains a small amount of iron but is very clear. The two pumps together deliver about 200 m3. per hour, so that the basin - the capa-city of which is about 14000 m3. - can be completely filled in three days. It can be emptied into a nearby canal through special drain-pipes.
It is to the credit of the builders that there is practically no leakage, which would cause a lowering of the water-level. Up till now it has not been found necessary to empty the tank for cleaning purposes, although it was originally estimated that this would be required every two years. The water is kept free of gnat larvae and water insects by a number of goldfishes, which appear also to feed on the sparse weed growing on the walls.
2. Towing Carriage and Rails
The towing-carriage is a steel construction and runs on four wheels along the rails on the sides of the basin. The carriage has several uses: in the first place it serves for towing the model in the basin, for keeping it on its course,
EQUIPMENT AND WORKING-METHODS
l. 32. Towing-carriage.
for bringing it up to speed, for keeping it at a constant speed while
measure-ments are being taken, to brake it at the end of the run and to tow it back again to the starting point in the basin. In the second place thevarious
mea-suring- and recording-appliances are mounted on the carriage, such as the resistance dynamometer for determining the towing-resistance of the model,
trim-apparatus which keeps the model on its course and with which the
sub-sidence and trimming during the run can be recorded, and apparatus for registering the course followed, the time in seconds and the number of re-volutions of the screw-models and vane-wheels. Lastly there are to be found on the carriage a Ward-Leonard aggregate which provides electric current for self-propelled models and for screws in open-water tests, a photographic installation for taking photographs of the model under way, and a floor and long table for the convenience of the staff.
The carriage (see fig. 32) consists of two transversely and three longitudinally
constructed girders. The centre longitudinal girder, which is slightly higher,
carries the measuring- and recording-apparatus: the models run underneath this girder. The table has been fitted to the south longitudinal girder, and the
floor lies between this and the centre girder. The variousgirders are constructed
36 EQUIPMENT AND WORKING-METHODS
made of tubes. It is essential that the whole shall be free of vibrations at all speeds between O and 81/2 m. (28 ft.) per second. The weight of the carriage with full equipment is a little more than 15 tons.
The four wheels are 1.2 m. (4 ft.) in diameter; there is no mechanical coupling
between them and they are flangeless. Notwithstanding thecarriage has proved satisfactory in keeping its course along the rails without guidance, four lateral conducting rollers on springs have been placed in a horizontal plane on either side of the south rail as a precautionary measure. Both rails have a planed running-surface. The south rail has moreover two vertically planed surfaces for the conducting rollers.
It is obvious that the rails must be mountedwith the utmost precision in order to ensure the carriage running at a constant speed while measurements are being taken. The rails lie on the walls of the basin on cast-iron sleepers cast into the concrete. The top surfaces of the sleepers are planed and are
placed at distances of about 70 cm. (2 ft. 4 in.) from each other. Before casting,
the vertical position of these sleepers wasdetermined by marks made in the concrete walls indicating the level of the water-surface during a first trial-filling. After casting and placing of the rails, the height of the running-surface above the water-surface was determined by means of electro-acoustic micro-meters; according to these measurements the requisite number of plates was placed between the rails and the sleepers until therunning-surface was parallel to the water-surface to within approximately 0.1 mm. (0.004 in.). In the horizontal direction, the rails were laid straight along a stretched wire with plumb-lines attached.
The rails consist of lengths of a little morethan l33 rn. (43 ft. 9 in.) made of heavy railway-profile (48 kg/rn.; 32 lbs/ft.). At the ends of these lengths the heads were planed off square and the web and flange slightly slanted off, so that it was possible to drive the rails firmly against each other by means of wedges in the fish-plates. Iii this way the joint is scarcely visible on the running-surface. It proved unnecessary to take into account the possibility
of rail-expansion due to variations in temperature, as these variations are
practically negligible.
The towing-carriage is driven by four 20 H.P. D.C. shuntmotors in series. Each motor is geared to one of the carriage-wheels. The energy-supply of the field-coils of these motors is maintained at a constant voltage of 220. The armature-coils can be given any voltage varying between 8 and 600, so that all
speeds between O and 81/2 rn./sec. (28 ft./sec.) are covered. During the
acceler-ation period, these motors can produce three times their normal torque. The 220 Volt circuit, to which the field-coils and some of the appliances on the towing-carriage are connected, is supplied by a converter consisting of
a 5 Kw. D.C. compound generator and a 10 H.P. synchronous3-phase motor.
The 600 Volt circuit, to which the armature-coils are connected, is supplied according to the Ward-Leonard system by a converter consisting of a 72 Kw.
EQUIPMENT AND WORKING-METHODS 37 synchronous motor of 135 H.P. and an exciter for supplying energy to the latter.
The synchronous motors of both converters are started asynchronously; they run on a tension of 380 Volts, converted in the NS.M.B.'s own high tension chamber from the 10,000 Volt net of the Provincial Electricity Co. of
Gelderland.
Results of the model tests within certain speed limits are obtained from the correlated results of a number of trips at different speeds, the speed remaining constant per trip. For this reason it is imperative that the speed of the towing-carriage shall remain constant during each measuring period. This is attained by laying the rails with the greatest possible degree of accuracy and by the choice of synchronous motors for driving the converters, and lastly by the provision of valve-regulators in the energy-supply of the 220 Volt circuit and
in the energizing of the 600 Volt dynamo exciter. By this means a constant speed of the towing-carriage is assured even during frequency-fluctuations of plus or minus 4% and load fluctuations occurring simultaneously also of plus or minus 4%. Formerly other model basins used ]arge accumulator batteries with charging equipment for this purpose. These however involved a far larger outlay in capital and were expensive in maintenance.
Electric current is supplied to the towing-carriage from four rails fitted to the roof.
The towing-carriage is driven and the converters switched on from the
switch-cabin (fig. 1, no. 25). When the carriage is travelling in the reverse direction and approaches the end of the track, a signal is given to the operator by two control lamps. Braking is electric for speeds up to about 3 m./sec. (10 ft./sec.). For higher speeds a frictional brake is used and the whole carriage is lifted slightly from the rails. To this end, very gently inclined rails with flanges turned upwards are laid alongside the ordinary rails at the east end of the basin (see fig. 30). Brake-shoes covered with brake-lining have been fitted alongside the wheels on the towing-carriage. These can be accurately adjusted in a vertical direction. As the carriage reaches the brake-rails, its weight is gradually transferred from the wheels to the brake-shoes. It then begins to slide first on the two foremost and subsequently on all four brake-shoes. On account of the gradual upward inclination of the brake-rails, deceleration commences gently but increases in force as the whole weight of the towing-carriage comes to rest on the four shoes. Before the towing-carriage can travel back the shoes are turned upwards and the carriage comes to rest on the wheels again. The conducting rollers maintain sufficient contact with the running-rails to prevent the carriage from slipping sideways over the brake-running-rails.
For emergency purposes there is a hand-brake fitted to the towing-carriage, and the current to the carriage-motors can be switched off. The brake consists of a brake-disc with l)rake segments on the shaft of each carriage-motor. As a precautionary measure, there are two buffers at each end of the tank; at the harbour end these are simple spring-buffers, at the other end hydraulic buffers with a stroke of 1.2 m. (4 ft.).
Fig. 33. Towing-carriage with one of the winches for the false bottom.
The east transverse carriage-girder is fitted to carry temporarily the winches for hoisting the false bottom (see fig. 33).
The following may serve to give an impression of the relation between ac-celeration-, measuring- and deceleration-distances: for a constant speed of 5 rn/sec. (17 ft./sec.), a distance of 30 m. (100 ft.) is covered during accele-ration and 15 rn. (50 ft.) during deceleaccele-ration; the remaining 195 m. (640 ft.) can therefore be utilized for taking measurements. For a constant speed of 7 m./sec. (23 ft./sec.) the corresponding figures are 55 m., 25 m. and 160 m.
(180 ft., 82 ft. and 525 ft.).
The speed attained by towing-carriage and model during the taking of measurements is calculated after each run from the recorded distance covered and the recorded time. Pins have been fitted on the north rail on the basin at equal distances of 1 m. (3 ft. 4 in.). These open a circuit-breaker on the towing-carriage for an instant as the latter passes. At the same moment a pen-point attached to a relay makes a small mark, thus breaking the straight line shown on the paper of the recording-drum. In a similar manner another line is traced showing the number of seconds indicated on a contact-clock. The speed can then be calculated from the relation between the distance co-vered and the time taken.
EQUIPMENT AND WORKING-METHODS
Measuring section er profiLes
-.--.----View from above
-
Lorgiludinai ectIOflC
.1Jy
-
Eg2l dh1iLt
¶5000
Fig. 34. General plan of the cavitation tunnel of the N.S.M.B.
E. The Cavitalion Tunnel
Cavitation phenomena occur on a propeller-blade section as soon as the pressure on the profile of this section at a certain point has attained such a high negative value that the absolute pressure obtaining at that point is equal
to the vapour pressure of the water. This causes the formation of cavities which
implode under increased pressure. The result is a hammering motion of the water on the propeller-blade which may cause erosion and detrimental vi-bration. These phenomena often occur at the ends of the blades where the section profiles are thin and the circumferential speeds greatest, or close to the boss where the profile sections are unfavourable.
With the increase in the load on the propeller and in the number of revolutions
per minute inherent in the greater speed and power of present-day passenger
and war-ships, the danger of cavitation erosion becomes more and more evident,
View indirection P
40 EQUIPMENT AND WORKING-METHODS
and the lack of equipment for examining these phenomena hadmade itself more and more strongly felt at the N.S.M.B.
The existing basin could not be used for the purpose because the atmospheric
pressure, which is a part of the absolute pressure on a given propeller-blade element, cannot be reproduced in it to scale. To this end a completely enclosed
canal is required, in which reduced air-pressure can be maintained. The most satisfactory construction is an enclosed tubular canal, through which water can flow at varying speeds and in which the model screw under observation stands at rest in a translational sense. In this way, the propeller-shaft can be
carried outside the canal and the measuring apparatus for torque, thrust and number of revs, per sec. can be set up outside the canal.
In 1938 it was decided to build a cavitation tunnel for theN.S.M.B. The
de-sign was entrusted to the Hamburgische Schiffbau-Versuchsanstalt, which had a great deal of experience in this field, having at that time already designed three cavitation tunnels, viz, in Hamburg, Russia and Japan.
The general plan of the tunnel is shown in fig. 34. The canal is rectangular with rounded corners. It is constructed in an upright position in order that
the impeller, which causes the water to circulate, could belocated at the bottom.
In this manner the impeller stands under sufficient staticwater-pressure to prevent its being subject to cavitation influences. The model screw under ob-servation is placed at the top so that the water can be adjusted to the required level above the propeller-shaft. The distance between the longitudinal axes of the horizontal parts of the canal is 7 m. (about 23 ft.) and that between the vertical parts 105 m. (about 341/9 ft.).
In the vicinity of the impeller, the tunnel section is circular in form with an interior diameter of 1500 mm. (about 5 ft.). The diameter of the impeller is a few mm. smaller. Guide vanes have been placed in a radial sense immediately behind the impeller. In the adjoining portion of the tunnel, the circular section gradually changes into a rectangular section with rounded corners; this portion is surrounded by a warm-water jacket in order to keep the water in the tunnel at a certain temperature (the tunnel water can be directly heated by letting
it circulate through a special heater in the boiler-room ofthe heating-installation
see fig. 2, no. 46). The water then flows round a bend, the top dimensions of which are 1780 x 2000 mm. (5 ft. 10 in. 6 ft. 7 in.) and which is fitted
with a guide vane in the centre. The sectional dimensions of the vertical portion above this bend increase upwards to 1900 x 2200 mm. (6 ft. 3 in. X 7 ft. 3 in.);
the aim of these gradually increasing sectional dimensions is to procure a tranquil flow, in which any air which may be present in the water can be evacuated by a vacuum-pump connected to the top bend portion; in this bend there are three guide vanes. There next follows a metal honeycomb through which the water is carried into the nozzle portion. Here the movement of the water is accelerated before flowing through the measuring portions at a homogeneous speed over the whole section. The section of the measuring por-tions is square with rounded corners, the area being 077 m2 (8.3 square ft.);
Fig. 35. Lower portion of the cavitation
tunnel with driving mechanism for the impeller.
42 EQUIPMENT AND WORKING-METHODS
the measuring portions are provided with a number of windows and Pitot-tube connections. In the top of the first measuring portion there is a lid for inserting symmetrical or asymmetrical bodies in front of the model screw in order to examine the influence of the inequality of the flow in the vicinity of the sèrew on the cavitation characteristics; in the second portion, the model screw under observation is placed. Behind the model screw, the bend in the tunnel is divided into two, so that the measuring apparatus can be set up as near to the model as possible; these two halves are joined up again at the bot-tom of the following vertical portion. A guide vane, in-erted in the top part of the bend, extends half-way down the vertical piece. Behind the measuring portions, the sectional dimensions are again increased in order to ensure that the water enters the impeller calmly. The bottom bend is provided with seven short guide vanes.
The tunnel rests mainly on two foundations under the bottom bends (fig. 35). One of the supports has been placed on rollers to allow for expansion due to changes in temperature. The nozzle and the two measuring portions can be replaced by pieces with different sections, e.g. for carrying out measurements on hydrofoil profiles. In order to be able to separate the two vertical portions of the tunnel for inserting the interchangeable parts and for holding them together when these parts have been removed, there is a horizontal girder with adjustable bolts. The measuring instruments and the motor for the model screw are placed on a platform that may be regarded as part of the tunnel construction. (fig. 35). The floor of the second storey is shown on fig. 34 by a dotted line; the tunnel and platform are in no way connected with this floor
and can move in it freely; the openings made in it are filled up with bituminous
material.
The tunnel is constructed mainly of cast iron, only the two vertical portions, the horizontal portion with warm water-jacket and the honeycomb being of
welded steel.
The maximum diameter of a screw to be examined is about 500 mm. (20 in.).
The model screw is driven by a 440 V. D.C. motor with an output of 250 H.P. at 1600 revs./min. This number of revs./min. can be increased to 3000 by a special feeding of the stator, but the 250 H.P. may not be exceeded. The motor is automatically ventilated. The maximum thrust is 1270 kg. (2800 lbs.), the maximum torque 110 kgm. (800 ft. lbs.).
The impeller for circulating the water is driven by a 440 V. D.C. motor with an output of 300 H.P. at 1200 revs./min. by means of a 4 : i gearing and
a Michell thrust-block which is capable of taking a thrust of 5 tons at 300 revs./ min.
Both motors are excited by a Ward-Leonard set, consisting of five machines,
viz, a 3-phase motor, two D.C. dynamos, a dynamo for phase compensation of
the 3-phase motor and an exciter-dynamo. The 3000V. motor is a synchronized asynchronous motor of 700 H.P. at 1000 revs./min. The accompanying dynamo
EQUIPMENT AND WORKING-METHODS 43
The D.C. shunt-wound dynamo for the impeller has an output of 247 Kw. at
55OAmp. and 45() V., the one for the model screw motor an output of 207 Kw. at 460 Amp. and 450 V. The exciter dynamo, with an output of 15 Kw. at 220 V, yields current for the fields of both dynamos and their motors and of the
phase-compensation dynamo; it also provides current for the ventilation-motor of the screw model motor and for the two vacuum-pump motors.
In order to be able to reduce the revs./min. of the impeller motor as much as possible, the 247 Kw. dynamo is supplied with a field winding with negative excitation, so that the residual magnetism is neutralized and the voltage may be reduced nearly to zero.
During a test the following quantities are measured: velocity of water,
temperature of water, pressure, number of revs. ¡min. of the screw-shaft, torque
and thrust (see III A and B).
The cavitation phenomena can be observed and photographed by means cf a stroboscope, which produces an effect as if the screw model were standing still (see III B).
III.
Measuring Technique
A. Review o! Tests and Measuremcnts1. Tests carried out in the basin
The following tests are part of the normal routine work carried out in the basin:
a. Resistance Tesis (fig. 37).
For these tests a paraffin-wax ship-model, made to a given scale, is attached to the resistance-dynamometer on the towing-carriage. The resistance of the model is measured at a series of corresponding model-speeds above and below the service- or trial-trip speed of the ship in question. According to Froude's law of comparison, the corresponding speed of the model is equal to the quo-tient of the given speed and the square root from the model-scale.
The resistance forces thus measured are appropriately converted so as to apply to the ship, and plotted in statistics in dimensionless form. By this means it is possible to judge the quality of the hull-form in question.
At the same time as the resistance is measured, it is possible to determine the
Fig. 38. Propulsion test.
46 MEASURING TECHNIQUE
trim of the model with the aid of trimming-apparatus which will be described below. The first run of the model, before the water in the basin has been dis-turbed, is utilized to engrave the wave-profile in the model. This is usually done at a previously estimated service- or trial-trip-speed. In order to assure a turbulent flow in the boundary layer along the model, a so-called trip wire 1 mm. (0.04 in.) thick is bound round the model at 1/20 of the model-length from the bow. The extra 2 to 3% resistance thus caused is not subtracted but is maintained as a safety margin.
The degree of exactitude of the resistance measurements may be considered
as 4to 1% for fine ships, 1 to 2% for full ships.
After a run during which measurements have been taken - these runs are
always made in the same direction - the towing-carriage travels slowly
back to enable the water to calm down again. This time is utilized for working out the recorded figures. Four to six runs can be made per hour, depending on the speed.
Resistance measurements can be taken with models travelling on the surface as well as with submerged models (submarines). For examining the latter, a special apparatus is required which will be described presently.
For ships travelling in restricted water, it is essential to reproduce the depth and breadth of the waterway to scale. This is achieved by means of the false bottom already mentioned, which has been inserted in the basin over a length of 200 m. (655 ft.).
In fig. 37 the attachment of the ship-model to the resistance dynamometer can be seen clearly. The motor and the propeller dynamometer have already been built in for the propulsion test, which usually immediately follows the resistance test. For the latter, however, they are not required.
b. Propulsion tesis (fig. 38).
In these tests the model propels itself by means of one or more propeller models driven by electromotors. The electromotors are placed in the model.
During the measuring period, the model is not connected to the towing-carriage, except through the trimming apparatus which serves to prevent the model deviating from its course. This apparatus exerts if necessary small athwartships forces on the model. The towing-carriage travels with the model and at the same speed.
Thrust and torque of the screw or screws are recorded during the measuring runs by means of dynamometers coupled between the electromotor and the screw. The speed of the towing-carriage (= speed of model) and the number of
revs, of the screw or screws are simultaneously recorded on the drum of the resistance dynamometer. For the latter purpose, the motor is provided with
an installation which enables a current-circuit to be closed every 25 revolutions,
thereby causing a deviation cf one of the recording-points.
By applying a certain towing-force to the model, the so-called frictional correction, on to which we shall not diverge at this point, it is possible to