CAVITATION-EROSION THEORY, TESTED IN PRACTICE,
WITH REGARD TO THE BLADES OF SHIP PROPELLERS
by J. A. van Aken and K. Tasseron, Drunen, Holland.
1. Introduction
The present paper discusses the cavitation erosion which was found to have occurred on the blades of a ship propeller seven months after it had been adopted for use.
Though a model of the propeller, before the latter itself had been manufactured, had been tested in a towing-tank in order to determine the relationship between revolutions per mi-nute, power, speed and efficiency, no tests had been carried out in a cavitation tunnel to deter-mine the non-cavitating properties of the pro-peller.
As a result of the cavitation-erosion pheno-mena which occurred on the blades of this propeller, cavitation tests were carried out in the cavitation tunnel in a homogeneous velocity field
at the Netherlands Ship Model Basin
(N.S.M.B.) at Wageningen.The results obtained from these tests are, in
the present article, compared with the ship
propeller. A few proposals made with a view to altering the blades of the existing ship pro-peller so as to render it non-cavitating, have been tested in the cavitation tunnel.Finally, a proposal made by the N.S.M.B. in order to attain «virtual» freedom from cavi-tation on the screw model by altering the shape of the curvature of the blade profile on the lead-ing edge (L.E.), is discussed.
II. Main data on the basis of which the pro-peller was designed
The propeller was designed for the propulsion of a single-screw motorship, having an engine
output of 6,800 SHP at 110 revolutions per
minute, and a speed of 15 knots in the full-load condition and under ideal weather conditions.
The shaft friction was fixed at 2 per cent.,
so
that the power delivered to
the screwamounted to 6,664 SHP.
The mean wake fraction was, in accordance with Taylor's method, estimated at 0.335.
The distance of the centre of the screw shaft from the surface of the water, inclusive of the height of the wave, amounted to 6,099 mm.
III. The design of the screw
A four-bladed Lima-bronze propeller with shock-free entrance of the blade sections was
chosen.
The design, which was made in December 1951, was prepared in accordance with [1].
The screw diameter was fixed at 5,500 mm; with this diameter, A was 0.1625, and with the
formula Aj = 0.978 A + 0.0810, valid for a
variable velocity field, A1 amounted to 0.2400. With a chosen minimum blade-area ratio = 0.38 and corresponding boss-diameter ratio 0.26, the maximum width of blade at 0.7 R was1,225 mm; see Fig. 26 in [1].
The other blade widths were at first deter-mined with the aid of this Fig. 26 as well.
By assuming an angle of rake of 9 degrees, the blade thickness at 0.2 R and 0.6 R was cal-culated with the aid of Romsom's strength for-mulae; see [2].
In carrying out these calculations a permis-sible compressive stress (inclusive of the com-pressive stress due to the centrifugal forces) of 6,750 p.s.i. for the maximum output of 7,200 SHP at 112 revolutions per minute, was
as-sumed.
As, owing to these stresses, the thickness-length ratio of the blade profile at 0.2 R was excessive, the blade width was increased by about 10 per cent. and the blade thickness re-duced by about 5 per cent.
Thus a thickness-length ratio of 0.216 at 0.2 R was obtained, which is permissible.
Next, the blade width was gradually faired from 0.2 R to the calculated value of 1225 mm at 0.7 R.
Table I
shows the blade
thicknesses, the blade widths and the thickness-length ratios of the blade profiles at the radii concerned.As no data of the variable velocity field were available, Fig. 80 from [31 for the radial distri-bution of the wake fraction was made use of for the purpose of designing with the modern circulation theory.
This distribution is represented in Table II
-while i - uY and ' have also been included in this table;
Table I.
Blade Thicknesses, Blade Widths and Thickness-Length Ratios of the Blade Profiles
Table II.
i -q.
Values of ,
I - "and fr'
being the mean wake fraction = 0.335; and being the wake fraction on the section
con-cerned.
As a starting-point for the calculation, the Baihan-Van Manen formula,
tan f3 1
=
. (see 1, p. 28),tan
ij
1_4j'
was chosen as a condition for the minimum loss of energy for wake-adapted screws.
Next, tan ß and tan Çj were calculated for the sections concerned in the conventional manner (see F 11). The value of ?J followed from
X : = 0.1625 : 0.2400 == 0.677.
For the calculation of the distribution of
i, Goldstein's reduction factor (x) (see [i], p. 9) was employed. To determine the effective
camber (f) of the
blade sections, Ludwieg-Ginzel's camber-correction factor (K), and the friction-correction factor (p), see [1], Figs. 19, 25 and 27.Fig. i
For the significance of the geometrical cam-ber (fo), see Fig. 1.
Table III shows the values of x, K, p., and the geometrical camber (fo) with regard to this screw design.
The results of the cavitation calculation
have been laid down in Table IV. This table
shows
p/q, a and the cavitation
factor ofsafety.
p/q was obtained by making use of Fig. 19 from [1], a being calculated in the ordinary manner.
It was decided to use the Lips aerofoil-shaped profile with asymmetrical mean line throughout the length of the blade. A description of this profile is given in [4], pp. 733 a
- 736 a.
Figures 2 and 3 represent this profile for
0.2 R and 0.7 R respectively.
The blade outline was kept symmetrical in respect of the generator line.
r As the pitch ratio H/D
R tan ßi, the
pitch as a result of the condition chosen for the minimum loss of energy, was constant for all blade sections and amounted to 4,150 mm. Experience showed that in practice the pitch value calculated in this way was generally a few percents too low, so that to test the screw
model in the towing-tank the pitch was
in-creased by 4 per cent. to 4,315 mm.
The principal data concerning the propeller designed may be summarized as follows:
Screw diameter D = 5,500 mm Pitch (constant) H = 4,315 mm Fig. 4 Fig. 5
1
-nRi -
1-1.000 0.870 0.765 0.235 0.975 0.874 0.761 0.239 0.950 0.878 0.757 0.243 0.900 0.886 0.750 0.250 0.800 0.905 0.734 0.266 0.700 0.945 0.703 0.297 0.600 0.991 0.67 1 0.329 0.500 1.094 0.607 0.393 0.400 1.281 0.519 0.481 0.300 1.654 0.402 0.598 0.200 2.350 0.283 0.717 r/R blade thick-nes S, in mm blade width 1, in mm s/i 1.000 16 0.975 22.5 750 0.0300 0.950 29 915 0.03 17 0.900 42 1,088 0.0386 0.800 68 1,213 0.0560 0.700 94 1,225 0.0767 0.600 120 1,215 0.0987 0.500 146 1,195 0.1221 0.400 174 1,165 0.1493 0.300 204 1,136 0.1795 0.200 238 1,100 0.2 160Number of blades Expanded-blade-area
ratio Fa/Fs
(corresponding with Fa ¡F5 0.38, if d/D = 0.26) Boss-diameter ratio d n ÍD Rake of blades=
9°Material = Lima bronze
Radial blade-thickness
distribution = Unistrength
Blade sections . . Lips aerofoil-shaped profile
with asymmetrical mean line. Table III.
Values of x, K, p. and fo
Table IV.
Values of- p/q, ci and the Cavitation Factor of Safety
IV. Tests carried out in the towing-tank
Based on the above-mentioned data, a screw model was, to a scale of 1 23.5, manufactured by a ship model basin abroad.
This screw model was tested in open and behind conditions.
According to the model self-propulsion test
the output, at the ship speed mentioned in
Chapter II, amounted to 6,734 SHP tank at116.1 revolutions per minute, with a propulsive coefficient of 0.742.
The open-water efficiency amounted to 0.58, the wake fraction to 0.364.
With 6,664 SHP tank, corresponding with 6,800 SHP engine, the ship speed amounted to 14.96 knots and the number of revolutions per minute to 115.5.
In designing the screw (see Chapter II), there-fore, no allowance was made on SHP tank for the trial run (full-load condition).
As the number of revolutions per minute had to be 110, the pitch was once more increased by 71/2 per cent. to 4,650 mm (l'/2 per cent. increase in pitch with i per cent. reduction in number of revolutions).
No allowance was deliberately made in pitch
for the influence of the difference
in wake value between ship model and ship.In the case of a single-screw ship this cor-rection normally amounts to about 2 to 2.5 per cent, of the number of revolutions or from 3 to 4 per cent, of the pitch (increase).
The screw model was not tested in the cavi-tation tunnel, as at the time (1951) such correc-tions of pitch were deemed permissible without running the risk of the non-cavitating proper-ties of the screw.
For an increase of the pitch by 7'/2 per cent. therefore, (from 4,315 mm to 4,650 mm), no correction was made to the geometrical camber (f0) of the blade profiles.
It has since been proved that this view was erroneous.
V. Service performances of the ship propeller After the ship had been put into service in 1954, an analysis was drawn up for the subse-quent seven months.
During this period the following results were obtained.
The mean indicated power during 11 obser-vations amounted to 7,685 IHP at 103.3
revolu-tions per minute.
The mean ship speed amounted, according to the log, to 14.23 knots for the full-load
con-dition.
With a
mechanical efficiency of 0.82 the engine SHP amounted to 6,300 and, if 2 per cent, shaft friction was assumed, the propeller SHP to 6,174.If these results are compared with the tank results for 14.23 knots, the allowance for the SHP tank will amount to 13.8 per cent.
According to the tank curve, corrected for the 4,650 mm pitch of the ship propeller, the number of revolutions per minute had to be
107.3. nR X K f0 in mm 0.975 0.390 0.390 0.897 18.45 0.950 0.530 0.530 0.896 19.30 0.900 0.700 0.700 0.893 20.30 0.800 0.8 70 0.835 0.887 24.40 0.700 0.930 0.860 0.876 31.80 0.600 0.965 0.865 0.858 40.50 0.500 0.975 0.870 0.828 55.50 0.400 0.975 0.950 0.789 72.10 0.300 0.990 1.360 0.746 74.00 0.200 1.045 1.900 0.696 74.80 nR p/q Cavitation factor of safety in O/o 0.975 0.154 0.2625 70 0.950 0.171 0.2780 62 0.900 0.208 0.3140 51 0.800 0.288 0.4040 40 0.700 0.400 0.5320 33 0.600 0.524 0.7329 40 0.500 0.708 1.0750 52 0.400 0.980 1.7450 78 0.300 1.414 3.380 139 0.200 1.996 9.140 357 4 0.42 0.18
The number of revolutions per minute made by the ship propeller was, therefore, 4 short, which is not in agreement with the prediction formerly made.
The pitch of the ship propeller, therefore, was too high by 5 to 6 per cent.
It would, after all, have been more advisable to make the ship propeller with the 4,315 mm
pitch originally calculated, as will be evident from what follows hereafter.
As it appeared during the first docking that cavitation-erosion phenomena had occurred
at about the same spots on all the propeller
blades, a more detailed investigation was made with the aid of tests carried out in a cavitation tunnel.
VI. The cavitation-erosion phenomena on the ship propeller
The photos Nos. 1, 2, 3 and 4 show very good patterns of the cavitation erosion set up on the
backs of the propeller blades Nos. I, II, III
and IV.
The photos Nos. 5 A, 5 B and 5 C represent this cavitation erosion on the propeller blade No. II at 1/4, 1 and 4 times its natural size
respectively, while the photos Nos. 6 A and 6 B show these phenomena at 1/4 and between 5 and
6 times the natural size.
These photos speak for themselves and re-quire no comment.
The experience gained with this propeller led us to decide upon having some cavitation tests
carried out.
VII. Discussion on the blades chosen for the bronze screw model tested in the cavi-tation tunnel
It was decided to have a bronze screw model, having 4 different blades and made to a scale of 1: 13, tested in the cavitation tunnel of the Netherlands Ship Model Basin.
Photo 2 Photo 4
Suction side bhide IT Suction side blade IV
Photo i Photo 3
Photo 5A
Caviation-erosiorr on suction side (see photo 2). full size.
Photo 5B
Cavitation-erosion on suction side (see photo 2). Full size.
Photo 5 C
Cavitation-erosion on suction side (see photo 2). 4 times full size.
Photo 6A
Cavitation-erosion on suction side (see photo 4). full size.
Photo 6 B
Cavitation-erosion on suction side (see photo 4). 5-6 times full size.
Blade No. i was manufactured in conformity with the blades Nos I,
II, III and IV of the
existing ship propeller.
Blade No. 2 was manufactured with the same type of Lips aerofoil-shaped profile with asym-metrical mean line, but with reduced pitch and corrected geometrical camber of the blade
pro-files.
Blade No. 3 was manufactured with the same type of Lips aerofoil-shaped profile with asym-metrical mean line and only reduced pitch.
Blade No. 4 was manufactured with a some-what modified Lips aerofoil-shaped profile at the tip of the blade and the mean line was kept symmetrical; all over the blade the pitch was reduced and the camber corrected.
The blade widths, blade thicknesses and blade outlines of all propeller blades were identical.
Table V.
Pitch Values for the Blades Nos 1, 2, 3 and 4
Design Existing Screw without correction Existing screw with pitch and camber correction Existing screw with pitch correction Table VI.
Geometrical Cambers (fo) for the Blades Nos 1, 2, 3 and 4 Geometrical camber (f0) in mm
nR
Blade i Blade 2 New screw Blade 3 Blade 4Therefore, blades Nos 2 and 3 might, if re-quired, be taken from the existing ship propel-1er by reducing the pitch and correcting the camber for blade No. 2, and by merely redu-cing the pitch for blade No. 3.
Blade No. 4, however, would demand an en-tirely new propeller.
Table V shows the pitch values of these blades.
Table VI shows the geometrical cambers for these blades.
Figs 4 and 5 show the Lips aerofoil-shaped profile with symmetrical mean line at 0.2 R and 0.7 R 'respectively for blade No. 4.
Fig. 2
LE
Fig. 3
The entire screw design of blade No. 4 will be dicussed more fully later; here it is sufficient
for us to give the value of .p/q,
and the cavitation safety factor of this propeller blade, as shown in Table VII. The condition for mini-mum loss of energy is the same as in the case of blade No. 1.V1II. Tests carried crut in the cavitation tunnel
The existing ship propeller (blade No. 1 L
II, III and IV) had, as said in Chapter V, an output of 6,300 SHP at 103.3 revolutions per minute.
The ship speed attained at this power was 14.23 knots.
With an engine output 'of 6,800 SHP the num-ber of revolutions per minute became
/6,80OV
X 103.3 = 106. The ship speed at
6,800 SHP may be taken to be /6,800\i
X 14.23 = 14.6 knots.
Blade No. 1 was thus tested for 106 revolu-tions per minute, whereas the blades Nos 2, 3 and 4 were tested for 110 revolutions per mi-nute.
Table VIII shows the conditions for which the blades Nos 1, 2, 3 and 4 were examined.
The results of the tests are illustrated
inphotos Nos 7, 8, 9 and 10.
It should be realized
that a homogeneous velocity field is created in the cavitation tun-nel, whereas all the blades were designed for a radially variable velocity field.Since, according to Table II in Chapter III,
i - ' from the tip of the blade to 0.7 R
ex-ceeds i -
(0.665), the loading of this part of the blade in the homogeneous velocity fieldof the cavitation tunnel will exceed that in
the radially variable velocity field behind theship.
Consequently, the cavitation phenomenon at the blade tips of the screw propeller behind the ship will be somewhat less pronounced than that observed in the cavitation tunnel.
0.975 18.45 25.00 18.45 25.00 0.950 19.30 25.50 19.30 25.50 0.900 20.30 27.00 20.30 27.00 0.800 24.40 31.00 24.40 32.50 0.700 31.80 37.00 31.80 40.50 0.600 40.50 47.00 40.50 50.50 0.500 55.50 59.00 55.50 64.00 0.400 72.10 72.10 72.10 76.50 0.300 74.00 74.00 74.00 74.00 0.200 74.80 74.80 74.80 68.50 r/R PITCH in mm
Blade 1 Blade 2 Blade 3 Blade 4 1.000 4,650 4,400 4,400 4,400 0.975 4,650 4,400 4,400 4,400 0.950 4,650 4,400 4,400 4,400 0.900 4,650 4,400 4,400 4,400 0.800 4,650 4,400 4,400 4,400 0.700 4,650 4,400 4,400 4,400 0.600 4,650 4,450 4,450 4,400 0.500 4,650 4,550 4,550 4,405 0.400 4,650 4,630 4,630 4,410 0.300 4,650 4,650 4,650 4,425 0.200 4,650 4,650 4,650 4,450 Corrected: New: Hj )R Pitch Existing: 4,650 mm Hi .0R HO.7R = HO.GR = constant 4,400 mm 4,400 mm HU.R = 4,650 mm HO.2R = 4,450 mm
Table VII.
Values of p/q, u and the Cavitation Safety Factor of Propeller Blade No. 4
Table VIII.
Data for the Condition in which Propeller Blades Nos 1, 2, 3 and 4 were tested
This conclusion, however, has no reference to the peripheral inequality of the velocity field. The peripheral inequality of the velocity field manifests itself as a rule as sheet cavitation on the backs of the propeller blades.
It is generally assumed that this sheet cavi-tation causes no erosion.
On the ground of the photos Nos 7, 8, 9 and 10 we arrive at the following conclusions Photo No. 7, blade No. i (existing propeller
blade)
Back. Fairly heavy sheet
cavitationat the
leading edge (L.E.) from 0.7 R to the tip of the blade.The sheet cavitation terminates in large bubles and vapour clouds.
Photo No. 8, blade No. 2 (reduced pitch, in-creased camber)
Back. Sheet cavitation at the leading edge from 0.7 R to blade tip. The sheet cavitation ter-minates in small bubbles and vapour clouds.
A slight degree of bubble cavitation on the
middle part of the blade between 0.8 R and0.9 R.
Photo No. 9, blade No. 3 (reduced pitch) Back. Sheet cavitation at the leading edge from
0.7 R to blade tip. The sheet cavitation termi-nates in small bubbles and vapour clouds. An initial indication of a slight bubble cavi-tation on the middle part of the blade between
0.75 R and 0.8 R.
Photo No. IO, blade No. 4 (entirely new design) Back. Sheet cavitation at the leading edge from 0.7 R to blade tip. The sheet cavitation termi-nates in small bubbles and vapour clouds. None of the blades had cavitation on their faces.
Theory and practice
If photo No. i (back of blade No. I of the
ship propeller) is compared with photo No. 7 (back of blade No. 1 of the bronze screw model) a striking agreement will be noticed between the spots where cavitation erosion occurs on the blade of the ship propeller and the cavitation phenomenon on the blade of the screw model. The sheet cavitation at the leading edge (L.E.) of the blade evidently does not lead to erosion. The blade of the screw model shows vapour bubbles in places corresponding with the spots on the blade of the ship propeller where erosionoccurs.
The vapour clouds, however, seem to have had no influence on the material of the blade of the ship propeller.
Even the fairly heavy tip vortex leaves no trace of cavitation erosion behind on the blade of the ship propeller.
The conclusion may therefore be drawn that it was the vapour bubbles which caused the cavitation erosion on the blades of the ship pro-peller.
Proposal made by the N.S.M.B. to alter blade No. 4 so as to obtain a non-cavitating propeller blade
In connection with the results obtained, the N.S.M.B. suggested the following alterations to be made to blade No. 4
NORMAL ROUNDING NEW SHARP ROUNDING
Fig. 6
r/R a Cavitation safe-ty factor in
0.950 0.192 0.2780 45 0.900 0.234 0.3140 34 0.800 0.320 0.4040 26 0.700 0.436 0.5320 22 0.600 0.567 0.7370 30 0.500 0.766 1.0750 40 0.400 1.060 1.7450 64 0.300 1.508 3.440 128 0.200 2.120 9.450 345 Blades Nos.
Data Blade No. 1
2, 3, 4
SHP engine 6,800 6,800
Revs/mm for the ship
propeller 106 110
Revs/mm for the screw
model 1,381 1,433
Ship speed in knots 14.6 14.6
Wake fraction 0.335 0.335
A (advance coefficient) 0.5135 0.4949
Velocity of the water in
the tunnel in rn/sec 5.0 5.0
Photo 7 Blade 1, N = 106 mm, 20 percent = 10.06 A = 0.5135 Photo 10 Blade 4, N = 110 min. - 20 percent = 10.06, A = 0.494f)
Fining down the curvatures of the leading edge of the blale from 0.5 R to the tip (see Fig. 6).
Shortening the blade profiles from 0.6 R o the tip at the leading edges (see Fig. 7). The alteration a aimed at rendering the
lead-ing edge of the profile less sensitive to the
deviations from shock-free entrance than is the case with an ordinary curvature.
The alteration b led to a slight increase of the geometrical camber of the profile.
The alterations were introduced in the case of blade No. 4, after which blade No. 4 a was tested in the cavitation tunnel once more.
The results of this test are
illustrated inphoto No. 11. Photo 8 Blade 2, N = 110 min - 20 percent = 10.066, A = 0.4949 Photo 11 Blade 4 a, N 110 mm, - 20 percent = 10.06, A = 0.4949 Photo 9 Blade 3, N = 110 mm, - 20 percent = 10.06, A = 0.4949 Photo 12 Blade 3 a, N = 110 mm, - 20 percent = 10.06, A = 0.4949
If next photo No. 10 (blade No. 4) is compared with photo No. 11 (blade No. 4 a) it will be seen that On the back of the blade the sheet cavitation at the leading edge from 0.7 R to the tip has entirely disappeared, as well as the small bubbles and vapour clouds. Nor did the face of the blade show any cavitation.
Blade No. 4 a may be called «virtually» ca-vitation-free.
The result obtained is astounding and clearly demonstrates the influence of the shape of the curvature of the blade profile at the leading
edge.
There will only be sense in introducing the alterations in blade sections having shock-free entrance, hence in screws which have been
de-Fig. 7
signed with the aid of the circulation theory. It would be no use introducing them with ordinary «Troost» B-series screws, since in this
case the blade sections have no shock-free
entrance.By further utilizing the favourable result
obtained, blade No. 3 (existing blade of the ship propeller with reduced pitch) was altered exactly as had been done with blade No. 4.The result obtained from this alteration is shown in photo No. 12
In comparing photo No. 9 (blade No. 3) with photo No. 12 (blade No. 3 a) a considerable im-provement is again manifested.
Blade No. 3 a shows only very little sheet cavitation.
From the above experiments it is evident that, by reducing the pitch and by the employ-ment of special curvature of the leading edges of the blade profiles, it has become possible to render the existing blades of the ship propeller, under disscusion in the present article, «virtu-ally» free from cavitation.
XI. Summary
The conclusions to be drawn from the investi-gations discussed, are the following:
1. In designing ship propellers with blade
pro-files having shock-free entrance (circulation-theory propellers) it is not permissible to carry out the correction for a considerable difference in the number of revolutions of the screw model examined in a towing-tank.
and the number of revolutions required for
the ship propeller as a correction of the
nominal pitch only.
The correction of the nominal pitch must not exceed 2 to 3 per cent.
With higher percentages part of the correc-tion has to be made by altering the geome-trical camber of the blade profiles.
In order
to obtain a higher degree ofinsentisiveness to cavitation at the leading edges of profiles having shock-free entrance,
the adoption is recommended of the curva-ture used by the N.S.M.B. and represented in Fig. 6 of the present publication.
Though in the large cavitation tunnel of the N.S.M.B. only a homogeneous velocity field could be generated, whereas the blades of the screw model had been designed for a radially variable velocity field, the agree-ment between the cavitation-erosion
pheno-menon on the backs of the blades of the
ship propeller and the cavitation phenomena on the screw model may be called very good Conclusion 3 does not apply to
the
peri-pheral inequality of the velocity field; theinfluence of this inequality could not be
investigated at that time. Only a few months ago (summer 1956) a cavitation tunnel with controllable velocity distribution over the screw disc (see International Shipbuilding Progress, Vol. 2, No. 16-1955), was estab-lished at the N.S.M.B.
In order to avoid disappointment it is of the
greatest importance that a screw design
caned out with the aid of the circulation
theory should be tested on its non-cavitating properties before the ship propeller is ma-nufactured.
B I B L JOG R A P H Y
Balhan, J., and Van Manen, J.D. «Het ontwer pen van cavitatie-vrije scheepsschroeven», Schip en Werf, 1950, 20 January and 17 February.
Romsom, J. A.: «Propeller Strength Calculation», The Marine Engineer and Naval Architect, 1952. February and March.
Van Manen, J.D.: «Invloecl van de
ongelijkma-tigheid van het snelheicisveld op het ontwerp
van scheepsschroeven»>. Thesis 1951, Publication No. 100 of the N.S.M.B.
Van Aken, J. A., and Tasseron, K.: «Over de
invloed van de vorm van de skeletlijn van draagvleugelprofielen, toegepast bij
scheeps-schroeven op hat rendement en de cavitatie-eigenschappen». Polytechnisch Tijdschrift 1954, 1
and 15 September. Translation in English has been published in International Shipbuilding Progress, Volumes 12 and 14, 1955.
