Model tests with icebreakers

FRAN

STATENS SKEPP.SPROVNINGSANSTALT

(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TANK)

Nr 20 _{GOTEBORG 1952}

MODEL TESTS WITH

ICEBREAKERS

BY

H. F. NORDSTROM, HANS EDSTRAND AND HANS LINDGREN

GUMPERTS AB GOTEBORG

Model experiments with two icebreakers have been carried out in

recent years

at the

Swedish State Shipbuilding

Experimental Tan k.

Since this type of ship is unique as regards both form and propulsion arrangements and since, _moreover, there is only a very limited amount of published data on the subject, an account of these experiments and their results is given in this paper.

The experiments consisted of model tests on a triple-screw icebreaker (two after propellers and one forward propeller) with a displacement

= 2200 m3 for the Swedish Government and on

a quadruple-screw icebreaker (two after propellers and two forward propellers) with a displacement 17

_{4350 m3 for the Finnish}

Ministry of Commerce and Industry.

Both ships were designed for diesel-electric _{propulsion. The} arrangement with one forward propeller has hitherto been the general rule for large icebreakers, particularly for those designed for service in the Baltic Sea and nearby waters. The forward propeller is not intended in the first place to contribute towards propulsion but its purpose is rather, when icebreaking, to break up and sweep aside the loosened ice. The latter applies mainly to work _{in pack-ice, but} the forward propeller can also be of use when breaking hard ice, since its suction helps to break the ice and the slipstream _washes the sides of the ship, thus tending to prevent the ship_{becoming fast in} the broken channel.

It will be evident from the above that the forward _{propeller can} be subjected to considerable strain and it must _{therefore be of very} robust construction. Icebreakers which are designed for more severe ice conditions than those normally found in the Baltic Sea are gene-rally not fitted with a forward propeller. This is thecase, for example, with the Russian icebreakers which are designed for service in Arctic regions.

In recent years, opinion has tended in favour of fitting icebreakers with two forward propellers, the main purpose being to increase the

intensity of the aforementioned washing of the ship's sides. When only one forward propeller is fitted, experience has shown

that the

slipstream runs mainly along one side of the ship (the port side with a right-hand forward propeller, the starboard side with a left-hand propeller). With two forward propellers, however, the slipstream becomes symmetrical and the washing effect is thus the same on both sides of the hull. In this arrangement, the starboard forward propeller should be left-handed and the port one right-handed, i. e. the forward propellers should be inward-turning.

This idea has

already been applied in practice in the Canadian icebreaking

train-ferry AB EGWEIT.

2.

Symbols, Units and Methods of Calculation

['lie symbols have been chosen in accordance with the recommendations made by

Sixth International Conference of Ship Tank

Super-intendents.

Ship Dimensions

AP after perpendicular

FP = forward perpendicular

= length on waterline (= Lpp, length between_ perpendiculars, in present cases)

= breadth on waterline draught

Am = immersed midship section area

= wetted surface area (including wetted surface area of rudder and bossing)

volumetric displacement

1(20,e half angle of entrance on waterline

Propeller Dimensions

= propeller diameter = propeller pitch

(a I

Ao = propeller disc area _{4 i} Ad = developed blade area

= maximum blade thickness referred to centre of propeller

Kinematic and Dynamic Symbols and Ratios

- speed in general v, = speed of advance

= ship's speed in Metric knots = resistance - = propeller thrust the L S = = = = V = =

= propeller torque

= rate of revolution (revs, per unit time)

P, = effective power

P, = shaft power (at tail end of shaft) E Ps = total shaft power

=- towing force (pull)

V

wake fraction (TAYLOR)

T

thrust deduction factor

--_ density of water (102.0 kg sec.2/m4 for fresh water, 103.5 kg sec.2/m4 for Baltic Sea water)

kinematic viscosity of water

V.IPe (m3, Metr. knots and HP)

C2 = V213 PIP, (m3, Metr. knots and HP)

Fnv = Vg v4I3 = FROUDE'S number, displacement

Coefficients and Ratios

= block coefficient

L BT

Am

I3

BT

midship section coefficient

V prismatic coefficient --- length-breadth ratio = breadth-draught ratio 17113 = length-displacement ratio = pitch ratio D.

= disc area ratio

= blade thickness ratio

KT --= thrust coefficient e-D4 n3 Q torque coefficient eDs n2 Dimensionlessfl

re -= = advance coefficient Dn KT

J

KQ 2 a P, = propulsive efficiency

Units and Conversion Factors

Metric units are used throughout.

metre --- 3.281 ft. (recipr. 0.3048)

metric ton 1000 kg = 0.984 British tons (recipr. 1.016)

metric knot --- 1852 rn/hour = 0.999 British knots (recipr. 1.001)

metric HP = 75 m kg/sec. = 0.986 British HP (recipr. 1.014)

For g (acceleration due to gravity) the value 9.81 m/sec.2 has been used.

Methods of Calculation

The model-scale results from the resistance tests have been converted to the scale

of the full-sized ships in the conventional way in accordance with FROUDE'S method.

The frictional resistance has been calculated using the formulae decided upon at

the Tank Superintendents' Conferencel) in Paris in 1935. No

length correction has been employed.

All the self-propulsion experiments were carried out according to the so called

Continental method (GEBERs)2) with the skin-friction correction applied as a towing

force. The results have been converted to full scale in the conventional manner In the towing power experiments, the corresponding towing force in ship scale has been obtained by assuming that it varies directly as the cube of the model scale.

In converting the measured values to ship scale, corrections for scale effects, air resistance, hull condition etc. have only been applied where it is expressly stated

so below.

3.

Ship and Propeller Data

Both the ship models were made of paraffin wax. The triple-screw model was designated No. 337 and the quadruple-screw model No. 451. The main particulars of the ships are given in Figs. 1 and 2 and

= propeller efficiency in open water

') Modified here. See Some Systematic Tests with Models of Fast Cargo Vessels, II. F. NORDSTRoM, Publication No. 10 of the Swedish State Shipbuilding Experi-mental Tank, Goteborg, 1948, p. 7.

9 See Congres International des Directeurs des Bassins, Paris, 1935, p. 86.

77prop.,

1

1 1

0

Model No. 337

Fig. 2.

Table 1

(BIT, 6, # and cp, are calculated using Tinean)

9 Fig. I. Model No. 451 9 10 10

Unit Ship Model No.

337 451 Model scale 1:12.5 1:17.5 Number of propellers 4 L m 58.53 77.50 B m 15.44 18.70 Tfoed m 5.04 6.00 Tait. m 5.34 6.40 Tmean m 5.19 6.20 V m3 2200 4350 S m2 943 1520 LIB 3.79 4.14 BIT 2.98 3.02 LIvils 4.50 4.75 5 0.469 0.484 13 0.807 0.817 (P 0.581 0.592 1/2 a, degrees 26 25 .. ,,

...

. . ..

Propeller No P346

Arvilf4-Ford

Propeller No. P345

Aft. Fig. 3.

in Table 1. Both models were fitted with a rudder and shaft bossings. No turbulence stimulators were used in any of the tests.

Particulars of the propellers are given in ship scale in Table 2 and their outlines are shown in Figs. 3 and 4. The right-hand example is illustrated in each of the figures.

In addition to being tested with the after propellers P 345 (see Table 2 and Fig. 3), the triple-screw model (No 337) was also tested with another pair of after propellers, P 369 (see Appendix 1). The

latter propellers had the same dimensions as P 345 except as

Prtch P-2.00 I -\ st Soo , 370 .770, fr545 0,6 v Pitch P.2300 /I

80 Ford

Propeller No. P 439

Pitch Propeller No. P 440 tAft. Fig.. 4.,

regards pitch, r, Which was increased (about 15 '%) from 2300 mm to 2650 mm

Finally some tests were carried out with Model No. 337 fitted with another forward propeller, P 375. This propeller differed from P346

only in that it was left-handed. The

reason for also testing this

model with a left-hand forward propeller is ,dealt with further in Section 6. 567 56 7 \ 0 Pitch P 2 600 F8 --.;;;\ I/ _.'c98 I_' .547,9 rrA'nWAPA 5 7.5 .57.3. a iso PrZAi -2570 2273 k 510 1 ₆₀ H 604

r

580 I so° 5.47 6-.< P

Table 2

I. The Icebreaker with Two After Propellers and One

Forward Propeller (Model No. 337)

4.

Experimental Arrangements

The resistance tests and open water tests were carried out in the usual manner. For the resistance tests, dummy bosses and cones were fitted in place of the propellers.

As already mentioned in Section 1, the ship was designed for

diesel-electric propulsion with three diesel engines each driving its own generator, the latter in turn being electrically coupled to its own propeller motor (one motor for each propeller shaft). It is not possible here to give a more detailed description of this machinery arrangement since it lies beyond the scope of this paper, but reference can be made to Development of Ice-Breaking Vessels for the U. S. Coast Guard, HARVEY F. JOHNSON, Trans. S. N. A. M. E. Vol. 54, New York, 1946 and Statsisbrytaren Ymer, GUSTAF HALLDIN, Teknisk

Tidskrift, Stockholm, 1932.

When each propeller is fitted with a separate motor, freedom is

1) The pitch is not constant over the whole radius and the figure given refers to the outer sections.

3) Starboard propeller right-handed, port propeller left-handed. 3) Starboard propeller left-handed, port propeller right-handed.

Unit

Ship Model No.

337 1

451

Propeller Model No.

Aft. For'd Aft. For'd

P 345 P 346 P 439 P 440 Model scale 1:12.5 1:17.5 Number of propellers . . . . 2 1 2 2 D mm 3650 3000 4200 3500 P mm 2300 2800 41001) 26001) PID 0.63 0.93 0.98 0.74 Number of blades 4 4 4 4

Right or left handed . . . . 2) right 2) 3)

Ad/A0 0.55 0.60 0.53 0.58 tID % 6.8 7.3 6.1 8.0 Rake degrees 0 0 5.4 0

...

- --.,-. . .... .

. ...

.. . .. . .. .

provided to select for each occasion the most suitable direction of rotation and the most suitable revolutions for each propeller within

the performance limits of the respective motors. In ships of this

type it is possible, for example, when breaking pack-ice, to run the two after propellers full astern and the forward propeller full ahead. This produces the aforementioned effect of washing the ship's sides, whereby the frictional resistance against the ice is reduced and the liberation of the ship from the ice is made easier. Or, on the other hand, when going ahead in hard ice, it is possible to run the forward propeller astern while the two after propellers work full ahead, the

purpose being to generate, by means of the forward propeller, a

powerful bow wave and a disturbance in the water ahead of the

vessel and so facilitate icebreaking.

It will be evident from the above that there are in practice a large number of possible combinations of the ship's speed and the directions of rotation and revolutions of the respective propellers. Self-propulsion

experiments with the model were, however, necessarily limited

mainly to an investigation of the case when all three propellers are working ahead at the same revolutions. In addition, however, self-propulsion experiments have been carried out with only the after propellers working and the forward propeller disconnected from the propelling machinery and free to rotate, with the forward propeller disconnected as before but locked in a certain position and finally with the forward propeller removed and replaced by a dummy boss.

In the main tests, all three propellers were driven at the same

revolutions through gearing from one electric motor. Self-propulsion dynamometers, which registered revolutions, thrust and torque in

the usual way, were in this case coupled to the starboard after

propeller and to the forward propeller. Check recordings made with a dynamometer coupled to the port after propeller showed, however, that the two after propellers were loaded differently, due presumably to the fact that the forward propeller race is divided unsymmetrically between the port and starboard sides (see Section 1 regarding the distribution of the washing effect between the two sides of the ship). Since only two suitable propeller dynamometers were available, there-fore, the tests were duplicated so that all the results would include recordings from all three propellers. The aforementioned asymmetry in the forward propeller race distribution and the consequent diffe-rence in loading between the two after propellers are discussed in greater detail in Section 6.

Experiments with the forward propeller free, locked and removed in turn were carried out in a similar manner to the above, i. e. the

two after propellers were run at the same revolutions, driven

through gearing by an electric motor. Revolutions, thrust and

torque were measured on both after propellers and, in addition, in the case when the forward propeller was rotating freely, its revolutions were determined by means of a stroboscopic tachometer.

It was not possible, of course, to carry out tests which would

illustrate directly the icebreaking qualities of the ship. A ship's

performance as a tug can, however, give an indication of its suitability as an icebreaker. Various towing conditions have, therefore, been investigated by applying different extra resistances to the model

from the carriage in the course of ordinary self-propulsion tests. The additional resistance was applied to the model by means of a wire stretched horizontally between the stern of the model and the resistance dynamometer, which in this case was placed behind the model on the carriage. By means of this arrangement, the towing force could be continuously recorded during the experiments. Towing power experiments, running both ahead and astern, were carried out with all three propellers working at the same revolutions.

The aforementioned asymmetry, due to the slipstream of the

forward propeller, causes the ship to have a certain tendency to yaw. This has been confirmed in practice in ships of similar type. This

tendency has, therefore, been the subject of an investigation in

conjunction with the model experiments in question and the results are given in Appendix 2.

All the self-propulsion and towing power experiments mentioned above were carried out with propellers P 345 and P 346 (see Table 2). In addition, however, as stated in Section 3, self-propulsion tests were carried out with a second pair of after propellers with 15 % greater pitch than P 345, but otherwise exactly similar (see Appendix 1). Another forward propeller, left-handed but otherwise similar to P 346, was also tested (see Section 6).

5.

Resistance Tests and Open Water Propeller Tests

Resistance tests with Model No. 337 were carried out over the speed range 4-16 knots and the results are shown in Table 3 and Fig. 5. This diagram shows Pe and C, as functions of the correspond-ing ship speed and the correspondcorrespond-ing C, values obtained from Model

600 400 200 LC' qq 4 6 8 Model No. 337 Model No. 451 /0

Ship Speed, V, in knots (Metr.) Fig. 5.

No. 451 have been included for comparison. The comparisons should

however, be judged with caution, since, as already stated in Section 2, no correction for length of ship has been applied.

The results from the open water tests with the propellers P 345 and P 346 are given in Fig. 6. Of the after propellers (P 345), only the right-hand propeller was tested in this case, since, of course, the left-hand propeller is of the same design.

6.

Self-Propulsion Tests

Self-propulsion tests were carried out over the speed range 9-14 knots and the results are given in Table 4 and Fig. 7. In the latter diagram, the total shaft power for those propellers which were working

/2 /4 13

ir-16 2000 1500 1000 500

11

Table 3 Resistance Tests'

during the respective tests is shown as a function of the corresponding ship speed. The corresponding revolutions are also included together with the revolutions of the freely rotating forward propeller in the appropriate case.

It is evident from Fig. 7 that the state of the forward propeller is of considerable importance, from an efficiency point of view, when the ship is moving in open water. Voyages of some length under such conditions are conceivable, especially in going to or from an icebreaking job. Besides fuel economy, the maximum speed which can be reached with the power available must be considered. This is so, particularly if, for example, an ice-bound ship must be freed with the minimum delay.

Over the whole of the speed range investigated, probably the Most favourable case from the point of view of efficiency is when the forward propeller is removed and all the available power is absorbed on the two after propellers (see Fig. 7). This case, however, is of no practical interest for the ship in question, since the forward propeller is essential for icebreaking purposes.

It appears from the results from the other alternatives that when

Ship Model No. 337

V Env Pe CI knots HP ( NI et r .,), (Metr.), I 4 0.182 I 23.6 459 .... 0.228 0.273 ; i 1 43.8 74.5 483 490 ti. 0.319 II 114 508 8 0.364 175 496 9 I 0.410 254 485 10'' I 0.455 359 471 11 0.501 i 503 448 12 0.546 I 684 427 13 0.592 1 - 959 388 14 0.638 I 1400 332 15 0.683 1904 300 16 i 0.729 2519 275 6

'10 Kr. 100 K 5 4 3 2 , 10

Propeller Water Temp.

I

li n _{n 0}

No. , t °C m2/sec. ' r/sec. V

____--- 1 0 345 Aft. Prop. 12.9 L208Id6r 8.2 5.79-106 P346 Ford Prop. 12.9 1 1.208-10-6 10.6 5.05-105 0 I

/

...-0--\ 1 I

...

_ik.

.

.

.

. N 1 1 . , I 1 11 II. I, _ _

4

, .

I

q $ li 11 1

.

_\

,

t I 1 II 1 K Q 1 NO ; II

\

l' \i, t\ I ). "` 1 0.2 04 e 0.8 Ve n Di Fig. 6'.

moving in open water, as much of the available power as possible should be absorbed on the after propellers. With the forward propeller rotating freely, there is an average reduction in total power of 10 % as compared with the case when all the propellers are working at the same revolutions. 60 5(1 40 30: 20 6

__----460'

/20

80

40

0

Model No. 337

Propellers Nos. P345, P346

Propelters Working fnfor,d / no: I )

FOrward Propeller Running Free' Forward Propeller Fixed

Forward Propeller Removed'

4000 3000 2000 mod II 1 I -...., ,./ ..., ...-' I.

--...' .,..., ,...-...., ..." 1 ..." ...01 ._../.."'''....". ... ' 1 ...-- ...-- --- ....--,1,---* --...- ij --I I...--...,' ...-- ...!.. ...< "'"-r.,...,... n -_ ... - --1 , free') 1 ... running _...-- ' n fore_

-

--L, --- --_-

/

.

/

/

.

/

/

1.-- -- _ _ ____

/

../ -<ZP

//

...-//

..,...

/

/

/.

I 1 1 ...: .-...-. --- --....- ...

.--,

...---''27) soft. , ....-. ...., ----

,

- --.--- .. -1 _ _ _. 9 10 '112' 13 /4

Ship Speed, V, In knots (Metr.)

Fig, 7,

All

Table 4 Self-Propulsion Tests

1) The propeller was fixed with two blades Vertical and the others horizontal., 21) The propeller was replaced by a dummy boss and cone.

.

Ship Model No. 337

_-,,-.-1 , After Propellers P345 Forward Propeller P 346

Total' V F4v Ps II w 1 t P1. Ps ?I E Ps .___.::__ _{C2 t} Port n Strbd. Port. , Mean

Strbd. E P,

'

knots -

HP HP (Metr.), jr..11.111- 1 % HP (Metr.) r. p,. m. % % HP (Metr.), 1i 1

All Propellers Working with the same r. p.

m-9 0.410 284 249 109 11.3 8.0 240 109 22.1 60.9 773 32:9 1160 43.2 10 0.455 391 346 122 11.3 8.1 5.3 333 122 23.0 60.2 1070 33.6 158 43.0 11 0.501 536 487 135 10.9 I 8.5 7.8 453 135 23.0 61.0 1476 34.1 153 41.6 12 I 0.546 719 624 148. 11.1 7.7 10:0 602 148 23.1 61.4 1945 35.2 150 42.4 13 0.592 914 843 161 10.3 8.2 11.5 812 161 23.8'60.5 2569 37.3 145 40.9. 14 0.638 1266 1190 178 9.3 7.4 12.3 1150 178 24.5 58.6 3606 38.8 129, 38.4 After Propellers Working. Forward Propeller Running Free

91 0.410 348 :330 111 14.5 12.2 _{78 678 '182} 1 10 0.455 493 46,7 124 14.8 12.6 85 960 170 11 0.501 680 63'0 , 138 14.1 111.6 94 1310 172 12 0.546 940 825 , 153 13.8 9.7 104 1765 166 13 0.592 1238 1137 168 12.6 9.4 113 1 2375 156 14 0.638 1689 1559 185 11.3 8.8 I 122 3247 143

After Propellers Working. Forward Propeller Fixed')

, 9 0.410 461 450 1 117 14.8 13.2 911 135 10 11 0.455 0.501 656, 873 646 855 132 146 j13.7 13.5 12.7 12.3 1302 il 1728 130 tad 12 0.546 1132 1086 160 12.7 11.4 _{2218 132} 13 14 0.592 0.638 1505 20251 1455 1959 175 191 12.5 12.4 11.2 11.2 ' 2960 , 3984 126 116

After Propellers Working Forward Propeller Removec12)1

9 0.410 321 321 111 12.3 21.8 642 39:6 192 10 0.455 458 463 124 12.0 23.8 r 92[ 390 184 11 0.501 631 636 138 11.7 24.5' ' 39.7 178 12 0.5461 831 802 151 11.5 I23.8 , 1633 41.9 ,179 I 13 0.592 1085 1072 165 11.1 23.4 2157 44.5 172, 14 0.638 1503 1489 182 10.5 21.3 1 2992 I 46.8 155 t (Metr.) % 1267

For the tests with a free forward propeller, the latter was fitted on a short piece of shaft which ran through the bossing in the usual way. There was therefore a certain amount of friction in turningthe propeller and this in turn caused resistance. Of course, this resistance

cannot simply be assumed to be in scale with the resistance which would result in the ship if the forward propeller were to be discon-nected in a corresponding manner. Similarly, the revolutions of the uncoupled forward propeller, as measured with a stroboscopic tachometer (see Section 4) and converted to full scale in the usual way, cannot either be considered as being true to scale.

However, less resistance and therefore less power on the after propellers can presumably be expected if the ship's forward propeller is provided with enough power to rotate it without thrust (T 0).

On the other hand, some account must also be taken of the power which is supplied to the forward propeller in this case.

The maximum speed attainable in open water naturally depends on the available power and the possibility of utilising it under the particular loading conditions. The probable performance under trial conditions in open water is discussed further in Appendix 1.

The distribution of power between the three propellers when they are all working at the sam& revolutions is illustrated in Fig. 8. As stated in Section 4 above, the loading on the two after propellers is clearly unequal, for the power transmitted by the starboard after propeller is 10 to 15 ()/0 greater than that transmitted by the port after propeller. As also mentioned earlier (Section 4), this is due to the unsymmetrical distribution of the slipstream from the forward propeller. The right-hand forward propeller (possibly in combination with the special form of the ship) probably causes a greater mean speed of advance at the port after propeller, which is therefore less heavily loaded than the starboard after propeller. Further support for this theory is provided by the observed difference in the washing effect on the two sides of the ship in a triple-screw icebreaker. With a right-hand forward propeller the slipstream is stronger on the port side than on the starboard side.

In order to throw further light on the effect of the direction of

rotation of the forward propeller upon the loading of the after propel-lers, some experiments, as already mentioned in Sections 3 and 4, were carried out with another forward propellerwhich was left-handed but otherwise similar to the first.

Model No.

337

All Propellers Working (nforv nap.' I

Strbd.

Aft. Propellers No. P345

Port

For'd Propeller No. P346

/000

750

500

250

9 /0 ii 13 /4

Ship Speed, V, in knots (Metr.)

Fig. 8.

propulsion arrangements is illustrated in Figs. 9 and 10.

These

diagrams show the percentage differences in power and propeller thrust respectively as functions of corresponding ship speed.

The magnitude and general tendency of the differences in power and thrust between the two after propellers

are about the same,

regardless of the direction of rotation of the forward propeller.

cb

All Propellers Working ( _for'd

n aft.' /.)

Forward Propeller Running Free

Forward Propeller Fixed Forward Propeller Removed

/

J

/

/

1 .0...

\

\

z

...--0' For'd

/

1

/

/

Aft. Prop. Prop. No. No. P345 ,P346 (Right-blonde.

---.'-.. .I.1 ...-... 1:as, ../ I 1 ...-- 0

/

,

AdlilliMil

N . Aft. Prop. Ford Prop. No. No. P375 I I (Left-Handed) d I /5 Q 10 -0 -ti C.--C C-, 5 Q-sz.s. C

0

Q_ 5 /0 9 13

Ship Spe;ed,V,"a kno,ts (Metr.)

Fig.. 9.,

However, with 'a left-hand forward propeller, the starboard after propeller is less heavily loaded, while, as stated above, with a right-hand forward propeller the situation is reversed and the lower power and thrust are developed by the port after propeller. It seems obvious, therefore, that the recorded differences in power and thrust must be

P345

7D-0:]

Model No. 337

Propellers Nos. P 345, P346, P375

All Propellers Working (nforv /nait.= I )

Forward Propeller Running Free

._ Forward Propeller Fixed

Forward Propeller Removed

/0 20 -... --_{- -o--} ...--No. P345 .

-,

.-, ./ .., Aft. Prop. Ford

-0---Prop. No --- - .... , . P346 (Right-Handed) -0- _{0--- .} __ __0 ..,' . .-... ... -...-

=

,/

-

/

-

----

:---No. P345 No. P375 1 1 (Left-Handed) Aft. Prop. For'd Prop. 30 CL 0 ₂₀ CL. L-(77) 10 9 10 Ii 12 /3 4

Ship Speed, V, in knots (frletr.)

Fig. 10.

caused by the asymmetry which the one forward propeller arrangement entails.

The results given in Figs. 9 and 10 also show that the differences between the values from two after propellers can be as much as 3 %, even when the forward propeller is removed. These differences can

-Q

-be wholly attributed to unavoidable inaccuracies in the manufacture of the model hull and propellers, errors of measurement and other such causes, since, of course, the hull-propeller system is completely symmetrical in this case.

It is remarkable that, according to Figs. 9 and 10, greater differ-ences result when the forward propeller is rotating freely than when it is locked; the latter alternative, however, was only investigated with a right-hand forward propeller.

It should be pointed out, in connection with the observed differences in loading on the after propellers caused by the forward propeller, that all the propellers were designed for towing (icebreaking) at low speed.

They were therefore loaded very lightly during the

self-propulsion experiments in question; so lightly, in fact, that they

were working at efficiencies on the steep parts of the curves at the right-hand side of Fig. 6. Consequently, any slight alteration in the speed of advance while the revolutions remained constant caused comparatively large percentage changes in power and thrust. This is clearly illustrated in Fig. 6. (At high values of J, even smallchanges

in J give rise to large percentage changes in KT and KQ).

The

reason for the differences in speed of advance at the after propellers causing such large percentage differences in power and thrust can therefore be sought in the light loading on the propellers.

The aforementioned larger differences obtained when the forward propeller was rotating freely, as compared with those resulting when the forward propeller was locked, can possibly be partly explained in the same way, for the ship resistance, and therefore the loading

on the after propellers, is

considerably lower when the forward

propeller is free than when it is locked.

The exceptionally low loading on the propellers in unrestricted self-propulsion gives rise, as stated above, to low propeller efficiency

which in

turn lowers the propulsive efficiency (Pelf PO; see

Table 4.

Finally, it should be pointed out that the curves in Figs. 9 and 10 tend to a maximum and a minimum respectively at about 12 knots. This tendency is particularly marked in Fig. 9. Since the tendencyis

evident even in the results obtained when the forward propeller was

removed, it seems that it must be attributed to the unavoidable

inaccuracies in the model. Possibly an unsymmetrical breakdown in flow at the after end occurred at just this speed.

Table

Towing Power Tests

The values, refer to the starboard propeller. (Port propeller gives almost the same values in this ease).

Ship Model No. 337

After Propellers') ForwardPropeller _Total

P,(For'd) v P 345 P 346 'P8 P, (Strbd.) to P, w E P, a F knots HP I % HP HP _{r. p. in.} tons I%

(IVIetr..): (IVIetr.) (Metr.) (Metr.),

--Pulling Ahead 0 0 854 1174 i 680 l 914 ' 2388 3262 , 1 112 , I 124 I 35 44 28.5, 28.10 0 .1 1704 1329 ' 4737 1 141 56 28.1 01 4.0 2090, , 641 , 1 1583 516 , 5763 1797 V 150 111 64 21 ' 27.5 28.7 4.0 937 763 2636 1'95 28 28.9 4.0 4:0 1380, 1691 1 -27.0 1113 I 1369 27- 7 3873 _ 4 /51 141 1 150 ,1 39 46 .28.7 28.8 5.0 527 ' I , 425 ' 1480 Ill , 13 28.7 6.0 810 649 1 2269 125 I 20 28:6 6:0 6.0 1251 1638 -- 7.81 979 , 1304 24,9 3481 4580 141 -I 151 1 30 38 28.1 28.5, 1 I 8.1 381 V 316 1078 111 I 5 29.3. 8.1 8.1 649 1046 I 518 818 1 H 1817 2910 125 , 141 1 II 19 28.5 28.1 8.1 8.2 1471 2075 1.6, l 1 1123 1623 , 28,6 4065 5773 154 I 171 I 1 27 :39 27.6, 28.1 1111 10.2 686 788 1 , 556 637 i 1928 2214 137 , 1 141 i 6 8 V 28.8, 28.8 10.1 1205 I 934 I 3344 154 16 27.9 10.2 1723 68 . 1378, 1 23.3 4824 i 170 1 25 28:6 Pulling Astern 0, 0 0 1 , 639 , 882 P>51 1 619 870 II P35 , 1898 I 2633 1 :3737

Ill

124 ' - 1 140 21 28 34 32.6, 33m, 33.0 0 1596 1678 I 4870 156 42 I 34.5 0 I P01 P - 2326 '6728 1731 52 34.6

Model No. 337 Propellers Nos. P 345 P346

Pulling Ahead (All Propellers Working)

I Shaft Power, P at n

/n

=I

s for'd aft.

Corresponding Pull Force, F

0 170

,

,

'

1041.1.1

WAffift.

E"--"m"-"""

Emir

A

NIVAIMWAI

AM

,

Ogrill

Ar

'.

_-'41!

10 knots

t

4116

Mil 111

/20 130 /40 150 160 n in r.p.m. Fig. 11.

7

Towing Power Experiments

Towing power experiments were carried out in the manner described in Section 4 at speeds corresponding to 0, 4, 6, 8 and 10 knots with all three propellers driving ahead at the same revolutions. Similar tests were carried out at 0 knots with all three propellers working

astern at the same revolutions.

Four different revolutions were

investigated at each speed.

ca_" 6000 5000 4000 3000 2000 1000 60 50 40 30 20 10 knot

3000

Model No. 337 Propellers Nos. P 245

,

P346

Pulling Astern at Zero Speed (All Propellers Working)

I Shaft Power, P , at n

/n

=1

lard aft.

Corresponding Pull Force, F

7000 6000 5000 4 000 -0 /000 Limit-Curves ...,

Engine-r

....-..., .., --- ...--., /10 ₁₂₀ 130 140 150 160 170 n in r.p.m. Fig. 12.

The recorded values of shaft power, revolutions and towing force are given in Table 5. In addition, curves of total shaft _{power and} towing force at the various speeds have been plotted against revolu-tions in Figs. 11 and 12. The measured values of _{towing force have} been converted to full scale in accordance with the statements in Section 2.

In each of the Figs. 11 and 12, two curves of available shaft power have also been plotted. The lower _{curve in each diagram represents}

70 60 50 40 30 20 10

the maximum power which may be developed continuously, while

the upper one corresponds to the

overload power which may be

sustained for a maximum of six hours. The curves have been obtained by summing at each revolutions the powers developed by the three ship propeller-motors and allowing a small percentage deduction for losses in the shaft bearings and bossings. The curves in question

can therefore be taken as

representing the total maximum power

available at the ship propellers. (No corrections for scale effects etc. have been applied to the measured values.)

The towing force and the corresponding revolutions for each speed can be obtained by means of the points of intersection in Figs. 11 and 12 between the aforementioned curves of maximum available power and the curves of measured shaft power. The values of maxi-mum towing force thus obtained are shown in Fig. 13 plotted, to a base of corresponding ship speed.

The curves in Fig. 13 are, however, only approximate, for in the ship the total maximum available power, as given by the limit curves in Figs. 11 and 12, is not divided between the different propellers in the same proportions as in the model experiments, where all the

propellers were working at the same

revolutions. Three similar

propeller motors are to be fitted in the ship, so that one third of

the value given by the limit curve will be available for each propeller. The extent to which the actual distribution differs from the values

measured during the tests can be

estimated from Table 5, which

.shows the relation between the power

absorbed by the forward

propeller in the experiments and

the corresponding total power.

However, the approximation introduced in Fig. 13 in order to show the maximum towing force is probably not of any great importance here. In the ship, of course, a smalladjustment in the revolutions of the forward propeller or of the two after propellers will, if required, enable maximum power to be developed on all three propellers.

In connection with the differences in power and thrust between the after propellers, which were observed in the self-propulsion tests and were due to the influence of the forward propeller (Section 6, Figs. 9 and lO)., an investigation was

also carried out under the

conditions of towing ahead. In this case,

however, the measured

differences, which apply at the corresponding speeds and revolutions indicated by Fig. 11, are proportionately much

smaller than the

corresponding differences. shown in Figs. 9 and 10. Actually, they only slightly exceed the differences

which can be attributed to

,60

201 1

,Model No. 33.7 Propellers Nos. P345,, P346

AN Propellers Working Pull Three, F, at

nfor,doft.= Putting Ahead

-Pi Pulling Astern

40, iaUpper-Limit-Curve

6_.Lower- Limit-Curve

0

2 4 6 a

.Ship Speed, V, in hoots (Mor) Fig. 13.

Unavoidable inaccuracies This is probably due to the

_{fact that the}

after propellers are more heavily loaded under towing conditions, so that the effect of the difference in speed of advance becomes pro-portionately much smaller (compare the statements _{in Section} 6).

For this reason, it was considered unnecessary to give details of the shaft power on the port after propeller in Table 5, since it differed only very slightly from the corresponding power on the starboard after propeller..

It may be mentioned here that. the after propellers were designed for towing at about 5 knots at 140 r. p. m. and the forward propeller for towing at zero speed and the same revolutions.

When icebreaking,, the towing forces shown in Figs. 11 and 12 are employed in overcoming the increased resistance caused _{by the ice.} Furthermore, at low propeller revolutions, the available power above the propeller power curves (up to the machinery limit curves) can be assumed to be absorbed, since the resistance to the rotation of the,

propellers is increased by the ice. This applies particularly to the forward propeller, which, as mentioned earlier, is mainly intended for clearing the broken ice.

8. Wake Fractions and Thrust Deduction Factors

Wake fractions have been calculated in the usual way using the propeller as a wake integrator. Values of wake fraction were worked out, both on the basis of torque identity and on the basis of thrust identity, with the aid of the curves of results from the open water propeller tests (Fig. 6). A mean between the two values so obtained was then taken in each case. This method of calculating wake fraction is the normal practice at the Tank.

The wake fractions obtained in the self-propulsion tests are shown in Table 4 and Fig. 14. The differences between

the values at the

port and starboard after propellers respectively are of particular interest and they confirm the earlier assumption (Section 6) that the forward propeller induces a greater speed of advance at the port after propeller than at the starboard after propeller. It is also noticeable that the differences in wake between the after propellers are greater when the forward propeller is running free than when it isfixed, which helps to explain the greater differences in power and thrust (as shown by Figs. 9 and 10) which were obtained with the former alternative. Only one value of wake fraction for the two after propellers is given at each speed for the alternative with the forward propeller removed, since the hull-propeller system is symmetrical in this case. For the towing power experiments, the wake fractions at each speed have been calculated at the revolutions which correspond to the intersection between the appropriate propeller curve and the lower machinery limit curve (maximum continuous loading) in Fig. 11. The values obtained are given in Table 5. A mean value for the two after propellers is given here since, as has already been pointed out in Section 7, the differences in loading between the port and starboard propellers were small in the towing powerexperiments in question.

Thrust deduction factors have been calculated from the results of the self-propulsion tests with all three propellers working at the same revolutions and with only the after propellers working and the forward propeller removed. In both cases, the values calculated for the after propellers are a mean between port and starboard.

25

10

5

0

Model No. 337

Propellers Nos. P345, P346

All Propellers Working

'° ford "aft.' )

Forward Propeller Running Free

Forward Propeller Fixed Forward Propeller Removed

For'd Prop. Aft. Prop. ,Strbd. V --._____ N , Port 9 10 11 12 /3 14

Ship Speed, V, in knots (Metr.)

Fig. 14.

For the case when all three propellers were working, firstly a total thrust deduction factor (referring_{to 2' T from both the after propellers} and the forward propeller). was calculated and secondly separate

thrust deduction factors for the after

_{propellers and the forward}

propeller respectively were determined _{The latter necessitated a} special test in which the after propellers were removed and replaced by dummy bosses

_{so that the model was driven by means of the}

forward propeller alone. The revolutions of the forward propeller

20

4'.

were chosen at each speed to agree with the corresponding value in Table 4. At the same time, however, a towing force was applied to the model (from the carriage) equal to the skin friction correction, which is usually applied in sell-propulsion tests, plus the effective thrust provided by the two after propellers at the same speed and revolutions in the previous tests with all three propellers working By this means, the forward propeller was loaded to the same extent as when all three propellers wereworking. (If, instead, the experiment had been carried out with the after propellersworking and the forward propeller removed, the loading on the after propellers would not have been affected by the slipstream from the forward propeller and there would therefore have been a risk of error in computing the respective thrust deduction factors).

Allowing for the suction on the hull created by the after propellers, the towing force applied to the model to compensate for the absence of the after propellers is given by:

= Ra

( 1 tot.) f Taf1.

where le ---= the towing force (measured)

Ra = skin-friction correction

22 T 0. = thrust on the two after propellers

taft.

= the required mean

thrust deduction factor for the

after propellers.

F, Ra and 22 T known, tot. can be calculated.

Knowing the resistance and the thrust measured on the forward pro-peller, the thrust deduction factor for the forward propeller can be calculated from:

R Ra + (1 - taft.) Tot + (1 - tiwd.) T fwd.

F

where, in addition to the above notation, R -= resistance

T f,d = thrust on the forward propeller

ti,4

-= the required thrust deduction factor for the forward propeller

As R, F and Tim., are known, tf,d can be calculated. (N. B. All dimensions in model scale.)

+

Model No 337

_{Propellers Nos. P345, P346}

All Propellers Working (nf0rd/not=1)

Ford Propeller Removed

Total For 'd Aft.

- t-o 60 50 40 30 20 10 70

---

---'-9 10 11 12 13 14

Ship Speed, V, in knots (Metr)

Fig. 15.

These tests were carried out in such a way that several revolutions were investigated at each speed and the towing force F was measured

in each case. A curve of F as

a function of revolutions was thus

obtained and the value of F was then read off

_{at the revolutions}

corresponding to the speed in question according to Fig. 7.

All the calculated thrust deduction factors are given in Table 4 and Fig. 15. The high values of the thrust deduction factor for the

forward propeller, as shown in Fig. 15, are of particular interest

and they contribute, of course, to the low

propulsive efficiencies

(Table 4)

referred to in Section

6.

They can also be said to

illustrate the unsuitability of the forward propeller as a means of propulsion.

It may also be noted that the mean thrust deduction factor for the after propellers becomes considerably lower when all the propellers are working, compared with when the forward propeller is removed and only the after propellers are working. This would appear to be due to the fact that the after propellers are more heavily loaded in the latter case, but it may also possibly be a result of the influence of the forward propeller in the first case.

No thrust deduction factors have been calculated for the towing power experiments.

II.

The Icebreaker with Two After Propellers and

Two Forward Propellers (Model

No. 451)

9.

Experimental Arrangements

The arrangements for the tests were largely the same as those for Model No. 337, as described in Section 4.

Resistance tests both with free and with fixed forward propellers were carried out in addition tothe usual tests of this type. This was in order to determine the increase in resistance caused by the forward propellers.

As mentioned previously, this ship also was designed for diesel-electric propulsion. In this case, however, there are six diesel engines each with its own generator. Of the totaldeveloped power, therefore, a maximum of twothirds can be transmitted to the two afterpropeller motors while one third is supplied to the two forward propeller motors. Alternatively, the opposite distribution may

be adopted, i. e. the

after motors may be supplied with onethird of the total power and the forward motors the maximum two thirds. In the former instance, each of the after motors receives the current from two generators, while each of the forward motors is supplied from one generator. In the latter case, the arrangement is reversed, i. e. two generators

The above power distribution alternatives _{are considered limiting} cases at maximum power output. Any possible combination between these limits can also, of course, be adopted_{at power outputs below} the maximum. The latter combination, with two thirds of the total power on the forward propellers is intended in practice for use under particularly difficult ice conditions.

On account of the freedom which exists for_{distributing the available} power between the after propellers and the forward propellers, self-propulsion experiments were carried out at two different revolutions combinations.

_{These combinations were such that the relation}

between the forward propeller revolutions and the after propeller revolutions (njoedlnatt.)

_{was 1.25 and 1.80 respectively. With the}

propellers in question, it was calculated that these revolutions ratios would correspond with the aforementioned power distribution limits;

e. P

P.

_{8 1/}IPA_-aft. .= 0.5 and 2.0 respectively.

As in the case of the previous experiments, all the propellers were driven from one electric motor. The two different _{combinations of} revolutions, nmedlnaft.

_{= 1.25 and 1.80, were obtained by means}

of two different gearing arrangements. Two _{self-propulsion} dynamo-meters, recording revolutions, thrust and torque, were coupled up, one to the port forward propeller, the other to the starboard _after propeller. Since, in this case, the hull-propellersystem is symmetrical, no check measurements were made on the other two propellers _and

the total thrust and torque

_{on the forward and after propellers}

respectively have been obtained merely by doubling _{the respective} recorded values.,

Self-propulsion tests with the forward propellers_{running free were}

also carried out, but no such tests

_{were made with the forward}

propellers either fixed or removed.

All the ordinary self-propulsion tests (without tow) were carried out with inward-turning forward propellers and outward-turning after propellers.

Towing power experiments _{were run at both revolutions ratios,} with all propellers working and with the_{same directions of rotation} as above. In addition, however, similar tests

were made at

nfor'dinalt. = 1.25 with the forward propellers also outward-turning. This was arranged by exchanging one forward propeller for the

other and reversing the directions of rotation of the respective

shafts.

10.

Resistance Tests and Open Water Propeller Tests

The results of the resistance tests are shown in Table 6 and Fig. 16. For comparison, as mentioned previously, C, values from the experi-ments without propellers are included in Fig. 5. As mentioned in Section 9, resistance tests both with free and with fixed forward propellers were carried out in addition to the usual tests of this type.

It is particularly interesting to note from Fig. 16 that the two

forward propellers give rise to a relatively high resistance, especially when locked.

Open water tests were carried out with the right-hand propeller of both the forward and the after pair and the results are given in Fig. 17.

Table 6

Resistance Tests

Forward propellers running free (inward-turning).

The propellers were fixed.

Ship Model No. 451

No Propellers For'd Propellers P 440 Free') Fixed.)

V Fnv

Pe Ci Pe PC

knots HP HP

HP

(Metr.) (Metr.) (Metr.)

2 0.081 6.2 344 3 0.122 17.3 416 4 0.163 38.4 444 5 0.203 74.7 446 6 0.244 124 464 260 280 7 0.285 191 479 8 0.325 278 491 9 0.366 393 494 10 0.407 542 492 840 1310 11 0.447 730 486 12 0.488 979 470 1470 2370 13 0.529 1326 442 14 0.569 1774 412 2500 3950 15 0.610 2444 368 16 0.651 3428 319 4440 6570 17 0.691 4533 289 1)

Model No. 451

Propellers No. P 440

Forward Propellers Removed

6000 5000 4000 3000 2000 1000 Resistance Forward Forward Tests Propellers Running Propellers Fixed Free

/

/

/

.

.

z.

.

111

10 II /2 13 /4 /5 16 17

Ship Speed, V, in knots (Metr.) Fig. 16.

11.

_{Self-Propulsion Tests}

The results of the self-propulsion tests _{are shown in Table 7 and} Fig. 18. Unlike the results obtained with the triple-screw_{model, no} large differences in shaft power between the alternative with _{all the} propellers working (ni,,,/nait.

_{1.25) and that with the forward}

propellers running free were evident in this case. In fact, according to Fig. 18, the way in which the power is distributed _{between the} forward and the after propellers is to _{a large extent unimportant, as} far as unrestricted motion is concerned, so long as the smaller portion

/

10 KT 100 KQ

Fig. 17.

of the total power is transmitted to theforward propellers; (compare Figs. 7 and 18).

The reason for this is presumably that when the forward propellers are free, they and their shaft bossings produce a proportionately much greater increase in the resistance of the ship than the corresponding arrangement in the triple-screw vessel. It has already been observed (Fig. 16) that the resistance of the freely rotating forward propellers themselves is considerable. Unfortunately no exact comparison can

Propeller Water Temp. 1) n 02 No. t °C m2/sec. r/sec. V

._._._--- P439 Aft. Prop. 12.7 1.215.166 9.3 4.40.106 ...---- P440 Ford Prop. 12.7 1.215.10-6 /5.1 4.97-105

7

PAM

Pi

7.

IN

EN

\

Efflail MI

i

MU WREN

Oil

-

-,7611111

'MN

II

0 02 04 0.6 08 1.0 7 6 5 4 3 2 80 70 60 50 40 30 20 10 a

Model No. 45/

Propellers Nos. P439, P440

All Propellers Working,

'for'd raft.= 1.25

All Propellers Working, nfor,d nap.= 1.80

Forward Propellers Running Free

6000 5000 4000 3000 2000 1000 0 17 1 11/11/1w'A

In

or'd Ilk- n aft.

/

/

/

/

FAA

II

I P/

s

/

P.=

1111/// .7

_d

10 11 12 /3 /4 15 16

Ship Speed, V, in knots (Metr.)

Fig. 18.

120

80

40

Table 7 Self-Progillsion Tests

1) These values were obtained by using the Pe-values for the eaSe with freely rotating forward propellers from Table 6 and Fig. 16.

. . .

Ship Model No. 451

.

'ItY Env

Aft. Propellers P 439 For'd Propellers P440 Total P, 2 - P8 h W . 1 t 2 Ps .7/ w E P, C t E Ps 1 HP HP HP

knots

-(Metr.) r. p. % % (Metr..) r. p. m. % (Metr.) 1, % 1

All Propellers 'Working nietwinai. = 1.25 1 1 6 _{0.2441 '} 198 44 19.4 101 55 18.2 299 41.6 193 19.7 7 0.285 312 51 19.5 147 64 17.3 459 41.6 199 27.4 8 10.325 448 58 18.7 182 73 15.7 630 44.1 217 27.7 9 0.366 624 66 18.6 1 238 83 14.9 862 I 45.6 225 28.7 10 0.407 844 72 19.1 , 310 90 15.0 1154 47.0 ' 231 27.2 11 0.447 1114 79 1.9.91 . 410 99. 16.1 1524 1 .47.9 j 233 27.4 12 0.488 1488 87 20.4 548 109 16.1 2036 i 48.1 226 30.1 13 0.529 1966 95 20.3 746 119 16.6 2712 48.9 216 31.2 14 I 0.569 2458 101 21.6 926 126 17.7 3384 52.4 216 29.9 15 0.610 3412 111 21.9 1360 139 18.1 4772 51.2 188 32.1 16 0.651 4386 121 21.0 1888 151 19.1 6274 I 54.6 174 31.2 17 0.691 5732 131 I 19.8 2672 164 1 19.8 8404 53.9 156 32.2

All Propellers Working 92f or (010E.1.80=

10 0.407 ; 300 63 19.5 1120 113 17.2 1420 38.2 188 50.4

12 0.488 I 520 75 20.1 2020 135 19.8 2540 38.5 181 50.0

14 0.569 990 ' 90 19.8 3550 162 20.2 4540 39.1 161 50.5

16 0.651 1980 106 19.0 6550 I 191 21.2 8530 40.2 128 47.5

Forward Propellers Running Free

1) . 11 1 1 i [ . 6 0.244 380 48 28.8 8.4 36 380 68.4 11151 I 10 0.407 1302 77 23.2 20.8 . 72 1302 64.5 205 12 0.488 2114 ' 92 29.3 20.4 91 2114 69.5 218 14 0.569 3690 109 22.3 20.3 105 3690 67.8 198 15 0.610 4630 117 23.8 18.0 111 4630 7L5' 194 L 16 0.651 6310 128 23.0 19.2 119 6310 70.4 173 m.

be made, since resistance tests with the forward propeller running free were not carried out with the triple-screw model.

In Fig. 19, the total power, as given in Fig. 18, has been divided between the forward and after propellers respectively. This distri-bution is also given in the form of a ratio, so that it can be seen to what extent the adopted revolutions ratios (niedlnai(.= 1.25 and 1.80 respectively) correspond with the required power distribution

ratios (Pf'd!

_or

_IPft.

,_at.= 0.5 and 2.0 respectively). Fig. 19 indicates that there is a marked discrepancy between the selected revolutions

ratios and those corresponding to the actual power distribution

ratios.

This is so particularly for ni,dinot.= 1.80. It should be

pointed out here, however, that this revolutions ratio is really only suitable for towing conditions (icebreaking), where it gives a power

distribution ratio of Pf' IPft.

_{8 ordi} ,_a 1.8 (see Table 8, Section 12) and thus approximately satisfies the assumption.

As regards the maximum ship speed attainable in open water at full power, it can be stated that a total power at the propellers of 8400 HP is available for continuous working and 10500 HP is available for overload conditions for a maximum of six hours. These powers, on account of the character of the machinery, may be considered as

independent of propeller revolutions between fairly wide limits In converting the model results to the ship under ideal trial condi-tions, a correction should be made to allow for scale effects, air resi-stance and the hull condition of the ship. This correction is generally

applied in the form of an addition to P, and n, but as far as the

power is concerned, of course, it can equally well be applied as a subtraction from the available machinery power. The revolutions do not require further consideration in this case, since, as mentioned above, the maximum available power is more or less independent of propeller revolutions.

In view of the large number of shaft bossings and their relative size, there is reason to suppose that the scale effect must be consid-erable.

The total correction has therefore been taken as a 15 %

reduction in machinery power. The figures for available power given above are then reduced to about 7150 and 8950 HP respectively and, according to Fig. 18, these values would result in ship speeds of 16.4 and 17.2 knots respectively at ni,..dInaft.=- 1.25.

The above power values, of course, presuppose the aforementioned

power distribution ratio, P

_sped!IP,_aft. = 0.5.

If this condition

is

2

Model ,No. 451

Propellers Nos',

P4391

P440

6000 50000 40001 3000 0 2000 mob, ''forV soft. ---....

/

/

/

Ill

111

I

11/11/1011

PIP

' I ,

r

..---- ..---,---

r-

2,P ( soft .... 1

2.P

I _ I _____ sfor'd 1 Propellers Working Aft. Propellers n0ft.'115 "ford Ford Propellers

= = --

Aft. Pro petters

nford

1

naft= 1.80 _.c

Ford Propellers

Fl 12 13 '44 .1,6 47.

Ship Speed ,V, in knots (Metr.)

Fig. 19: All IAft. 0 -

/

/

P P 'for'd

/

2.P for'd aft.

r

15

not satisfied (see Fig.

19) the ratio

_n/,,,'d/aft.

can be adjusted

in the ship and any adjustment will possibly influence the speed obtained.

12.

Towing Power Experiments

Towing power experiments were in this case only carried out going ahead. The test results are given in Table 8 and Figs. 20, 21 and 22. By plotting also the maximum available power (overload power permissible for six hour periods), which in this case can be taken as constant over the range of revolutions, the maximum towing (ice-breaking) force has been obtained in a similar manner to that described in Section 7. The values so obtained are given as a function of ship speed in Fig. 23. (No corrections for scale effects etc. have been applied to the measured values.)

An interesting fact which is clearly indicated in Fig. 23 is that, at the same power, a slightly greater towing force is obtained with

outward-turning than with inward-turning forward propellers. On the other hand, the washing effect along the ship's sides is presumably much stronger with inward-turning forward propellers. This was to some extent verified by means of a film, which was taken during the tests in order to investigate the water flow along the sides of the model with the two alternatives. Some parts of this film also illustrate the washing effect. In Fig. 32 at the end of this paper, are shown two

photographs of the fore part of the model taken while towing at

4.4 knots.

In the upper picture, n!,/nft.

= 1.25 and the

for-ward propellers are outfor-ward-turning, while in the lower picture, nfor,Inalt. = 1.80 and the forward propellers_{are inward-turning. Apart} from the fact that the forward propellers_{were differently loaded in} the two instances, some idea can be obtained of the difference in washing effect between inward-turning and outward-turning forward propellers.

The differences between the assumed values of the power

distri-bution ratio

_(P8

_f'lj

_{IPlt. = 0.5 and 2.0}

_{respectively) and those}

ore .a

obtained during the tests are evident in Table 8. The reservation regarding the power distribution ratios, which was made in Section 7 in connection with the values in Fig. 13, also applies to the values in Fig. 23.

In order to obtain some idea of the relative properties of the_two

ships tested, curves of FIE Ps for each ship have been brought

"

Model No. 451

Propellers Nos. P439, P440

All Propellers Working

2- Shaft Power, P , at nfor'd/ naft.= 1.25 Corresponding Pull Force, F

(Ford Propellers InwardTurning)

16000 /4000 12000 10000 L.: 8000 6000 4000 2000 0 80 90 /00 n aft. r. p.m. i t I I t I I 100 120 140 160 in nfor'd in r.p.m. Fig. 20. 160 140 120 100 80 60 c 40 20 0

A

,

MIIIMIWANK,IIA

larl

A Engine-Limit-Curve

ANII,

pi

A

/

/

For

iks,

knotsilAik

Alik

knots

/ /

/

MEI

I/O 120 130

Model No 451

Propellers Nos. P439, P440

AN Propellers Working

2 Shaft Power, P ,at h / n =1.80

s farV ,a ft..

Corresponding Pull Force., F

(Ford Propellers InwardTurning)

14000 ;12000 10000 4000 2000 0 11

km

II

Mrs=

Engine-limit-Curvet- Curve

NE

AI,

IpArd11 1

/

1

A

r

i , 60 70 80' 90 400' naft. in r.p.m. I It A II :I, _ _ A fr__ _-4 - 4 100 120 140 160 ma. 200 .nfor,d.in r. p.m Fig.. :41.. 8000 Q 6600 /40 120, 1100 I 40, 201

/

/

/

/

s 80 60 0

8000

z

6000

Q."

Model No. 45/

Propellers Nos. P439, P440

All Propellers Working

Shaft Power, Ps , at nfor,d/naft= 1.25

Corresponding Pull Force , F

(Ford Propellers OutwardTurning)

/4000 12000 10000 4000 2000 0 100 120 /40 160 nfor'd in r.p.m. Fig. 22. 140 120 100 80 60 40 20 0

.m.

,

Ap

A

Engine- Cm t- Curve

NMI. I YAM

MAN

,

.

,

0

knotsdj

row

A

80 90 100 I/O 120 130 °aft. in r. p.m. 1

/

Table 8

Towing Power 'rests (Pulling Ahead') Ship Model No. 451

17

After Propellers Forward Propellers

P440 Total. Ps (For'd)

P8 (Aft.) 1

2 Pg n w 2 - Ps n w E P, 1 F

knots HP r. p. m. % HP r. p. m. HP tons

(Metr.) ' (Metr.) I ,(Metr.) (Metr.)

nI or'ia it.n = l2-5, For'd Propellers Inward-Turning'

I 0 2992 80 ' 1662 100 4654 67 0.56, '0 5658 100 '' 3232 125 I 8890 102 0.57 0 9708 , 120 1 1 5456 149 15164 148 0.56. 4.4 , 4.4 3670 6390 91 , 109 91 1964 3476 114 136 1 1 -8.4 5634 9866 I 61 93, 0.54 0.54 4.4 8828 121 4822 151 13650 ' 120 055 9.0 9.0 3862 7406 99 120 18. 0 1944 3936 124 150 19. 0 58061 ' 11342 43 81 0.50 0.53 9.0 12898 140' 6640 17,5 19538 124 0.51 1 12.0 4268 109 2054 137 6322 311 0.48 12.0 8586 131 21.1 4472 163 20.3 13058 ?72 0.52' 12.4 13860 149 7432 187 , 21292 115 0.54

nmedinait. = 1.80, Ford Propellers Inward-Turdng

0 1150 60 2060 108 1 3210 48 1.79 ' 0 0 2786 5320 , 80 99 4528 8902 144 178 7314 , 14222 85, 133 1.62 1.67 - 1 4.4 1554 71 I 2832 127 4386 I 1 47 1.82 4.4 3448 90 , 6044 161 9492 86' 1.75 4.4 6420 109 'I 15.1 197 .7.9 17604 135 1.74 1 9.0 1934 85 4208 154 6142 1 39 2.18 9.0 4330 , 104 15.8 8554 187 2:3,31 12884 ' 82 1.98 9.0 8150 125 15330 225 23480 135 1.88

ntwdlnaft. = 1,25, For'd Propellers Outward-Turning

' o 2686 79 1632 98 4318 63 4161 1 0 5164 99 3106 123 1 8270 99 0.60 l 0 4.4 8960 3354 118 ,90 5326 1986 147 113 14286 , 5340 142 59 0.59 0.59 4.4 6052 109 1 3638 1:36 1 9690 '95 0.60 4.4 8112 120 -15.4 4898 150 19.5 13010 118 0.60, 00 3234 99 1926 123 5160 39 0.60 9.0 7582 125 I 1.9 4600 156 21'5 12182 86 0.61

-11184

120

100

40

20

Model No. 45/

Propellers Nos. P439, P440

All Propellers Working Pull Force, F, in tons

nford no/ t 1.25 For'd Propellers InwardTurning

nfor'd /flOft. 1'80

ford naft. 1.25 Ford Propellers OutwardTurning

n''

... ... ... ..s. ....,..

,

-...., .... ... ... ... ... '.... 0 2 4 6 8 10 12

Ship Speed, V, in knots (Metr.)

Fig. 23.

80

L.

12

10

2

0

Pulling at Max. Power

All Propellers Working

Mod. No 337 Prop. Nos. P 345, P346 (n for,d nap.= I) Mod. No. 45/ Prop. Nos. P439, P440 inforid naft..1.25)

...

Ford Propellers OutwardTurning

I

For'd Propellers InwardTurning

,.

_..

'.,-.-. .

,

...

0 01 02 03 04 05 F -n [7 Vg V.-3 1 I I I i I I 0 2 4 6 e lo /2

Ship Speed, V, in knots (Metr.) Mod. No. 337

0 2 4 6 8 10 /2

Ship Speed, V, in knots (Metr.) Mod. No. 451

Fig. 24.

4

,

- -1- - ----1 - - I

together in Fig. 24. The material for them was taken from Figs. 13 and 23 and applies at maximum overload power. Furthermore, the

power distribution was to a large extent the same in each case,

i. e. one third of the power forward and two thirds aft. From Fig. 23, the results with both inward-turning and outward-turning forward propellers have beeR iiSed. As abscissa, both FROUDE'S number and corresponding speed for the two ships are employed.

It must be pointed out, in connection with Fig. 24, that the ordinate

Fif P. is not dimensionless, so that the comparison between the

two is not strictly true and should therefore be judged reservedly. However, the results in Fig. 24 seem to indicate that the two ice-breakers are, to a large extent, equivalent in performance from this point of view.

13.

Wake Fractions and Thrust Deduction Factors

Mean values of wake fraction for both the forward and the after propellers have been calculated in the same way as that described in Section 8 for the triple-screw icebreaker. The results referring to the unrestricted self-propulsion tests are collected in Table 7 and Fig. 25, while the values from the towing power experiments are to be found in Table 8. The latter have been calculated at each speed for the revolutions corresponding to the intersections of the respective propeller curves with the machinery limit curve in Figs. 20, 21 and 22; i. e. the same method as that applied in the previous case and described in Section 8.

It is apparent from Fig. 25 that the value of the wake fraction at the forward propellers is somewhat below that at the after propellers. In the case of the triple-screw icebreaker (Fig. 14), on the other hand, the value at the forward propeller was considerably higher than that at the after propellers. This, of course, can be fully explained by considering the position of the two forward propellers in relation to the hull in comparison with that of the single forward propeller in the previous ship.

It can also be observed from Fig. 25, as from Fig. 14, that the wake fraction at the after propellers assumes a lower value when the forward propellers are working, compared with when they are rotating freely, fixed or removed respectively. This must be due to the race from the forward propellers, which decreases the wake aft.

4. 20 30

15

5

Model No. 451

Propellers Nos.. P439 P440

- Aft. Propellers Ford Propellers } Aft. Propellers Ford Propellers

--,-n 1.25

for'dn of

n ford °aft.' 1 1 i i i 1 1 ,

_

,,--. 1

---.

IIII._

_

-- ---1 1 1 1 1 I 11 -1 1 -

-1 Aft. Propellers, Ford Propellers Running Free

V.0 IL 13 1,4 116

Ship .Speed .1/, in knots (Metr.)

Fig. 25.

10

0 25

deduction factor at the forward and after propellers respectively proved to be unsuitable in this case. Unfortunately, therefore, only

the total thrust deduction factor (applying to

/1 from all four

propellers) has been calculated for the alternatives with all propellers

working. On the other hand, the thrust deduction factor for the

after propellers has been worked out for the alternative with the forward propellers running free. The results obtained are shown in Table 7. It should be emphasized that, in the latter case, the value

T R

of R in the expression for thrust deduction factor,

t

T was taken from the results of the resistance tests with freely rotating forward propellers (Section 10).

14.

Acknowledgement

The authors wish to express their gratitude for the grant made

from Hugo Hammar's Foundation for Maritime

R e se ar ch which enabled the results of these investigations to be published. The authors also wish to express their appreciation of the

courtesey shown by the Swedish Government

Author-ities and the Finnish Ministry of Commerce and

Industry in making the test results available for publication.

Thanks are also due to the staff of the S wedish St ate

Shipbuilding Experimental Tank in Goteborg for

all their assistance and to Mr. DACRE FRASER-SMITH, B. Sc., who translated the paper from the Swedish.

APPENDIX I

Maximum Speed in Open Water (Model No. 337)

As mentioned previously, the original propellers (P 345 and P 346) for the triple-screw model were designed for towing (icebreaking) at low speed. In fact, the after propellers were dimensioned for towing at 5 knots at 140 r. p. m. and the forward propeller for towing at zero speed and the same revolutions. This meant that they were not so suitable for unrestricted movement in open water (see Section 6). With a view to obtaining propellers better suited to the characteristics of the pro-peller motors, some tests were carried out with a new pair of after propro-pellers, P 369.

The propellers P 369, as already mentioned in Section 3, had the same dimensions as P 345 except as regards the pitch, which was increased by about 15 % from 2300 mm to 2650 mm. These propellers were thus also more suitable for open water

Model

No.. 337

Forward Propeller No.,P346, Running Free

After Propellers No. P 345

After Propellers No. P369

Fig. 26.. 4.0 00 3000 2000 l' I --- ---- .---ft. - --I I .

/

/

/

/

/

/

-Z P 1 soft ....-- .. ---.. ---- "--- II...--__11 . ...-1 9 if 0 8,1 12 14

Ship Speed, V, in knots (Heir)

#6 0

120.

8 0

4 0

Model No. 337

Forward Propeller No. P346 Punning Free

Aft. Propellers No. P345

Aft. Propellers No. P369

3000 2000 1000 0 Engine-Limit-Curve Power of the I Propeller-Engines) Two After 1

/

/

,(Z. Max. Continuous

The shaft powers from the self-propulsion test results increased

by 15 'X and the corresponding

1..

/

/

13

revolutions by 4 %, according

/

/

to scale effect, roughness etc.

/

. 12 knots

/

12

/

/

/

11 11 ,---,0

/

.,, ---- 10 ---"""111111111."-10, .

..--,

_{. 9 knots} 110 120 130 140 150 160 170 n in r.p.m. Fig. 27. -

/