Manoeuvring experiments with large models in open water

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ARCHIEF

(41IIRODARSKI

IIISTITUT SHI PBUILOINUIESEARER INSTITUTE

HOREB

Manoeuvring Experiments with Large Models in Open Water

by

DOCt. eng. R. De Santis

Istituto Nazionale per Studi ed Esperienze dl Architettura Navale, Roma

Lab. v. Schez:pshowv...

Technisch eplineiszcohrl

Delft

TPaper io e presented il at the Symposium onthe

Towing Tank Facilities, Instrumentation and

Measuring Technique

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MANOEUVRING EXPERIMENTS WITH LARGE MODELS IN OPEN WATER

by doct. eng. R. DE SANTIS

In 1915, after three years of theoretical and experimental work, doct. NINO PECORARO - a

lieutenant-colonel of the Italian Navy - read before the Collegio degli Ingegneri Navali e Meccanici in Genoa a paper on a new method for researches on the turning qualities of the ships.

The method, in its general lines, _{was a simple}

one: a big model of the ship, weighing some three tons and carrying five experimenters, propelled by batteries and electric motors and equipped with steering and

recording instruments, was put to turn in a cove of the Gulf of La Spezia, while at shore a few attendants

stationing at the ends of a measured base continuously observed the running boat through angle recording telescopes. By subsequent triangulations the position of the boat at known time intervals could be fixed and therefore the path followed by the model and its velocity drawn and calculated.

- This method though apparently simple was really

complex and doct. Pecoraro had to overcome many difficulties before being able of collecting reliable results from

his tests. The principal difficulties lay on the design and construction of the recording instruments and

particularly of a reliable torque measuring apparatus. Notwithstanding the failure of the early attempts Pecoraro at last succeeded - in 1915 - in his endeavours and thenceforward the Italian Navy adopted his method as

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a running prediction system of the turning qualities of

the battleships under cOnstrction. The Froude law of

dynamical similarity was supposed to hold good and no allowance for frictional effect was introduced in the

calculations.

In 1929 GIUSEPPE ROTA - who 40 years before had been the founder of the Experimental Basin at La

Spezia - founded in Rome the "Vasco. Nazionale per le

Esperienze di Architettura Navale" which after the second world war changed its name into "Istituto Nazionale per Studi ed Esperienze di Architettura Navale".

It was Rota's opinion that the Pecorari system of investigation was good in its general lines but apparently had two drawbacks: it was lengthy and expensive. He maintained that sometimes the results of the model trials had been acquired when the construction of the ship was so advanced that it was impossible to take the results into account for any improvement of the

ship forms and her steering appliances. With smaller models, possibly the same models employed in tank tests,

time and money, he thought, could be saved.

But it was not an easy problem; it was

essentially a matter of weight and volume. Batteries and electric motors were undoubtedly too heavy and bulky. Rota devised to make use of a light gasoline motor - like those employed in motor-cycling - directly geared to

the propeller shaft or shafts. This simplified the problem

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. .

but not so much as to bring the solution at hand and not earlier than 1935 we were able to launch the first

modelboat, powered with a,2 HP and 3000 RPM gasoline

motor, into the lake of Albano. The model was a scaled reproduction of the battleship VITTORIO VENETO. It was 7 meters long and carried two experimenters in addition

to the propelling and steering medhanisms.

Looking back to that first attempt it appears now a poor thing. But at that time it was a real

achievement. It enabled the designers of the VITTORIO VENETO to choose the best solution for higher manoeuvring

qualities of the battleship. Aboard the model there were the following instruments:

a revolutioncounter connected with the motor shaft;

a graduated level to read the model list when manoeuvring;

a differential gauge on the rudder head to measure the twisting moment;

a tiny rudder at some distance forward the bow to read the local drift angle.

Although the first goal had been attained the'tank staff was not satisfied with the experimental

technique employed. We were not sure that the correspondence between the model and the ship navigating conditions had been realised as strictly as necessary for reliable results. Correct metacentric height and period of roll could not be obtained. Two experimenters on board were giving each other trouble and possibly influenced the

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regularity of the trials, the gasoline motor developed a too small power and it was difficult to get it regulated as finely as desired. The starboard and portboard propellers were directly connected to the motor shaft and that prevented

-having them rotating at different revolutions when the boat was turning or when a different rate wes required by the particular programme of researches under examination.

The small rudder of the drift angle indicator was somewhat unstable and generated. an additional

disturbung drag. Propeller thrust and torque were not measured.

The frictipn correction - as used in tank propulsion tests - was not applied and it seemed

extremely difficult it could be; therefore the propeller number of revolutions, slip, thrust,

torque and efficiency were not in accordance with the corresponding ship measures as required by the law of dynamical similarity.

All these and other minor defects were subsequently eliminated or reduced by adopting

new experimental procedures and particularly by

designing and building lighter, simpler and more efficient apparatus.

After the first set of trials the experimental station was displaced from the lake of Albano

-which is 21 km far from Rome - to the more distant lake of Sabaudia which was considered to be

more favourable for its general characteristics of shape, size and location . Recently the station has been transferred to the lake of.Nemi which is

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MANOEUVRING EXPERIMENTS. WITH LARGE MODELS IN OPEN WATER

30 km from Rome, is 1.3 km wide., 1.8 knHlong and has a

maximum depth of 34

m.

The lake of Nemi lies on-the bottom of two'

adjacent

Volcanic

craters whose :top ridge is 100-400 m

higher than the

water

level.:This encircling elevation

offers a good shelter against winds. (Fig.1)

The experimental station (fig.2)

has

a laboratory

at ground level to fit up the models arriving

from Rome

and a small room at the first floor for drawing's and

'caloulatione. Outside the station, at right angles with

the declining' shore, there is a trimming dock - 1.20 in

large,

2:05 m deep and 16.00m long. 7'.. Across the_doek

a removable iron frame with a top running tackle. helps

lowering /lifting up the model

and.

putting it afloat

(fig.3),.

The models

are

made at the Rome tank laboratory

after the usual building technique. They are made with

cembran pine or white spruce. Their

average

length and

weight are 8.00 m

and 400-500 kg. No

putting,nails and

screws are used but only synthetical glue and treenails.

If some putty is necessary to stop leaks or correctly shape

small recesses a special paste is used which is water

and rot-proof and becomes as hard as

stone

in a few minutes.

The boats are 'minted with three:' coats of hard paint in

two colours: light

green the hull

and

yellow-white the

upper-work.

Hull appendages are made of zinced iron - as bilge keels - white metal - as propeller-shafts and

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P

ibY;6046/ A ',74';

+

. 14,4 d /

"V

/,40,40147.2,

'

a

(9)

8 Fig. 3

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--MANOEUVRING EXPERIMENTS WITH LARGE MODELS IN OPEN. WATER .

brackets - aluminium alloy - as rudders and propellers - bronze - as propellers.

Rudders are balanced in water at the correct draught. This is obtained by acting on the thickness of the rudder plating or fitting; adjustable counter-weights on a pulley keyed to the rudder-stock head. Rudder-stocks are of stainless steel and turn on rustproof ball bearings.

Propeller shafts are made of stainless steel

or monel-metal. The bearings are worked

up

with a high

degree of precision and their watertightness and lubrication are guaranteed by cOunterpreasuring oil.

An electric :motor is geared to each propeller shaft which at the inner end is connected with a

switch inserted in the circuits of

the

revolution

recording and reading instruments. The electric motors are mechanically independent and receive the current

A

from a d.c. generator driven by a gasoline motor. A smaller Coaxial d.c. generator - assisted by an accumulator battery which acts as well as a starting device - supplies the main generator and the auxiliary

services with the exciting and feeding current.

The following manouevring appliances are on board (fig.4):

rudder-wheel with adjustable clutch and clamping device to fix the wheel at any position as desired;

steel wires connecting the rudder-wheel With the rudder -head pulley through an appropriate recording apparatus;

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resistors to regulate the voltage of the main d.c. generator and thence the rate of the propelling motors;

resistors to change the rate of any separate motor; switches to reverse motors.

The measuring instruments are:

a special apparatus putting on record: a) the screw

propeller revolutions; b) time; c) twisting moment on the rudder-head; d) lateral rudder resistance; e) rudder angle or rudder deviation from the fore-and-aft direction;

propeller revolution-counters or, inversely, time-indicators starting and stopping every 25 revolutions;

screw thrust-and-torque recorders;

long-distance indicator of the rudder true angle; levels to read the transverse and longitudinal boat inclinations;

current controlling instruments on motors, generators and dynamometers.

The friction correction is performed as

follows:

a small screw propeller developing an additional

thrust equal to the pre-calculated friction correction at the boat speed of advance is placed 10 am underkeel right at the boat c. of g. vertical axis4fig.3). The thrust is checked on board by reading the number of

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revolutions of the screw propeller. This is previously calibrated at the experimental basin taking due

account of the boat trim when under way.

The signalling system consists of:

red flashing light at 10 seconds intervals

from a bow point placed 60 am over deck; white fixed light at the stern;

white bow light working as a Morse signalling telegraph.

The recording apparatus included in the steering transmission deserves some explanations

(fig.5). Pulley Pi is keyed to the rudder stock 0 that turns on ball bearings supported by the leverage

L: this is equilibrated by springs Mi and Mi . Pulley

Pi is the the driven pulley of the system PiN P2

which has _{P2 as} driver and the steel band N as

transmission line. In this line springs Mo and M: are inserted. A third pulley

P3 coupled with the driver

P2 is steered by the rudder wheel R.

The forces acting on the rudder are: the resultant N of the normal forces applied at a distance "b" from 0;

the resultant T of the horizontal tangential forces;

the resultant of the vertical tangential forces

which need not be taken into account;

forces R' end R" on the driven pulley with their

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-MANOEUVRING EXPERIMENTS WITH LARGE MODELS IN OPEN WATER

Fig. 5

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-transverse components Ricos.f and R"cos:5 ;

the transverse component A of the support pressure; the longitudinal component of the same pressure vhich may be neglected;

the residual forces of the connected devices which may be neglected as well.

For the equilibrium of the forces we have:

T = A - (R - R") cos;1 (1) Y

Y

= N.b = (R' - R")r (2) hence - T = A - K.M (3) Y

Y

where cos.5 K =

is an instrumental constant to be obtained by calculation or calibration.

If force N instead of acting on the after rudder plate acts on the fore rudder blade - case of a balanced rudder - formula (3) becomes:

-P7 = A + K.M

giving the final general formula:

- T =.;K.M

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Equations (2) and (4) can be used to calculate the torque and the transverse drag on the rudder. To

calculate the total normal drag - whose measure must be known for particular strength controls - the approximate

formula

N = (N - T )/cos.04

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associated with formula (4) can be used observing that

component T compared with N is so small as to be

negligible. Should a higher precision be desired the

correct formula N = Ny/co8.o4 can be employed

separately computing T as theoretical and practical

considerations may suggest.

Arm b can be obtained from equation (2). The instrument gives the measures of the

moment M and the reaction A through the differential

tension of the springs M0 and

mg ,

and springs M1

and M. The relative displacement s of the points A and

C when the helm is put over affords the calculation of the rudder angle o4 by the formula

04=

8.180/0 r

The Gebers dynamometers, which are in

running use for propeller thrust and torque measurements in our tank, have been recently discarded, when at

the lake, on account of their big volume and weight and other defects. Now we make use of dynamometers working after the strain-gauge principle (fig.6).

The thrust bearing is backed by a spring and the

propeller shaft is operated through an idle gear-wheel equilibrated by a second spring having, both, strain-gauges. Volume and weight of these dynamometers are exceedingly small and they can be placed close by the inner bearing of the propeller shaft.

The model path is plotted on a chart through the spots of the subsequent triangulations.

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046

V,

MANOEUVRING EXPERIMENTS WITH LARGE MODELS

IN OPEN WATER -Vef FiR4P747;;;Zi, A fitrirftr , '714.7*To779"7.7"-r-r, rr,^2rrrg,"77. ,1:;,ez=p, _,d,,ogioyor '?//' "if ,370 777 7,7, 777 -' ' AAAA,trror L '4q7/il:F," J912F;;';:iii/j;JY,0;14j4041/. ffi'/ fl,,,;;;?,4,/,14,947Y14.;!;_ 4. , 044:144 06z , ,44,/ 1 7 445'1

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The electrical connection between the two stations ashore - for signalling the instant when simultaneous bearings are to be taken - is realised by radio-control.

Knowing the length of the shore base and the cross bearings of a target rod put on board the model the rod position is easily found. The model position is fixed with the aid:of the drift angle at the rod point. This is now obtained with a cine-camera in-lieu of the early small outbow rudder. Every time the boat falls in line with the observation stations a shot is taken and the corresponding bearing calculated on the developed film.

The boat rod point is not found as cross point of the observation angles, but transforming the angular coordinates into rectangular coordinates. These are usually referred to the left station as center of the system. To perform these operations we make use of a special sliding rule which permits calculating the results in exceptionally short time.

Many interesting investigations can be conducted with a free running boat. So far our

Institute has carried out experiments on these subjects: speed trials;

turning trials; stability of route;

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ahead-and-astern trials;

stopping and reversing engine;

efficiency of propulsion with Voith-Schneider propellers;

efficiency of bow-rudders, lateral rudders and under-keel rudders;

sailing in shallow-water and canals; sailing in rough water;

rolling and pitching.

The system of experimenting big models on a lake or elsewhere on a large natural basin instead of small models in artificial manoeuvring ponds has advantages and disadvantages. The principal advantages are:

big measures involved so that more reliable and tougher instruments can be used;

correct metacentric height and period of roll can ne easily realised;

no wall-effect;

smaller or null scale-effect; water left calm after each trial.

The disadvantages are:

need of waiting for extremely good weather conditions; more time and money required;

any movement of the man on board influencing the trials.

The third drawback can' be eliminated by

I

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-18-MANOEUVRING EXPERIMENTS WITH LARGE MODELS IN OPEN WATER

replacing the experimenter on board with a radio-control from ashore.- This system is undoubtedly attractive

but we understand that it is not always possible or

-convenient to substitute an experienced person, who can judge and decide and watch how all on board is

working, with an electromechanical device.

CONCLUSIONS

We have not yet received sufficient and fully reliable results from sea trials to check the

exactness of our model predictions and we hope the gap will be filled in the near future. But, in the

same time, we have not received any notice about

unsatisfactory controls eventually made by shipbuilders or ship designers interested in the same experiments. Recent model predictions on the steering and

manoeuvring qualities of a big tanker Bailing on the Suez Canal route a very puzzling problem

-have been checked and proved correct at sea. Contemporary predictions made on a base of experiments carried

out with small models in a foreign tank turned out to be erroneous.

In the writer's opinion both systems - large models on a lake and small models in an artificial pond - are worthy of consideration, the choice depending on many technical and practical factors connected with the kind or nature of the

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MANOEUVRING EXPERIMENTS WITH LARGE MODELS IN OPEN WATER .

must be as large as possible and be fitted with wall wave-absorbers of top efficiency. Even with big models we have found that very smooth water is absolutely necessary for reliable results and this smoothness is usually ascertained with the lake surface clearly

reflecting the moored model.

The models tested till now may be classified

REFERENCE: "Esperienze su Lago con Modelli di Navi" by R. de Santis (Congresso Internazionale di Tecnica

della Nave e della Navigazione, Naples 1954).

2o -as follows: passenger ships 4 cargo Ships 1 tankers 2 military ships 4 miscellaneous 2

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APPENDIX

CALCULATION OF THE DRIFT ANGLE AND RADIUS OF GIRATION

With reference to

fig.71

which is a

diagrammatic plan view of the boat, the drift angle

of the target point A is

r.

When the longitudinal

plane of the boat is in line with a shore station

as is the present case angle E

is given

by:

7=

rvq)

(1)

where _{1A0 is the bearing of A from 0' and} _El _an

auxiliary angle given by:

or tg f = Xo - X - Yo X0 - X 21 -(2) (2')

where X0 and Yo are the coordinates of the center of curvature C referred to the abscissa axis 00' and the center of coordinates 0, and X, Y are the

coordinates of the terget point A.

Formula (1) is to be changed into:

r=

Or

according to the particular case under consideration. To solve equation (1) it is necessark to

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Angle

A

and coordinates X, Y can be obtained

as described before: by a eine -camera placed at the observation point 0' for angle ft and by the bearings taken from 0, 0' for the coordinates X, Y.

Coordinates X0, Yo of the center C may be directly

measured on the drawing or calculated by the following

formulae should the coordinates of three points 1, 2, 3 on the circle be experimentally known. Be:

X.1,Y1; 2,Y2; X3,Y3 abscissae and ordinates of points

1, 2, 3, respectively; then we have:.

(X42+Y)(Y2.-Y3)+(Xt+Y2.4)(Y1-Y, )+(Xt+Y;')(Y, -r.) ₍₃₎

Xi (rt -Y3)+X (YI-Y1 ) (YI -Y )

If we can read with sufficient accuracy the length

y of the cord between points 1 and

3,

and the rise x

of arc 1,2,3 radius R can be calculated by the well-known formula:

R 4 xl + r3f4 (6)

8x

The radius of gyration is:

23

-(7)

X:+Y4a)(X1-Xz)+(XZ+Y,2.)(X1 -X3)+(*Y:)(XL

(4)

Xi(Y1-Y3)+X4(Y1-Yd+X3(YI-Yz)

The radius R of curvature is:

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If the boat travels along a small arc of circle

never putting her head to an observation point it is

impossible to know the auxiliary angle

p.

In this

case the problem can be solved by the following procedire (fig.8).

Choose on the boat two additional points P

and P. respectively placed at the stern and the bow

at knOwn distances p and p' from A. With the cine-camera placed at 0' take the bearings of Pt A and-P'. The

position of points P and P' will be found as intersection of O'P and O'P' with tne circles centered in A - whose coordinates are known - and radius equal to p and p'.

The drift angle gcan be directly read on the drawing. Obviously it is sufficient to work.with only one point P or I" but the double way offers a convenient cross control.

If angle AT is too small to feel satisfied with

the foregoing procedure we can calculate in the

following alternative way. From fig.8 we get: (8)

Nowt angle 91can,be calculated by formula (2) and

angle (% by

fk=

So,

-

(fl-e)

(9) where:

son. APO' - V03-304+ sen. (10)

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24MANOEUVRING EXPERIMENTS WITH LARGE MODELS -IN OPEN WATER -i

\

0

I

\

1 .

Fig. 8

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or by

po= 180 - (AP'O' 4.(1+)

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where

Ben. iPbe

V(13-361+ Y1

sen.e

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p.

EXAMPLE. Xi = mm 10.0 abscissa of point P = " 33.0 ordinate El II Xs = " 23.5 abscissa A = 45.0 ordinate Xa = * 65.0 abscissa P'

Y;

= 59.5 ordinate.

(34 = 38030' observation angle of point A

e

= 10030' angular decrement of point P

EP = 240 " increment n

p

s= mm 20 distance PA

p' = 30 ". P`A

b = " 80 base length 00'

Applying formula (3) we obtain: X0 = mm 70 applying formula (4):

yo = mm -20.5 and applying formula (6):

R = mm 80

Remembering that:

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- MANOEUVRING -EXPERIMENTS WITH LARGE MODELS IN OPEN WATER

Sen. y,

we get

son. T

Roma, feb. 1959

x

K

70 .- 23.5

80 f?i 35°30°

Applying

formula (10) we have

APO° 211 41030'

and applying formula. (9)

= 13°301

Therefore the drift angle

5" is

3-=

22°

and the radius of gyration

R =

nim 74

27