Apparatus for recording motions of ship model in waves

SII(PU11IR RESRC lUST hUlE ZA E B

Apparatus for Recording

Motions of Ship MoeI

io Waves

by

Kaname Taniguchi

Chief of Mitsubishi Nagasaki Experimental Tank

Kyoji Watanabe

Assistant to Chief of Mitsubishi Nagasaki Experimental Tank and

Jitsu Shibata

Mitsubishi Nagasaki Experimental Tank

Delfi

taper to be presented at the Symposium on the

Towing Tank Facilities,

Measuring Technique

APPARATUS FOR RECORDING MOTIONS OF SHIP MODEL IN WAVES

by

K. Taniguchi, K. Watanabe and J. Shibata

The details of BPS Recorder /heaving, pitching and

surging! is presented. This is currently used in Mitsubishi Nagasaki Experimental Tank for recording ship model motions in waves with excellent results.

The BPS recorder consists of /1/ a small and light sub-carriage which is placed on horizontal and longitudinal tracl$ on the towing carriage and can freely travel back and forth together with model and /2/ a vertical rod which is fitted on the sub-carriage and constrained to it to make free vertical motions together with the center of gravity ol' model. This recorder is capable of recording heaving, pitching and surging motions of model independently from the relative motions of these components. Low-torque-potentiometers are used as transducers and an electro-magnetic oscillograph takes recordings.

Preface

At the saine time with the reconstruction of our large

tank in

_1953,

waves by means of selfpropulsion test of models in waves, and since then a total of 50 models including tanker, cargo ship, fishing boat and various naval craft have been tested.

The reasons why we took up self-propulsion test in pre-ference to resistance test in the study of ship's motion in waves are that the latter was considered tc be an indirect means from the standpoint of applying the test results to

the actual ship's design, and that the former was not neces-sarily difficult as compared with the latter when using testing apparatuses of our rwn device.

1. Measurements in the tests in waves.

The measurements in our experimental tank may be suinma-rized. as follows:

A/ Speed

The ground speed of the carriage is adopted as the speed of ship model for tests in waves, of course, it should be corrected for the surging motion of the model relative to the carriage, while the water speed of carriage measured by a screw-type current meter is regarded as the speed of ship model for tests in smooth water. The ground speed is

nìeasu-red. by the period of encounter of the carriage and the dis-tance pieces attached to the rail side at the disdis-tance of every 2 meters. The encounter is recorded continuously by pen-chronograph or automatic printer combined with an electric counter.

B/ Rotating speed of propeller

The rotating speed of propeller are measured as the mean revolutions by cumulating the revolution signal of 200 pulses per revolution transformed by a magnetic pick-up. Otherwise, they are recorded continuously by an automatic printer.

C/ Thrust and torque

The thrust and torque are measured by the self-propulsion dynamometers /ISD-type and SDD-II type/ devised and manufac-tured in Mitsubishi Nagasaki Experimental Tank, and automa-tically recorded. by pen-writing oscillographs, pen-writing milliamnieters, electro-magnetic oscillographs and self-balancing type strain meters, etc.

D/ Wave profiles

The electric wave height recorder with parallel plates has been used since the tests in waves were started. Recently,

APPARATUS FOR RECORDING MOTIONS OF SHIP MODEL IN WAVES

the digital wave height recorder using the multiple contacts has been used together and the measurements are expected to be more accurate. The new wave height recorder indicates the

wave height as the multiples of unit height mow

_{5 mm!.}

E/ Encounter marks

By using an encounter marker, encounter marks are inserted to the records simultaneously.

F/ Accelerations

As the data of comfort bleness on board acual vessels, the vertical accelerations of models at bow and. stern are measured.

The accelerometer is of unbonded wire strain gauge type.

G/ Motions of ship

In the test in waves in our tank, the motion of ship is restricted within the vertical plane which contains the di-rection of advance, because the ship didi-rection is always perpendicular to the crests of regular waves. Therefore, the ship's motion is determined by measuring three components of motions, i.e.«pitching, heaving and surging. We made an

instrument of our own device to measure the motions. This

report deals with the instrument - so-called BPS Recorder

in the following chapters.

2. On the design of BPS Recorder

Generally the instruments to measure the motion of ship in waves must have the following performances:

a/ The measured values should indicate the ship's motions perfectly in general expression.

c/ The results of respective ineasurenients, should

autoniati-cally be written on the same paper simultaneously as

continuous records in order to make it possible to

in-vestigate the relation among the phases.

BPS Recorder was completed as a result of our

investi-gations satisfying the aboie mentioned conditions.

The motion of ship which advances perpendicularly to

wave crests can be expressed perfectly by three components.

erierally they are heaving

¡vertical motion of the center

of gravity/,

pitching /rotational motion around the

center of -gravity/, and surging

¡for-

and afterward

motion of ship/.

Our instrument measures directly these three components.

So we call the instrument BPS Recorder. Here, H, P and S

mean heaving, pitching and surging respectively.

For the condition b/, electric measurement was adopted,

whose principle is that the bridge unbalance induced by

angular movement of simple potentiojneter is transformed

into electric current, from the standpoint of the

availa-bility of low torque potentiometer and ease o

handling.

c'/ For the condition cl, electro-magnetic oscillograph is

suitable. This method satisfies the conditions of the

linearity for transformation and the simultaneity for

records.

.

Structure of BPS Recorder

The BPS recorder consists of two sub-carriages which are

placed on horizontal and. longitudinal tracks on the towing

carriage and can freely travel bac1

and. forth together with

model. Each sub-carriage has a rod constrained to make only

free vertical motions. The lower ends of the rods are

con-nected. to ship model at the two points on the centre line.

4-pick up potentiometers

mounted on the rod --..

and sub - carriage

vertical rod

guided L3' 2 sets of 3 ball bearing and only vertical motion is permitted

\

sub -carriage guided by guide rods guiderods fixed on lowing carriage

P.

41

A

dl

arma

centre of gravity of the ship model

Fig. 1.

Schematic Drawing of the Arrangement of HPS Record.er

Fig. 1.shows the schematic drawing and Fig. 2. and 5. the arrangement.

t

p2g

I--.4

_p...

-

--Fig. 2. Arrangement of lIPS Recorder

The connections between the rods and ship model allow

mo-derate angular movement in any direction /including rolling!,

and. the forward one of the connections is located at the centre of gravity of the ship model. So the centre of gravity of the ship model moves vertically with a rod, and sways fore and aft with sub-carriage. Therefore, by the forward set, vertical movement of the rod coincides with the heaving of the ship model, the longitudinal displacement of the sub-carriage shows the surging, and the variation o±'ang1e between

APPARATUS FOR RECORDING MOTIONS OF SHIP MODEL IN WAVES

-i

the rod. and the hull indicates the pitching. The pick-ups

for heaving, pitching and surging are mounted together on

this set. The other set of sub-carriage and rod. is used

merely as a guide for direction. This instrument is suitable for ¿4 n ship model used as standard model for the

self-propulsion test in waves in our tank, and all parts are made from light metal in order to decrease the errors induced by

friction and inertia force. Two sub-carriages supported on

the horizontal guide rails weigh about

4,5

the increment of mass for surging is about onJ.y 1 % of the

mass ai' the ship model. The vertical rods weigh about

_2,7

kg. But as this is included in the mass of model which is balanced with the buoyancy of ship model, the vertical rods

do not influence on the motions of ship model.

The three components of motion - heaving, pitching and surging - are transformed into the angular motion of the

pick-up wheel, and the angular motions of wheels are measured and recorded remotely by using low-torque potentiometers as

the transducers. The arrangement of the pick-up wheels are

schematically shown in Fig.

4,

_5.

vertical md

guide boll bearing

heave pulley mounted on

sub - carriage

'Nwlre passes once around heave pulley

ends of wire are fixed

-8

to vertical rod

sub-carriage

APPARATUS FOR RECORDING MOTIONS OF SHIP MODEL IN WAVES

Bottom of the ship mode!

Pig. 5.

wire posses once around surge wheel. pitch wheel

axis of wheel are rmun ted on vertical rod.

wire posses once around pitch pulley and ends of

wire are fixed to fan

shaped plate.

tan shaped piole

fixed to the ship model

surge wheel

axis of wheel is rncv.,,ted

on sub-carriage

-9

Pig. 6

Arrangement of Surge Wheel

ends of wire ore

fixed to main

carriage.

1/ Heaving

The vertical rod.

moves vertically according to the motion of the centre

of gravity of ship model, and the _{motion of rod}

is transformed into angular motion

of the heave wheel mounted

on the sub-carriage, _{by means of a cord}

as illustrated in Fig. 4.

2/ Pitching

The relative angular motion between _{rod and. ship model}

-pitching - is transformed into angular

motion of pitch wheel,

as illustrated in Fig.

_5.

3/

A cord fixed to the main carriage

at its both ends passes

once around surge wheel mounted on the _{sub-carriage,}

conse

quently the surge wheel rotates in _{proportion to the}

for-and. backward motion,

being identical to the surging of the

centre of gravity _{of ship model.}

The ratio of the rotating angle of _{the pick-up wheel}

to

the ship's motion can be chosen freely _{by changing}

_the

diameter 01' the wheels.

The values of above mentioned ratio of our apparatus

are selected as follows:

Heaving - _{500 mm heaving results in one revolution}

of surge wheel.

Surging - _{440 mm surging results in one}

revolution of surge wheel.

Pitching - _{a complete revolution of pitch wheel}

corres-ponds to the 36° of pitching.

-4. The transducer and recorder

We make use of low torque _{potentiometers as the}

transd.u-cers which transform the angular motion to the electric

quantities. The construction of transducers and the recording apparatuses for the three components of motion are all the

same. The particulars of the potentiometer are shown as

follows:

resistance

500fl

electric current _{20 mA.}

frictional torque _{under 0.3 g-cm}

dimension /diameter _{x length/ 22 mm x 22 mm}

The electric circuit used for measurement is an ordinary

bridge circuit as shown schematically in Fig.

_7.

For the recording apparatus, _{we used the electro-magnetic}

oscillograph, whose vibrators are directly used as the de-tectors of bridge circuit. In daily work, it is most impor-tant that the instrument should be very stable, and this is

the reason why we adopted such a simple transducer _and

electric circuits The power supply for the bridge is the stabilized 220 V direct current, which is used on the towing carriage. The balance of the bridge circuits can be checked by the millianuneter mounted on the panel of the control unit, and can be adjusted by the potentiometer on the panel, if necessary.

The sensitivity of the records can be selected _{by a switch}

as shown in Fig.

_7.

the potentiometer results in 65 mm movement of the recording spot.

-o 5O ¡rwn 5Omm bow down surge pitch encounter mark length of model 4.2 m Vm: I 044 m/5

we length

Pig. 8.

Typical Record of the

Motions of the Ship Model

base of heave

o lb o

direction

-APPARATUS FOR RECORDING MOTIONS OF SHIP MODEL IN WAVES

Fig. 11. A Self-Propulsion Test in Viaves

on a 4.2 Passenger Ship Model

A typical oscillograin by this recorder is shown in Fig. 8.

Fig.

_9.

and Fig. li. is the photograph of a passenger boat model with this apparatus during self-propulsion test in waves.