A rotating arm for towing models of ships and other forms in circular paths

Experimental Towing Tank Stevens I n s t i t u t e of Technology

Hoboken, New Jersey

A ROTATING ARM • . "

FOR TOWING MODELS. OF SHIBS AND OTHER FORMS IN CIRCULAR PATHS

Walter Fried

Prepared f o r the David Taylor Model Basin

Navy Department • •VTashington, D«C»

under Bureau o f Ships Contract NObs 22087 Job Order 9

TABLE OP CONTENTS

ABSTRACT

NEED FOR A ROTATING ARM

DESIGN OF THE ROTATING ARM APPARATUS General Specifications

Alternate Design Considerations Features of the F i n a l Design OPERATION OF THE ROTATING ARM

ABSTRACT

A " r o t a t i n g arm" apparatus, i n s t a l l e d i n the Ifeneuvering Tank at the E&cperiinental Towing Tank, Stevens I n s t i t u t e o f Technology, provides a means of towing models o f ships and other forms i n c i r -ouiar paths* The diameter o f the c i r c l e . t r a v e r s e d may be adjusted from ten to s i x t y - f i v e f e e t . The

speed o f r o t a t i o n may be adjusted from one to. s i x revolutions per minute.

The r o t a t i n g arm equipment, constructed under Contract NObs 22087 between the United States Navy and the Experimental Towing Tank, was put i n t o op-e r a t i o n i n Octobop-er, 1945,

NEED FOR A ROTATÏITG ARM . ^

For ttatiy yeeo-s considerable a t t e n t i o n has bee^n giveix t o the hydrodynamics o f ships moving i n c i r c u l a r paths. I n p a r t i c u l a r , , the David Taylor Model Basin and the Esperimental Towing Tank have |. j o i n t l y f u r t h e r e d these studies both by a n a l y t i c a l work and model

t e s t s . As the investigations progressed, i t became apparent t h a t ! e x i s t i n g t e s t techniques d i d Ppt supply s u f f i c i e n t data t o deter¬ ' mine completely the forces a c t i n g . Furthermore, the a n a l y t i c a l ;, approach could advance f u r t h e r i f supplementing experimental data ' could be s u p p l i e d .

I n 1942^ a s e v e n t y - f i v é f o o t square maneuvering t a i i k , known as Tetnk No. 2, was b u i l t at the Ëxpefimöntal Towing Tank under the ij sponsorship o f the NDRC f o r t a c t i c a l t e s t s of ship icaodels. The

; t e s t teclmique developed f o r these studies, ijiyolved the use o f f r e e -i runn-ing models carry-ing t h e -i r am-i propuls-ive and steer-ing equ-ipment.

' I t was thus possible t o observe both the steering and t u r n i n g char-a c t e r i s t i c s óf models d i r e c t l y . I n char-a d d i t i o n , mechar-asurements of t h é " l a t e r a l force acting on the rudders were made w i t h a dynamometer

carried i n the model. However, no other force measurements could be obtained r e a d i l y while the model was i n c i r c u l a r motion.

I t was apparent t h a t need existed f o r a type o f instrumenta-, t i o n comparable t ó t h a t used i n a towing tank where a model can be towed by a carriage equipped w i t h a system o f dynamometers f o r meas-u r i n g the l a t e r a l and l o n g i t meas-u d i n a l forces acting on t h é h meas-u l l . To state the problem another way, a towing carriage capable o f t r a v e l -ing i n c i r c u l a r paths was r e q u i r e d .

This heed had been recognized at the Experimental Towing Tank i n 1939, at which time a small-scale r o t a t i n g , arm apparatus was con-!-structed and used i n a l i m i t e d number o f t e s t s which were conducted in. a swimming p o o l . The arm could be extended t o cover a maximum diameter of 20 f e e t . I t ceurried i n d i c a t o r s at i t s . outer end, but was u s e f u l only f o r work w i t h models o f about 27 inches length ahd a l i t -t l e more -than one pound'displacemen-t.

I t was decided, t h e r e f o r e , t o i n s t a l l i n Tank No. 2, a new and enlarged rotatinig arm, large enough t o tow the f i v e t o eight f o o t mod-els o r d i n a r i l y used at the Experimental Towing Tank. I t should also be able t o carry the necessary complement o f dynamometers needed f o r . a complete p i c t u r e o f the hydrodynamics of a form t r a v e l i n g i n a c i r -cular p a t h .

DESIGN OP THE R0TAIIN6 ARM APPARATUS

General Specifications

I n January 1945 the Eöcperimental Towing Tank vmdertook the de-sign and i n s t a l l a t i o n óf a r o t a t i n g arm apparatus under the f o i i o w i n g general s p e c i f i c a t i o n s :

(1) Provision f o r fiynamometers.

The arm should be equipped t o carry s u i t a b l e f o r c e . measuring dynamometers. Either e x i s t i n g "towing

tank" dynamometers or instnmients especially de-slgnedi f o r the arm might be used,

(2) Turning Diameters

Minimum diameters o f about two ship lehg^ths were des i r e d ten f e e t f o r a f i v e f o o t model. Maxlm\mi d i -ameters were l i m i t e d by the sevehty-five f o o t diamet e r o f diamethe diametank and by diamethe mechanical problems i n c i -dent t o b u i l d i n g an arm o f great r a d i u s ,

(3) Speed p f Rotation

The speed p f r o t a t i o n óf the arm was t o be adjustable from one t o s i x revolutions per minute, so t h a t the speed ranges f o r models t r a v e l i n g at the extreme maxi-mum and minimaxi-mum diameters would be amply covered. (4) • Apceleration

I n order t o obtain a complete t e s t n m w i t h i n 360** o f t u r n , and thus avoid running a model i n i t s wake, s u f -f i c i e n t d r i -f i h g power was t o be^provided t o accelerate the arm t o t e s t speed w i t h i n 9 0 ° , O r i g i n a l l y i t was . f e l t t h a t some sort o f catapult loight be required t o ' _ accomplish t h i s , but a c t u a l operation showed t h a t

prop-er manipulation of the speed controls obviated t h i s need

Alternate Pesigi Considerations

A number of alternate designs f o r the arm were considered. Prom the standpoint of l i g h t n e s s , a l a t t i c e construction appeared a t t r a c t i v e . However, t o insure a s a t i s f a c t o r y degree of s t i f i f n e s s f o r the structure as a whole, unduly complicated systems o f guy wires were i n d i c a t e d . , Furthermore, none of the l a t t i c e arrange-ments gave much promise o f t o r s i o n a l s t i f f n e s s of the h o r i z o n t a l

ö l

D I A G R A M M A T I C S K E T C H OF T H E

;i a m . Such' s t i f f t i e s s would be needed to counteract the drag o f the

jj model being towed, especially i f t h é modél were an underwater body

such as a torpedo ó r submarijiie. . Design considerations f i n a l l y pointed t o the dé3ifabili^y o f

constructing the arm of durailuminum t u b i n g i O r i g i n a l thoughts t h a t s u i t a b l e m a t e r i a l would be Unobtainable bécausé of the war were d l s -' polled when a quantity of tubing o f the pfopef type was located i n : "excess stock". The t u b i n g , prepared f o r a p r o j e c t which had since ; been cancelled, measured f i v e inches i n outside diaiaeter ahd had a

I quarter inch w a l l . I t s outer surface had been ground^ making i t ad¬ I ; mirably suited f o r the a p p l i c a t i o n .

Features o f the F i n a l Design

A general assembly drawing, bh the opposite page, shoWs the. p r i h c i p a l features o f the r o t a t i n g arm a t r u o t u r e . They a r e :

(1) a h o r i z o n t a l duraluminum arm w i t h a simple system o f s t i f f e n i n g ' s t r u t s ,

(.2) a V e r t i c a l s t e e l s h a f t w i t h bearings top and bottom and w i t h s u i t a b l é keyways f o r t r a n s m i t t i n g the d r i v -ing torque t o the arm,

(3) a welded s t e e l supporting base, mounted i n the center o f the tank,

(4) a d r i v e assembly, located at the top of the v e r t i c a l e h a f t i including a gear box and motor.

The h o r i z o n t a l arm i s f i t t e d on either side w i t h p a r a l l e l r a i l s to which a carriage can be attached^ at any a t t i t u d e and any r a d i u s , The carriage, i n t u m , carries dynamometers and other t e s t equipment. The arm^ together w i t h the s t i f f e n i n g s t r u t s , i s mounted on the v e r -t i c a l .drive shaf-t i n such a way -t h a -t -the arm can be moved v e r -t i c a l l y . This provides an adjustment accomodating various arrangements o f dy-namometers and models w i i c h might involve d i f f e r e n t spaoings between the a m and the water surface.

The v e r t i c a l d r i v e s h a f t can be properly plumbed by s h i f t i n g I t s lower bearing h o r i z o n t a l l y . The upper bearing o f the s h a f t i s support-ed by a frame-work fastensupport-ed t o the roof trusses of the b u i l d i n g . Near the top of the v e r t i c a l s h a f t , a s l i p - r i n g and brush assembly supply the means f o r cohnécting e l e c t r i c a l instnamènts on. the arm w i t h a s t a -t i o n a r y

point

ad-d i t i o n a l c i r c u i t s can be aad-dad-dead-d at a l a t e r ad-date.

At i t s lower end, the v e r t i c a l s h a f t i s supported by a t h r e e legged s t e e l supporting base, bolted t o the tank f l o o r f o r easy r e -moval.

The driT© meohaniem,. which i s located at the top o f the s h a f t , i s a variable speed motor and a reduction gear. The motor speed is. . e l e c t r o n i o s L l l y c o n t r o l l e d , the des i r e d . speed-being selected by a po-tentiometer, The accuracy i n speed r e g u l a t i o n i s of the order o f ifom The s p e c i f i e d speed rangé p f the arm and the dharacteristios. o f the d r i v e motor c a l l f o r reduction gear r a t i o , of 652.5 : , 1 , However, a • pa.ir pf e x t e r n a l l y moünted .gears between the motor and gear box allow an extension of the speed range above 6 r . p . m . An ö v e r - r i d i n g » or "free-wheeling", c l u t c h protects the h i g h - r a t i o gear box from damage which might be caused by the arm's coasting^ the motor can thus d r i v e

the arm, but the arm cannot drive against the worm of the gear box when the power i s cut o f f . As. an a d d i t i o n a l p r o t e c t i o n , a shear p i n ,

interposed between the arm and the c l u t c h , serves as an o v e r a l l guard against damage t o any part o f the d r i v e system caused by a shock or impact l o a d .

OPERATION OF THE ROTATING ARM

The r o t a t i n g arm has been i n use since October 1945, I t s op-eration has not only been quite s a t i s f a c t o r y , but has exceeded expec-t a expec-t i o n s .

The outstanding operating c h a r a c t e r i s t i c i s i t s extremely smooth motion and freedom from v i b r a t i o n , tJse o f c e r t a i n "towing tank" dyna-mometers on the arm, f o r example, reveal greater steadiness o f scale readings than those encountered on the towing tank carriage.

The p f a c t i c a l outcome o f t h i s s t a b i l i t y of operation has been the successful vise o f "small motion" dynamometers especially designed f o r use on the arm. These instnments embody s t i f f springs which de-f e l c t over a maximum range ode-f a de-few hundreths ode-f an i n c h . Their m.o-t i o n is dem.o-tecm.o-ted and m.o-transmim.o-tm.o-ted by e l e c m.o-t r o n i c means m.o-through m.o-the s l i p rings t p a shore s t a t i o n . Simultaneous readings o f resistance, l a t e r a l , 1} and rudder forces acting on the h u l l may thus be obtained while the I model ( w i t h or without s e l f - p r o p u l s i o n ) i s being towed by the arm.