On unusual phenomenon in manoeuvrability and its successfull countermeasure of a fishery research vessel

ARCHIEF

Lab.

y,, Scheepbouwknd

Technische Hogschoj)

Delfi

ON UNUSUAL PHENOMENON IN MANOEUVRABILITY AND ITS SUCCESSFUL

COUNTERMEASURE OF A FISHERY RESEARCH VESSEL*

**

**

**

by Míchio Nakato

,

Kuniji Kose

,

Kazuhiko Ilasegawa

* * *

and Hisayoshi Tatano

Summary

These days we sometimes have experienced or heard of fu1i-bodi.ed ships with so-called "uusua1 phencrnen&' in rnanoeuvrability,. and these ships seem to increase in nuimber rapidly because of growing the fullness of hulls.

Herewith in this paper an example of such a phenomenon appeared

in a fishery research vessel is dealt. Turning tests and zig-zag tests were held, using both the ship and the 1/10 scale model. Oblique towing tests, s-varying tests (changing drift angle slowly) and observation cf the stern flow, furthermore, were held in a

towing tank using the model.

These experiments certify that there are two states in the separate

flow fields around the hull and the state altca,nntes one to another.

The large clearance above the propeller and thc rudder as well as the full stern form permits the flow passing from starboard to port

or vice versa through the clearance. By this flow, two types of

unsymmetrical flow separation may iccur. The alternation of the flow direction through the clearance accompanies the changes in the

separation, the flow field and the force acting on the hull. Basic characteristics of the "unusual phenomena" oberved both

in the ship and the mo.ol can he explained by this unsyiraiatrical separation nd its alternation. The apparent difference betueen

the ship and the ode1 cari be also treated by the conception of

"switching line". The "switching line" is defined as the critical

state of motion ai: which the alternation of the flow occurs.

In consideration of those e.perLnents, a center fin shutting the

clearance was adopted to both the ship and the model. The results

are satisfactory. In model tests the unsymmetrical separation is

no longer observed. Thereforc, the course stability recovrrs

almost completely and both the model and the ship can keep the' course without any difficulty.

* Presented at

hc 3.S.N.A. confe:ence on May

9, 1978 at

Tokyo

** Hiroshima UnivE.rsity

Osaka tjniverisij

/

2

t'/.c;7

:

INTRODLICTION

Generally, a fishery boat has comparatively a larger deck area and a smaller value of L/B, so that she can keep stability

and permits deck works. In spite of a smaller CB boat, it

tends to be arranged an engine room as backwards as possible

and be rather fat locally in aft body.

In such a fishery boat, it is often observed strange behaviours

in manoeuvrability, so-called "unusual phenomena" l)4) 1)

In this paper "unusual phenomena" appeared on a fishery

research vessel and also on its model ship are fully investigated. In the results, strange behaviours in these cases are explained as "two states of flow field corresponding to one state of ship

motion". To eliminate the unusual phenomenon and to improve

rnanoeuvrability of the ship, a simple and effective countermeasure i found.

GENERAL VIEW OF THE SHIP

In Table 1 the principal particulars of the ship are listed. She is a fishery research vese1 with an ordivary fishery boat

form, though her L/B is a little bit smaller than usual.

Fig. i shows her lines. The remarkable features of her lines

are the fat stern, especially of the upper-fore part of the propeller, the small curvature of 2.0 m water line, and the large clearance and the flat frame lines above the rudder and

the propeller.

RESULT OF FREE RUNNING TESIS

The

tets of the ship

ro held under i.he condition or

_{Table 1.}

As the tests of the rnodel are held at the designed draught, the ship is comparatively stern-trimmed.

Fig. 2 shows the turning characteristics of the ship. In the

large range of rudder angi, it shows fairly stable. _within

the smaller rudder angle, however, it looks ciiit strange.

Fig. 3 is the record of yaw rate with constant rudder angle,

hut it looks like as if the rudder angle changed alternatively. lt shows the hydrodynamic forces acting on the ship change

stepwise in turn.

The condition and the result of the model tests are also shown

in Table I and Fig. 4. In this case, the tirning characteristics

corresponds to about 10 degrees in rudder angle. lIowever, as to the frequency of appearance, there is a difference between the

two lines.

Comparing the turning tests of the ship and the model, the apparent feature is somewhat different, although the basic

character is the same: two hydrodynarnic forces act in each

turning motion. In the ship, one of the curves fades away in

the larger range of rudder angle and the self-exciting alternation

of yaw rate occurs in the smaller range. In the case of the

model, there exist two steady states of turning.

Fig. 5 is prepared for the explanation of the difference

between the ship and the model. The full lines mean two turning

characteristic curves, which can be regarded the same between

the ship and the model. The "switching lines" described in

broken lines are defined that, if the turning motion (yaw rate in this fugure and, strictly speaking, the apparent drift angle at the stern might be quoted) grows across the line to the

hatched side, the flow field alternates wiciì another,

In the ship, within the smaller rudder angle the turning characteristic curves exist both the outsides of the switching lines, and therefore the motion exites between the switching lines. The difference between the maximum and minimum value in Fig. 2 coresponds to the gap of the switching lines. In the

larger rudder angle, as one of the two turnlng characteristic curves is placed between the two switching lines each, only

one state of motion is allowed.

Contrary in the caco of the model, below th medium value of rudder angle, switching lines surround turning characteristic curves, hence each state of motion is realizable. In the larger

rudder angle, switchino lines and turning characteritic curves seem to be adjacent or coincide each other, and therefore two

states of motion are not always coming up.

The "switching linet' is conceived through the observation of

the unsymmetrical stern flow separation, which will be described

later in this paper. It might be reasonable to consider the

slope or the gap of the switching lines is not the saiue between

the ship and the model in consideration cf Reynolds number etc.

The fundamental feat.ure of tho turning characteristics of this

motion of turning _{is still persuasive, however.}

In the followings, the results of zig-zag _{tests are described.} Nomoto's K-T analysis is well-known as a method of Z tests

analysis. But for ships having such a phenomenon, it _{is not}

convenient to apply it, because it only provides the average

(time-independent) characteristics of a ship. Here phase plane.

analysis is applied to t.he results of the model (see Fig. 6;

yaw acceleration for X-axis and yaw rate for Y-axis).

The full lines in the figure mean that the rudder angle is kept constant, and the broken lines in the figure mean under

steering. All of Fig. 6 (a) , (b) and (c) are obtained from the

same 7.5° Z tests, though they seem to be _{very curious. In}

Fig. 6 _{(a) starboard rudder works well and port-side less. In}

(b) vice versa and in (c) both cases happen to occur. These

results correspond to the two turning characteristic _{curves in}

Fig. 4: _{(a) shifts on the upper line, (b) on the lower line and}

in the case of (c) at the very time _{hen the 3rd rudder is} commanded, it moves from the upper to the lower suddenly and consequently both trajectories are piled _upon

_Cc).

The following expression of the yaw motion is provided to

describe the above ship motion. _{In the equation, an additional}

turning resistance acting on the hull is _{considered as well as}

usual turning resistance. _{This additional constant can be}

described in the term of rudder as ôa. The similar expression of the yaw motion is presented by Tagano and Asai5.

T +

= K(5 + 8a)

Applying this equation to the above results, _{following indices} 6)

are given

T = 1.77 sca,

_K=O.7l1/sec,

_6a5deg.

The dotted line in Fig. 6 _{(c) is the simulated trajectory,}

using the obtained K. T and _{5a and is in a fairly good coincidence}

with the observed one. _{The steady turning characteristics is,}

of course, able to Lake into _{account by this equation too.} Motora et al. presented several models of _{unusual phenomena7} and the model "system with _{abnormal moment around the origin"}

1L HYDRODYNAMIC FORCES AND FLOW FIELD AROUND STERN

It is doubtless that the additional hydrodyr'ìarnic _{force exists}

as well as usual hydrodyriamic forces, as explained before.

In this section, the results of Planar Motion Mechanism _(PMM) tests and of stern flow observation _{are presented.}

Fig. 7 shows the results of oblique towing _{tests. In the} figure Y'Y/(p/2)LdV2 and N'=N/(p/2)L2dV2 _{are nondimensional}

lateral force and moment around the center of gravity _{respectively.} From the results the stepwise alteration of hydrodynamic _forces can be observed about at -8°, +1° and +8° of drift angle and this fact verifies the idea mentioned in the first paragraph of this section and also corresponds to the results of _free

_running

tests dealt in the former section.

Are hydrodynamic forces acting on the hull affected _{by the} past history of the ship motion? _{Sometimes yes, and sometimes}

no. _{To certify the question in this case, a kind of tests in}

which drift angle is very slowly (w0.5dog/sec) _{changed by PMM}

is carried out. Fig. 8 is the results of the tests. The broken

lines in the figure represent the results of oblique _towing tests and the arrows attaching on the full lines _{mean the} direction of the alteration of drift angle respectively.

Within 100 of dr ft angle, it

is obvious that two

_hydrodynamic

forces work on the hull. _{Bccduse of the initial condition of}

setting or the different provability of _{occurence of two values,} in the oblique towing tests, both values do not appear at the

same drift angle. If the drift angle is over l00,l20, only one

of them acts.

Concerning to the working position of the additional _hydrodynamic force, which does the force act on, the hull _{or the rudder? To} certify it, the nominal rudder force is also measured in the tests, and it enables to separate the lateral force _{and moment} acting on the whole ship into two parts each; on the hull and on

the rudder. _{After this separation, it is 1ernt that to a large}

extent the additional force works on the hull, though _{to a little}

extent it does on trie rudder, and the apparent point of acting is found around A.P.

The flow field around the stern is observed _{to investigate} the mechanism of generation of the additional _{force. The flow}

surface and auxiliarily by tufts setting on the hull surface.

Owilig to the buoyancy, the tracer bubbles are apt to float.

This fault can be covered up, when Ernaller bubbles are used in

higher towing speed. This method still lias many merits such

that it is easy to make and to keep the tank clean.

It is verified from the observation of the flow field that

the unsymmetrical separation obviously occurs in the stern of

this model. In the case of a usual slender ship the face side

flow and the back side flow join at the center line of the stern

and pass backward by the action of the propeller through the rudcter. But in this ship, to some extent, flow crosses from

starhoard to port or vice versa over the rudder and the propeller.

Owing to the crossing flow, the flow pattern differs between the

starboard. side and the port side. In one side, it flows smoothly,

hut in the othsr side, a sep.-rated flow accompanied with vortices

is produced.

Explaining about the flow pattern versus the dif t ng1e more

in detail, an exarnDle that the drift angle increases slowly

from starboard 4° is quoted. Until the drift angle increases

about 9°, a large separation is observed around the port-side

stern (face side), when, in the clearance above the propeller

and the rudder, the crossing flow from starboard (back side) to

port (face side) is produced. But if the drift angle reaches

near lO°'l2°, the direction of the crossing flew suddenly alters and the flow passes through the clearance from port to starboard. This abrupt change of flow pattern does occur only near the

critical drift angle. Of course, at the same time when the

sudden alternation of flow happens, the hydrodynamic forces acting on the ship vary stepwise as shown in Fig. 8.

Considering the outstanding fatness around the s.s. 1/2 at 2

m witer une in Fig. 1, the lines of this ship, it may be

possible that the flow around this part separates. Separation

itself, if it occurs symmetrically in both sides of a hull,

does not produce any lateral force on a hull and has almost nothing to do with manoeuvrability.

ifl

this ship, however, the

crossing flow through the aperture induces rather larger

unsymmetrical separation, which is the problem. In spite of

some change in drift annie, this unsymmetrical separation and

vary.

Besides, it is observed that by steering or by the propeller loading, the sudden alternation of the crossing flow is

influenced better or worse, which is to be investigated but is

not done now, because of the urgent purpose of this research.

5. EFFECT OF CENTER FIN AS COUNTERMEASURE

Through the results of the several tests mentioned above, the

countermeasure to be took is almost coming up. The fundamental

measure is to alter the fat stern or frame lines, but it is more actual to equip a proper fin on the hull without changing

the hull itself.

The center fin shutting the crossing flow through the clearance above the propeller and the rudder is to he applied. By this

fin the unsymmetrical separation, at least, would fade away or

be weakened. Fig. 9 is an example of the center fin.

The results of oblique towing tests and the observation of flow field after fitting the fin on the model are successfull

as are expected: the unsyrtunetrical separation disappears and the forces acting on the model settle along one line as in

Fig. 10 respectively. The broken lines in Fig. 10 are the more

realizable part of the original model characteristics in

non-dimension. If the large fin (FIN I) shutting the clearance

over the center .lin almost completely to th stern is equiped,

the turning characteristic curve draws nearly a line.

The shutting area of

FIN II and FIN III

is the same, though the part of the fin above the rudder is attached on the rudder

(enlarged :udder) in

FIN II

and to the hull in FIN III. IrL the

case of FIN III, although within the small range of rudder

angle a little disturbance is left, it is sufficiently effective from the actual. point of view.

FIN IV and

FIN V

are lested to check which part of the fin has more essencial effect, above the propeller or the rudder.

Tn consequence, it is found that both parts rhould he shut and

that the part above the propeller is more important to improve the characteristics.

In consideration of the above results, FIN III is decided to

apply to the ship. To confirm the increase in resistance by

the same with that of the model without the fin. _{The photo of}

the fin attaching on the ship is shown in Fig. 11.

The sea trial of the ship with FIN III is done in _{the condition} of Table i and the result of turning tests is shown _{in Fig. 12} compìring with that of the ship without the fin. _{The result is}

truly satisfactory and the scattering within 10 _{degrees disappears.} In fact, the captain and the officers express their appreciations that they can at last navigate her without _{any anxiety. The}

little scattering of the values within the small _{range of rudder} angle is about the same with that of the model with _{the fin, and} the results of Z tests by usual K-T analysis are not different with those of normal stable ships.

As the fin attaching near the propeJler tips sometimes accompanies a severe hull vibration, several attenion as

possible are paid for the decision of the sectional form _{of the} fin and the detail works at the crmstruction. _{Fortunately, the} vibration is almost the same as that of the snip before _attaching the fin.

6, CONCLUSION

By several kinds of experiments, the strange behaviour

so-called unusual phenomenon in manoeuvrability _{of a fishery} research vessel, sepecial]y the cause of the 'unsteadiness" _is

investigated and verified. _{Besides, the countermeasure to be}

applied to the ship is certified from the model _{tests, and is,}

in fact, applied successfully.

The major conclusions to be announced are as follows:

The strange behaviour of this ship in rnanoeuvrability can

be explained by the existence of the additional _hydrodynamic force, which is almost constant against _{the yaw rate.}

This additional force is accompanied and influenced _{by the} unsymmetrical separation of flow induced by the crossing _flow through a rather large clearance above a rudder and a propeller

in a fat stern.

For this type of ships, the center fin liKe as applied in

this ship is quite effective.

As have heard or seen, this kind of "unsteadiness' _{in manoeuvre} is often observed in fishery boats, and they are hoped to recover their natural performance by the mentioned _{countermeasure.}

The correlation with unusual phenomenon appeared in large

full-bodied ships (one example of it is now prepared to report)

the relationship with the unstable phenomenon of thrust in the field of resistance and propulsion etc are to be studied further

in the future.

The authors would express their sincere acknowledgements to those who have collaborated with them in various stages of this work.

RErERENCE

K. Nomoto: Unusual Scale Effect on Manoeuvrability of Ships

with Blunt Bodies, 11th ITTC, Tokyo (1966)

T. Koyarna: On Unusual Phenomenon Appeared on a Tuna Fishing Boat, presented at 2nd Committee of JTTC (1968)

The Society of Naval Archiects of Japan; Proceedings of the

2nd Symposium on Ship Manoeuvrability, Tokyo (1970)

K. Kose, S. Matsui, K. Kawasumi, M. Nakato & Y. Yamasaki: Study on the Unusual Maneuvering Characteristics of Full Ships, Transtions of the West-Japan Society of Naval Architects,

No.54 (1977)

II. Tagano & S. Asai: On the Unusual Phenomena in Manoeuvring

Motions of a Full Ship Model, MiLsubishi Technical Bulletin,

No.116 (1976)

K. Nomoto, K. Kose & Y. Yoshimura: A New Procedure of Analysing

Zig-zag Test, Joi.irnai of the Society of Naval Architects of Japan, Vol.134 (1973)

S. Motore, M. Takagi, A. Kokumai, H. Kato & T. Koyama: An Analysis of the Maneuvrability of a Ship Associated with Unusual Characteristics under Steerage, Journal of the Society of Naval

Table i Principal par iculars of ship and model *) deíigned I T t -30 -20 -10 '1 -0.5

/

0/

0 20 20 S(deg) o i: aryirg therare

-1.0--Fig. 2 R*suit of turning te:t (ship)

'6 o : steady changed from low&r io upper changed from upper to tcwe r' g(deg) (10) characteristics /:wtching line ? _{of ship} switching tine o model Fg. 5 Skrtch of li'rr.ing charac;eria'ics

and switching line'

00261. rnnl with fin , n D (n 27.Oxí.00n3.00 i.7.6oO.4 (n 2.3 2.20 0.2660 dt (r.( 2.71 LOi 0.2268 (ml 3.70 3.30 0.0(68 Cn C.638' 0.039 O (tOn) 272.60 0.26615 in/iA 1/27.9 2/27.9 V (n/s) 5.64' lOt, Di,nt,-r no 1600 (60 :z o i (nmqlm) non 960 (15') rp(n 76 10 (0n) 370 690 2.0

-1(degIsec) 1.0 o tUf20\, 30 ¿.0 60 \Jtsec) -2.0 . SrO.2deg

Fig. 3 An example from tul-ning jest (original ship)

8F iz i /2

-r3-8

Fig. 4 Result of turning tcsts (nod)

Fig. ! Body plan and profile of ship

JJ.-L._J L. .-'--

4-4f-2

(deglsec2)

-10--()

0.4 -25 -20 15 -0.2 0. 2 0.2 - C. 4 Y.

-0.

Oi

L

_/ 5

i

r

I

t L!

1J

-15 -10 5 10 15

L

p(deg) :obitowing -0.1!--

tests

-->: varynci

tests

t I J 5 10 15 20 25 (deg)

4/

Çí) .cL _.À' -L -10 -ip (b) . (c)

Fig. 6 --ç pha'e planes of zig-zag tests (model)

I t

-25 -20 -15

-0.0J

Fig. 8 Results el 5-varying tests (model)

0.05 0.05

L,

L

1 -15

-i

10 15 20 25 (dea) 5 10 15 (deg) --- :obLtowing

tests

r-

-:-varyin

tests

_005L

Fig. 7 Reslutto of cdique towinq tests (model)

t I

-4 -2

t 0.4 0.2 -0.2 y, I- I I t J r J -25 -20 -15 5 10 15 20 25 (3(deg) _04L

Fir. 9 Fin roflle ond rrsults of oblique towing Ics's (model)

fr,P 4/2 W.L...

BL434;

LOIri=LIR 1.0 ci,'

o

L_.JJ__L, ')

-403D-20 ,'o' 10 20-20

Is. ,6 / 6 _A 9' -0.5

/ -0.5

-0.5 4' _I' -1.0- ° --10

i)

Fig. 11 Fin profile of ship

AP t/Z Ar 4/2 51' çtr' (Y (79' 0.5-

0.51-1

O,'

'o'

o'

,0,

_{I__L,} /.L_J

i

I 10 20-20 _,6 10 20-20 , 10 20 3Cì 40 &(deg) .P C2 s.

f,'

-0.5'-/ /

o,

_/

/ AP l/t 1.0

Fig. 12 Resub of turning te-as

(ship with fin)

(12) °-5r cf

,f

'--y L__L._L__.L.. rti-' I I j

j

i

-40 -30-20 _{t2' 10 20 30 40} s.(deg) -1.0

FIN I FIN FIN 1 FIN

Fig. lo in profiles and each result of turning tests (model)

101r'L/R ,,'

-- - :orianaI - - - :origir.t 9, 0.5 0.5 r r