Model tests with 3 bottom trawl otterboards

(1)

l.b. y.

Scheepbouwkundt

Technische

1-logeschool

Reprinted from Norwegian Fishing and Maritime

NewP-l963.

NORWEGIAN SHIP MODEL EXPERIMENT TANK

THE TECHNICAL UNIVERSITY OF NORWAY

MODEL TESTS WITH

3 BOTTOM TRAWL

OTTERBOARDS

BY

H. AA. WALDERHAUG

A. AKRE

NOR\vI:(;IAN SHIP MODEL EXPERIMENT TANK PUBLICATION NO. 69

(2)

LIST OF CONTENTS

Page

Abstract i

Definitions I

Scale effects 2

Free stream tests 3

Bottom tests 4

Comments to the test results .. 5

(3)

.9-MODEL TESTS WITH 3 BOTTOM TRAWL OTTERBOARDS

By H. AA. WALDERHAUG & A. ÄKRE

NORWEGIAN SHIP MODEL EXPERIMENT TANK - THE TECHNICAL UNIVERSITY OF NORWAY ABSTRACT

In this report are described some results of otter-board hydrodynamical tests carried out at the

Nor-wegian Ship Model Experiment Tank and sponsored by the Norwegian working group for development of one-boat pelagic trawl (APE). The test results were

presented at the first meeting of the International

group for pelagic fishing methods and gear (IF), the

Hague, 6. to 8. November 1962.

DEFINITIONS

The otterboards tested are all designed for bottom trawis, and following notation is used:

Rectangular otterboard of orthodox design (Fig. 1).

Oval otterhoard (Matrossow board) with one

vertical slot at the centre of the board (Fig. 2).

Oval otterboard with 3 slots (Fig. 3).

B 2 a: As B 2, but with 2 slots, one at the centre

and one at the leading edge (Fig. 4).

Section A-A Suct,on side. L31Qa n

4-Fig. 1. Otter Board B 1 (Rectangular otter board).

Weight: 1100 kilogram. Area: 456 m2. tu: 0,0355. Repnolds' number at towing speed 3,5 knot: 5,5 X 10°.

B 2 b: As B 2, but with a streamlined leading edge,

NACA wing section (Fig. 4).

B 2 c: As B 2, but without slot.

B I a and B 2 d: As B i and B a respectively, but

without fittings such as brackets, strengthening strips

and -edges, hooks, links for back strops, bolts, pints etc.

B 3 a, b, c: As B 3, but with slot angles equal to

40° 45° and 50° respectively.

The slot angle for B 3 is 42°, where the slot angle

is defined as the angle between the after slot corde

line and the otterboard corde line (Fig. 3).

The following definitions were made in order to

relate otterboard model tests to tests with the

com-plete trawl gear:

The tangent plane is defined as the piane tangent to the pressure side of the otterboard. Some examples of tangent planes are shown with dotted lines in the

sketch. For the otterboards described in this report

the tangent planes and pressure sides are synonymous.

Fig. 2. Otter Board B 2. Ovale otter board with one

slots. Weight: 950 kilogram. Area: 4.55 m2. tu: 0.055.5. Repnolds' number at towing speed 3.5 knot: 5,56 X 10°

(4)

The reference plane is defined as a vertical plane

parallel to the speed direction (speed vector).

During the tests described in this report the angle

of inclination and the angle of tilt, were both zero,

so that the tangent plane was always vertical.

The angle of attack, a is measured in a plane

through the speed vector and perpendicular to the

tangent plane.

The following nondimensional coefficients are used:

CL =

'/2p AV2

CD D dragcoefficient

p AV2

LID lift/drag ratio

where L = lift force in the plane of the agnle of

attack and perpendicular to the speed vector.

L

Suction Side.

= liftcoeffieient

I. 2OSu

Fig. 3. Otter Board B 3. Ovale otter board with three

slots. Weight: 290 kilogram. Area: 2035 m2. t/l: 0.0498.

Reynolds number at towing speed 3.5 knot 3.6 X 10'. 4

29p

3t20

btte,- board 02 a.

Fig. 4. Otter Board B 2 a, B 2 b. Modifications of

otter board B 2.

During the tests described in this report, the lift and spread forces are syllonymous.

D = drag force in the speed direction.

p = mass density

A = projected area of otterboard in the tangent

plane.

V = velocity.

Reynolds' number is defined as

Vc

Re =

V

where

c = length of otterboard (corde)

= kinematic viscosity.

SCALE EFFECTS - MODEL SIZE

The first problem in model testing is to find a rela-tion between model and prototype as regards forces

and moments. The forces involved when an

otter-board is towed deeply submerged in water, are fric-tional forces and inertia forces, and the streamlines around the model and prototype are geometrically si-Otter board 2 b

(5)

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Diagram no. 1. Freestream tests, otter board B I and

B 2. Force coefficient as function of Reijnolds' number.

milar if Reynolds' number is the same for model and prototype. This can not easily be realized in a model tank, but fortunately the force- and moment coeff

i-cienta are practically

in-dependent of Reynolds'

number 'variations for Re

larger than 0.5 X 10° i X 10°. As model scale

for B i and B 2 were

chosen i : 5, and for B 3, which is a smaller

otter-board, were chosen the

scale i 4. With these

models the critical

Rey-nolds' number mentioned

above, are reached in

the model tank.

Diagram no. 2.

Free-stream tests, otter board B 1, B 2 and B 3. Force coefficient and liftdrag

iatio of otter board as

function of angle of

attack.

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1 REE STREAM (OPEN WATER) TESTS

In Diagram i are shown the force coefficients of otter.boards B i and B 2 as a function of Reynolds'

number. B 2 is tested in this series at two angles of attack, i.e. 28° and 38°, and for this board the force coefficients will be approximately independent of Re for Re larger than 0.65 X 106. The critical Re for the

rectangular otterboard B i is lower, approximately

0.5 X 106. The higher critical speed of B 2 than that

of B i may be due to the shape of the otterboard as well as the effect of the slot.

Based on this test series and on results of airfoil

tests (see f. ex. the British Shipbuilding Research

Association, Reports No. 70 and 142) it is expected that full scale behaviour of otterhoard in open water

may be predicted from model test carried out at a

Re of at least 0.65 X 106.

The force coefficients and lift/drag ratios as a

func-tion of angle of attack is given in Diagrams 2, 3 and 4.

Only the angles of attack between 20° and 45° have

been considered of practical interest; however, the efficiency or lift/drag ratio of the otterboards will

have a maximum at an angle of attack less than 20°. It may be observed from the curves that otterboard

B 3 has the highest L/D ratio, whereas B 2 has the

highest lift-coefficient.

Concerning the modified otterboards, it is shown in Diagram 3 that B 2 a as well as B 2 b has a 12-13 %

higher lift/drag ratio than B 2.

In Diagram 4 is shown the effect of varying the

slot angle for B 3. The effect is quite marked with a maximum lift/drag ratio at a slot angle between

42° and 45°.

Results of the open water streamline tests are

4.5

(6)

Diagram no. 3.

Freestream tests otter board B 2. Force coeffi-cients and lift-drag ratio

of otter board as func-tion of angle of attacc.

shown in photograph no. i and 2. The

3-dimensio-nal flow at the top and

bottom of the otterboard

is easily observed. This

effect

will reduce the

lift considerably

compa-red with 2-dimensional

lift.

BOTTOM TESTS

During these tests the otterboards were towed

very close to the bottom (maximum distance about 5 mm) but without actually touching the bottom. Mechanical

fric-tion was therefore not involved, and the law

of similarity between model and prototype mentioned earlier is still valid.

In Diagram 5 is shown the effect of Reynolds'

num-ber variations on the force coefficients.

It will be

observed that the critical Re are higher than those

for the open water tests. This effect is more

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ex-pected since B i has a horizontal bottom line. The

critical Re for B i and B 2 will be approximately

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The effect of variations in angle of attack is shown in the Diagrams 6, 7, 8, 9 and 10. It will be observed

that the maximum lift

for the three parent

at-terboards (Diagram 6)

will be reached

at a

smaller angle of attack

than during the open

water tests. This effect

might have been expect-ed since stalLing is

de-layad by 3-dimensional

effects.

As for the open water

tests the maximum

Lift-drag ratio will occur at an angle of attack less

than 20°, and the

rectan-gular otterboard is still

inferior to the other two. Streamlining the

lead-ing edge of the

otter-board will increase the

Freestream tests, otter

board B 3. Force

coeffi-cients and lift-drag

ratio of otter board as

function of angle of

(7)

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Diagram no. 5. Bottom test, otter board B 1, B 2.B 2. Force coefficient of otter board as

function of RelJnolds' number.

Et-drag ratio about 6 í at the stalling point

(Dia-gram 8). Further the stalling angle is increased about 4.

The effect of the slot in B 2 is comparatively small as shown in Diagram 9. At the point of maximum

lift, the

slot seems to increase the lift/drag ratio

about 5 %.

Removing the otterboard fittings has a very marked effect on the lift characteristics as shown in Diagrams 7 and 10. The lift is increased by 10 % - 15 %, and

the lift/drag ratio is increased by 5 % - 7 96.

Fur-ther the stalling angle is reduced 3° for B 2. The

in-crease in drag may be the net result of a reduced

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Diagram no. 7. Bottom tests, otter board B 1. Force coefficient and lift-drag ratio of otter board as function of angle of attack. frictional resistance and an increased induced

resi-stance.

The streamlines for the otterboards at the bottom are shown in the Photos 3-10. From the pressure side streamlines of B i (Photo no. 4) it will be noticed that the flow at the keel of the board has more

2-dimen-sional character than at the top. This will explain

the increased lift/drag ratio during the bottom tests. At 28° angle of attack this increase amounts to about

6 %. The flow at the suction side is very confused due to eddies being developed here.

The difference in open water and bottom flow for B 2 may be observed from the Photos 1, 2 and 5, 6.

It is interesting to note that despite the slot effect,

there is reversed flow at the suction side of the keel of the board during bottom tests. However, these tests were run at the stalling angle, and large eddies may be expected at this point.

From Photos no. 7 and 8 it is observed that closing the slot of B 2 results in increased eddy formation at the suction side. The flow at the suction

side of B 3 seems to be quite stable (Photo

9 and 10). However, this test was run at an angle of attack of 29° compared with the stalling angle 36°.

COMMENTS TO THE TEST RESULTS

Let us first consider Diagram 2. It may

be observed from these curves, valid in open water, that otterboard B 3 has the highest L/D ratio, whereas B 2 has the

highest lift-coefficient. Which is the best

otterboard it not obvious. However, we

Diagram no. 6. Bottom test, otter board

B 1, B 2 and B 3. Force coefficients and lift-drag ratio of otter board as function

of angle of attack.

30 35 'ç

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may compare the boards on a basis of

constant area or constant lift-coefficient when the speed is regarded as constant.

In that case we draw a horizontal line through f. ex. the lift-coefficient of B 2 at a = 300. Where this line crosses the

CL curves of B 3 and B i we proceed ver-tically downwards to the respective L/D

ratio curves. We then f:ind that B 2 is approximately 9 % better than B 3 and

approximately 40 % better than B 1. The corresponding angle of attack of B 3 and

B i

is approximately 350 and 36.5° re-spectively.

We may also compare B 2 and B 3 on a

basis of constant angle of attack, f.ex. 30°.

If the two bards shall have the same lift at this angle of attack, we find by oonsi-dering the definition of lift-coefficients, that the areas of the two boards must be

inversely proportional to the

lift-coeffi-cient, i.e.

A3 CL2

A2 _CL

At a =

30° _{the area of} B 3 must be approximately 12 % greater than the area of B 2, but at the same time its LID ratio is approximately 7 % better. During this comparison the speed is of course kept

constant.

Proceeding to Diagram 3, we find that based on constant iift-coefficient (or

con-stant area) the otterboard B 2 a with a

leading edge slot lis approximately 25 %

better than B 2 at a

30°. Comparing

B 2 a with the rectangular otterboard B i of Diagram 2 we find that on a constant :area basis, the board B 2 a at a = 30° is

approximately 80 % better than B 1. The

corresponding angle of attack of

B i

is

approximately 39°.

In connection with Diagram 6 it may be

observed that the maximum

lift-coeffici-ents are smaller than those of the open water tests, and further that maximum

lift is reached at a smaller angle of attack. Both these results are in agreement with

the observations of Dickson on tests by

Gawn and Yakovlev.

Comparing with Diagram 2 we find that

at an angle of attack of 30°, the LID ratio

of B 2 is slightly better under bottom

con-ditions than under open water condition. The same CL can of course be obtained at

a lower angle of attack, i.e. 27°, and at

this point the LID ratio

is still much

'better.

20 25 30 ' 55 40 ' 45

Diagram no. 8. Bottom tests, otter board B 2. Force coefficient

and lift-drag ratio of otter board as function of angle of attack.

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(9)

B 2 e. STREAMLINES IN OPEN WATER. a = 28°. Re = 0.81 X 10°

Photo no. 1. Suction side.

B 1. STREAMLINES AT BOTTOM. a =

0°. Re = 0.805 X i0.

B 2. STREAMLINES AT BOTTOM. a = 28° Re 0.81 X 10°.

Photo no. 2. Pressure side.

Photo no. 5. Pressure side.

(10)

STREAMLINES AT BOTTOM. a = 28°. Re = 0.81 )< 1O.

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Photo no. IO. Pressure side. Photo no. 7. Suction side. Photo no. 8. Presszre side.

(11)