Further experiments with bulbous bows

(1)

MEDDELANDEN

FRAN

STATENS

_{SKEPPSPROVNINGSANSTALT}

(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING _{EXPERIMENTAL TANK)}

Nr 8 _GOTEBORG

1948

FURTHER

EXPERIMENTS WITH

_BULBOUS

BOWS

BY

ANDERS LINDBLAD

GUMPERTS AB GOTEBORG

(2)

GOTEBORG 1948

(3)

Synopsis.

This paper gives an account of some experiments which have been carriedout with

a group of models fitted with bulbous bows of moderate sizes. The models have

dimensions and fullness corresponding to a modern passenger ship or a high speed cargo liner.

The models of one of the series have been fitted with bulbous bows of the usual construction with a vertical stem, and on the other series the lower part of the bulb has been extended forward of the perpendicular to form a »ram bow».

Introduction

A considerable number of passenger ships and high _{speed cargo} liners now run at high speeds at which it seems possible that a large reduction bf the resistance might be obtained by the adoption of a bulbous bow. _{It is well known that for several ship projects} _some model experiments have recently been conducted with various types of bulb cohstructions. Previous investigations, notably _{those made} by TAYLOR, have shown that the best results have been _obtained with very large bulb areas, but for various _{reasons there are strong} objections to the use of such large bulb areas on merchant ships. In a few cases, however, bulbous bows with fairly 'large _{areas have}

been adopted also on merchant ships. In the present paper an

account is given of some series of model experiments in which the models have been fitted with bulbs having an area between 5 and 10 per cent of the .midship area. _{It is believed that such moderate} bulb areas might now be accepted by owners and builders of modern ships.

The present investigation is a continuation of the research described

inthepaper»Experiments With Bulbous Bows»,which

appears as No. 3in The Publications of The Swedish

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4 MEDDELANDE FRAN STATENS SKEPPSPROVNINGSANSTALT NB 8

Description of the Models

The dimensions and the fullness which have been chosen for the modeLs correspond to ships with a speed of about 18 to 20 knots; that is at speed length ratios between 0.9 and 1.0. Our experiments, however, have been carried out even to somewhat higher speeds.

The particulars of the models, as given for a 400 foot ship, are as

follows:

Length B. P 400 ft.

=

121.92 m

Length L. W. L.

412 ft. 54 in. =

125.41 m

Breadth 56 ft. -= 17.069 m

Draught at L. W L 21 ft. 61/9 in. = 6.565 m

Draught, light 18 ft. 8 in.

=

5.689 m

Block coefficient at L. W. L. ... .. . 6 --- 0.60

Midship section coefficient /3 = 0.974

Radius of bilge 6 ft. 6 in. =- 1.980 m

Rise of floors 6 in.

=

0.152 m

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A. LINDBLAD, FURTHER EXPERIMENTS _{WITH BULBOUS BOWS} ₅

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MEDDELANDE FRAN STATENS SERPPSPROVNINGSANSTALT NR 8

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Full particulars of the models are given in Table I.

A cruiser stern of moderate size has been used Its length is 2.65 per cent of the length between perpendiculars and its depth is 0.25

of the draught. The after body and the cruiser stern have been

constructed to snit a twin screw arrangement.

Figs. 1 and 1 A give the bcidy plans, and Fig. 2 shows the profiles

of the stern and stem. The sectional area curves for the variouS

models are shown in Fig. 3, and the corresponding offsets are given in Table II The load water lines are shown in Fig. 4.

The after body has been keptunchanged in all the models. It has a prismatic coefficient of 0.633. As Fig. 4 shows, the load water line has been Made rather full at the stern. This has been done in order to providC sufficient stability:

The models were fitted with streamlined rudders but had no other appendages.

The resistance tests Were carried out at two draughts. The load draught of 21 ft. 61/2 in gives a beam draught ratio of 2.6, and the light draught of 18 ft. 8 in. gives a beam draught ratio of 3.0.

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A. LENTUBLAD, FURTHER EXPERIMENTS WITH BULBOUS BOWS

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A. LINDBLAD, FURTHER EXPERIMENTS WITH BULBOUS BOWS 9 Construction of the bulb.

Although larger bulb areas would undoubtedly be advantageous, it was decided to use a maximum bulb area of 10 %. Fig. 1 shows

this bulb. On later models the bulb was reduced; to 7.5 per cent

on model 3 and to 5 per cent on model 5. Fig. 1 A gives the body plan of the fore body with the 5 (:)/0 bulb area. The sectional area curve forward was gradually sharpened to suit the smaller bulb areas, and the displacement was slightly reduced as Table I shows. The experiments of TAYLOR and of BRAGG have shown that a tangent value t = 0.65 would be suitable, and this t-value was used in both series. It is known from WIGLEY 's work that in several cases fairly large reductions of the resistance have been obtained by projecting the lower part of the bulb to form a »ram bow*. There are, of course, some practical difficulties connected with the use of large protruding rams, but it seems possible that shipbuilders might be willing to approve of a small ram. We have therefore also tested one series

of models - Series B - constructed with a ram bow of moderate size. In this series the forward ends of the water lines are rounded

off with half circles to form part of a rotational body. The stem

profile will thus have an outline of the same form as the half of the 'frame section at the forward perpendicular. Fig. 16 shows the streamlines around model 1 (10 % bulb with ram bow) at Bld = 2.6 and a speed length ratio of 0.95

Figs. 14, 15, and 17 show some photographs taken during and after the streamline test.

Table II.

Offse/s for .Sec/,ono/ /Freer Curves Fore body Sec/ion /0 // /2 /3 /4 /5 /6 /7 /9 . /9 /916 20 1 _{& ®} 46/ 26 3o /00 /00 995 394 96.5 957 8.97 88.6 78.4 ' 76.8 63.o 6/3 469 4376 332 32s 22.e 22.9 /5.3 /6./ 125 /3.3 /0.0 //.s

0

a® 2.6 3o /00 100 995 99.4 96.5 95.7 897 886 784 768 63o 613 46.9 456 324 31.e 20.9 210 /2.7 /3.5 .96 /01 75 8.6 C) & C) 26 3o /00 /00 99.5 99.4 96.5 957 897 886 784 76e 63o 613 46.9 456 -3/6 31./ /93 /9.3 /03 /0.9 72 8.0 So 58 A Herbody 5edio 17 0 ffr I 2 .3 7 ô 9 /0 A 7 // Plocleis a/d 2.6. 3.0 46 2.7 .94 6.4 /4-4 /05 273 226 4/.9 36.9 56.4 52.0 699 66.6 81/ 78e 895 88.5 95e 952 98.6 98.5 _ . /00 /00

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10 MEDDELANDE FRIN STATENS SKEPFSPROVNFNGSANSTALT NR .8 28 7 . grrfkiertce lefrrfica/ Sierns

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Series A with Straight Stem

The particulars of the three models in this series are given in

Table I.

Fig. 3 gives the sectional area curves. The water lines are shown

in Fig. 4. They are as mentioned above rounded off with

half circles and terminate on a vertical line through the forward

perpendicular.

The results of the resistance tests are presented as C) curves in Figs. 5 and 6. They are also given in Table III as C) values for a 400 foot ship, and the speeds when given in knots always refer to this length of ship.

The results o/ the tests at the beam draught ratio of 2.6 (Draught of 21 ft. 61/2 in.)

At low speeds up to 16 knots model 6 with 5 % bulb area gives

the smallest resistance. This is only slightly smaller than that of model 4 which has a bulb of 7.5 %. From 16 to 18.6 knots this

latter model is superior. At higher speeds again model 2, with 10 % bulb area, shows the best results.

It i

about 3 per cent better than model 4.

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A. LINDBLAD, FURTHER EXPERIMENTS WITH BULBOUS BOWS 11 Vet-lira/ Stems

Jrilkence of 6u/6oreo

070

Fig. 6.

At all low speeds the large bulb of model*, with 10 % area, causes a very large resistance. At 15 knots speed it gives 10 per cent and at 16 knots 5.5 per cent larger resistance than model 4 which has 7.5 % bulb area. These results are in line with previous

investiga-tions which have shown that large bulbs are generally very un-economical at low speeds.

The results of the tests at the beam draught ratio of 3.0 (Draught of 18 ft. 8 in.)

The results are shown in Fig. 6. Only two of the models 4 and 6, were tested at the smaller draught. It is to be noticed, that at this

draught the bulb area is a larger percentage of the midship section. Model 4 has a bulb area which is now 8.6 % of the midship area, and

model 6 has an area of 5.8 %.

TAYLOR'S experiments have shown that, in general, the larger bulb areas are most effective at the larger beam draught ratios. This is also the case in the present series.

Even at this draught the smaller bulb at low speeds givesa better result than the larger bulb. Thus model 6 with a 5.8 % bulb is the best up to a speed of 16.4 knots. But from this speed _{on model 4,} with an 8.6 % bulb, gives the smaller resistance.

At the lower speeds the difference is rather small, but at 18 knots there is a gain of 4.3 per cent in favor of the 8.6 % bulb area, and this gain is increased to 4.8 per cent at the speed of 20 knots.

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12 MEDDELANDE FRAN STATEN§ SKEPPSPROVNINGSANSTALT NR 8 Fig. S. rroruairy au/ as

_.

--'. _le _ . Influence of bu/borea . 1

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A. LINDBLAD, FURTHER EXPERIMENTS WITH BULBOUS BOWS 13

Series B with Ram Bow

As has previously been mentioned, the models of this series have all been fitted with a small ram. The profiles of the rams are shown in Fig. 2. It is to be noticed that the rams project forward of the

perpendicular in proportion to the area of the bulb, and that the

profile has exactly the same shape as the frame section of the bulb. The three models in this series have the same bulb areas as in series A, namely 5 %, 7.5 % and 10 % respectively. The sectional area curves are shown in Fig. 3 and the load water lines in Fig. 4. The results from the tests with series B are given in Figs. 7 and 8. A comparison between the results of series A and B can be made from Figs. 9 and 10. In Figs. 11, 12 and 13 the results of the effect of the ram bow have been given for each size of the bulb. Table III gives C) values for all the tests.

The results of the tests at the beam draught ratio of 2.6.

Fig. 7 plainly shows the same tendencies as have previously been noted in series A; namely that large bull; areas are unfavorable at low speeds but advantageous at high speeds.

Up to a speed of about 18 knots there is not much difference

between the 7.5 % bulb and the 5 % bulb, but above this speed the 7.5 % bulb is evidently superior. At 19 knots the gain is 2 per cent and at 20 knots it is 4.2 per cent.

The superiority of the 10 % bulb of model 1 stands out very

clearly. At 19 knots it is 7.2 per cent better than the 5 % bul(

and at 20 knots 8.2 per cent better.

The results of the tests at the beam draught ratio of 3.0. The results from these tests are given in Fig. 8.

As in series A it should be kept in mind that at this draught the bulb areas in proportion to the midship area have been increased from 5 % to 5.8 %, from 7.5 % to 8.6 % and from 10 % to 11.5 %.

Fig. 8 shows that even at this beam draught ratio large bulbareas cause large resistance at low speeds and low resistance at high speeds. The C) curve of model 1 shows a remarkable low resistance for all speeds above 17.5 knots for the 11.5 % bulb.

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14 MEDDELANDE FRAN STATENS SKEPPSPROVNINGSANSTALT NR S

Comparison of tIoefels ale 236r.Z6

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(15)

Table III. .

Os-values 6r a

406' ..517/;o

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node/ bab area.,

_17i/r1;,.

No.

Z1, .70 .75 .00 .85 .90 .95

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4

75 447 .658 .650 .667 .710 .636 .925 .96/ /00o

5

So .652 .659 £57 .666 .74o .858 .950 .992 /052 6 50 452 .649 .654 .674 .75-0 _{.908 .98/ /022 /057}

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1/5 .708 .709 .6931.6841.6941. 782 .737 .008 .875 2 Mg

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4

8.6 .678 .652 .662 .647 .694 .77o .827 .86/ .903

5

se _{658 .655 .658 .6.55 .7/4 .803 .852 .892 .348} G 58 _{.668 .662 457 .662 .72/ .804 .867} _.910 _.952

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16 MEDDELANDE FRAN STATENS SKEPPSPROVNINGSANSTALT NR 8 9 8 7 6 0 i

.inguenoe of RO1'77 BOW

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Comparison between Bulbs with Vertical Stem and

with Ram Bow

Fig. 11 shows the results for the 5 % and 5.8 % bulb. At the B/d of 2.6 there is a rather large reduction of the resistance due

to the ram bow. At 19 knots the reduction amounts to 5.5 per cent

and at 20 knots to 3.2 per cent. At the Bid of 3.0 the advantage of the ram is not so apparent. It only amounts to 1.8 per cent at

the speed of 20 knots.

Fig. 12 gives the C) curves for the 7.5 % and the 8.6 % bulb. At both the B/d ratios the ram bow causes a considerably larger resistance than the vertical stem at all speeds below 19 knots, but at higher speeds there is some gain due to the ram. At the Bid of 2.6 there is a reduction in resistance of 1.5 per cent at 20 knots and of 1.6 per cent at 21 knots, and at the B/d of 3.0 the reduction is 2.1 per cent at 20 and 21 knots.

Fig. 13 shows the resistance for the 10 % and 11.5 % bulb. At all speeds above 19 knots there is at the Bid of 2.6 some reduction

of the resistance. It amounts to 2.8 per cent at 19 knots and 2.2 per cent at 20 knots. At the Bid of 3.0 we can make no direct

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eo

Influence of Rom Bow

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1 8 MEDDELANDE FRAN STATENS SKEPPSPROVNINGSAN8TALT NR 8 1 Fig. 14. /4

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(20)

20 MEDDELANDE FRAN STATENS SKEPPSPROVNINGSANSTALT NR 8 not tested at the draught of 18 ft. 8 in. We may note, however,

that model 1, with a ram bow, gives a remarkably low resistance. This is particularly apparent from a comparison of the C) curves in Fig. 10.

Summary of the results

1'

experiments confirm the conclusions of previous investiga tions that by adopting large bulb areas very large reductions in thefl resistance can be obtained. Even bulbs of moderate sizes, 5 % to 7.5 % undoubtedly give considerable savings at higher speeds and

may permit a ship to be driven at speeds which are very high

compared with the fullness adopted.

2. The adoption of a ram construction of the bulbous bow gives, particularly at the B/d of 2.6, a fairly good further reduction of the resistance.