Experimental studies on the stability of inflatable lift raft

ARCHIEF

APERS

OF

SHIP RESEARCH INSTITUTE

' Experimental Studies on the Stability of Inflatable We Raft

By

Osamu NAGATA, Masayuki TSUCHIYA and Osamu MIYATA

March 1979'

Ship Research Institute

Tokyo, Japan

Lab, v1 ScheepsbouwkunfJe Technische Hogeschool

Afdeling Scheepsbouw- en Scheepvaarthunde

Technische Hogeschool, De/lt

DOCUMENTATIE I

: K56-6 61

DATUM'

EXPERIMENTAL STUDIES ON THE STABILITY OF INFLATABLE LIFE RAFT*

By

Osamu NAG ATA**, Masayuki TsucHIYA** and Osamu MIYATA**

ABSTRACT

There are few studies on the stability of life rafts, although it has

been often reported that life rafts were overturned in a strong wind and

waves.

To clarify the stability of life rafts some fundamental tests of inflata-ble life rafts of B-class were conducted in a water tank and a wind tunnel. The tested models were the type I of a prototype, the type II with a

hor-izontal skirt, the type III with a vertical skirt and the type IV with a bottom plate. _{Overturning tests of practical life rafts were also conducted} in a strong wind caused by using a helicopter.

1. _INTRODUCTION

The material and the construction of inflatable life rafts have been

reformed to improve the durability and the convenience in handling. In

practice, however, there have been reported a number of cases that life rafts were overturned in a strong wind and waves and a lot of human

lives were lost even though they had successfully escaped from the wrecks.

Nevertheless, systematic studies on the stability of life rafts are very few and much less the studies on the characteristics of life rafts in wind and

waves.

Therefore, we conducted some tank and wind tunnel tests of raft

models of four types. The materials of models used for these tests were

not flexible and so there might be effects resulting from the _{difference in} the rigidity of air tubes, canopies and a floor between the model and the

real life raft. _{In order to examine these effects, overturining tests of}

real life rafts were conducted on the sea in a strong wind caused by using a helicopter.

2. TEST PROCEDURES

The model rafts of a scale of one-fourth are shown in Fig. 1 and Table 1. _{The tested models are the type I of a prototype, the type II}

with a horizontal skirt attached to the air tube of_{a prototype, the type III} Received on November 21, 1978.

CEICEZ 690.7' _538 7 Ibottom moth tube .trig i d polyvinylchloride skirt '(rubber cloth ['horizontal skirt mrn)l ]Itvertical skrrt Is ffC ZDECCE111

Fig. 1. Model of Raft

with a vertical skirt and the type IV with a bottom plate. The prototype

model is the B-class raft capable of accommodating thirteen persons, and

it is composed of equilateral decagonal uper and lower air tubes, a canopy in the form of a decagonal cone and a floor. Human models and an adjustable weight are so arranged that the height of center of gravity and the radius of gyration are a quarter of the calculated values of real

rafts for seven or thirteen persons who lean their backs against the

upper tube and extend their legs, to the center of the floor. In the actual ILC

Table 1 Model of Raft

use of the real raft,, air .exists under the floor of a raft and the quantity

of air is determined by the, course of inflation of the raft.. A littleamount of air is escaped from the floor now and then when the raft rolls under the influence of waves, and the ,draft and the stability of the raft are changed. _{Accordingly, the rafts were tested in two manners, i.e, the tests} of type I and II where the undersides of the floors are completely filled with water and the tests of type I' and II" where the undersides of the floors are filled with air. Two sizes of horizontal skirts and vertical skirts are prepared as shown in Fig. 1., _{The test procedures are as} fol-lows.

Statical Stability Test

The model raft was floated in a test tank having. a Viewing window on the lateral side. A heeling moment of force was loaded on the raft by a pair of weights suspended through a fixed pulley. The angle of 'heel was measured by means of a vertical gyroscope placed on the raft

and scales marked on both sides of the raft. At the same time, the

quantity of air under the floor plate and the degree of deformation of the skirt due to the difference of the pressure across the skirt were ob-served through the viewing window.

Rolling Test

The height of center of gravity and the radius of gyration of the

model were examined by means of the apparatus for measuring the

mo-ment of inertia of model ships. _{And the free rolling period in a. still}

water was also measured: The maximum pitching angle in regular waves in the tank was measured both for drifting and non-drifting _{cases, as}

shown in Photo. 1, with a potentiometer, permitting the vertical _movement of the raft freely:.

i

load _{half 1} ' half 2 full

No. of persons 6 1 13 weight (kg) 8.39 9.56 16.60 KG' (cm) 14.7 14.8 15 2 projected area (m2) above W.L .172 1 1 . 168 . 155 under W.L .038 .041 _1-7 .055 I ---el (cm) 5.47 2.27 5.85 2.62 1 7.77 4.54 GM (cm) 118 58 100 ' 1 47 1 60, 26

condi. under the floor no air with air no air with air no air with air

Photo. 1. Rolling Tests

c) Wind and Water Resistance Test

The model was floated on the basin with a blower in addition to a towing carriage and a wave maker.

The horizontal component of force and the moment of force loaded on the model of full load condition with a constant heeling angle were measured by means of a differential transformer under the condition that

the raft was pulled with a constant speed and faced to a steady wind as

Photo. 3. Additional Wind Tunnel Tests

shown in Photo. 2. The prototype raft was tested also in a wind tunnel

and the horizontal component of force and the moment of force were

measured by using strain gages attached to vertically spaced two positions of the supporter.

Drifting Test in Wind and Waves

The model of full load condition was placed on the surface of water

without any constraint, and the drifting speed and the angle of pitching

were measured in a steady wind and regular waves.

Test in Strong Wind for Real Raft

This test was performed in cooperation with the Maritime Safety Agency.

Sea tests were done on real rafts in a strong wind caused by a heli-copter. Before these tests, the distribution of the horizontal wind velocity was measured on July 15, 1975 at Haneda International airport, using the Bell 212 type helicopter while varying the height above the ground from

five to twenty meters and the horizontal distance from ten to twenty meters. The horizontal component of the wind velocity was measured in

ten points on a vertical plane.

The sea tests were conducted on July 21, 23 and 24 offshore of Futtsu of Chiba Prefecture using the helicopter and a patrol vessel MATSUURA

and two patrol crafts belonging to the Third Regional Maritime Safety Headquarters. The real rafts tested were the type I of a prototype, the

type II and the type III with a skirt and the type IV with a bottom floor,

each having been tested in light and half load conditions. _Subsequently the overturning tests were done without any load of persons and

equip-ments.

f) Additional Wind Tunnel Test of Upper Part of Raft

In addition to the test c), wind tunnel tests of upper parts of models of a scale of one-tenth were conducted, as shown in Photo. 3, to confirm

the vertical component of wind force. Two tested canopies of the models were in the form of a right decagonal cone and of a deformed decagonal

cone. As the material of a canopy was not flexible, the recess

corre-sponding to the depression of a canopy in a strong wind was made in

windward side of the deformed canopy in advance.

3. TEST RESULTS

a) Statical Stability Test

The results of statical stability tests of models in half and full load conditions are shown in Fig. 2 and 3. These results show good coin-cidence with calculations. When the heeling angle Os is smaller than

the limit heeling angle 64, where the lowest part of the lower main-tube

is just exposed to the surface of the water, the water filled up under the floor of type I acts as a dead load. Therefore, the value of GZ for type

I is equal to that of type IV filled up with the ballast water between the floor plate and the bottom plate. And the value of GZ for type I is

larger than that for type I' where the free water effect takes place by

the air entrapped under the floor. When 8, becomes greater than 0,, the value of GZ of type I becomes smaller and it is equal to that of type I', because the underside of the upper part of the floor is exposed to the

atmosphere. The limit angle 0; for type II or III is larger than 0, for

type I. When 0.3 ranges between 01 and 0,', the portion of the flexible

skirt above the water line is recessed inwardly due to the pressure dif-ference across the skirt, and so the true limit angle 01' is smaller than

that which is geometrically calculated by the skirt size. Accordingly, the

size and the resistance-to-flexing of skirts should be great to improve the stability of rafts.

b) Rolling Test

The free rolling periods of models in still water are shown in Table

2. The virtual water mass for type III was extremely large due to the

entrapping of water within the cylindrical skirt, which led to a great

damping, and so it was difficult to obtain a definite period. But roughly the period was about three seconds for type III, although it was about

1. no. of persons 9.56 kg KG 14.8 cm do (no air) 5.85 c m do (with air) 2.62c m GM (no air) 100cm GM. (with air) 48 cm

with air under the floor

10 20 es(deg.) 30

Fig. 2. Staical Stability Curve (Half load)

results of pitching angle for drifting rafts in waves. Any significant

dif-ference from that obtained for rafts restricted to drift in waves was not

noticed. With type I or II, the change of the wave period Tw did not

effect the pitching angle of the raft when Tw>0.9 or the ratio of wave length to raft length was larger than 1.8, and the ratio of the raft

pitch-ing angle O. to the wave slope angle Ow was about 0.8. In the case of

type IV a phenomenon something like synchronism was admitted at Tw#

no air under the floor

r'

N 0

no. of persons 13 16.60 kg KG do (no air) 7.77c rn do (will) air) 4.54cm 0_ GM (no air) 60 cm 1,1 G (with air) 26 cm E a, 0 <3,"' N 0 , 0_ IE\__1(3

\

with olr under the floor

no air

under the floor

AB (with stiffener)

\u C (with stiffener)

load I IV JIB IIC IIIB I

half .85 .95 .95 2.9 3

full 1.05 1.0 1.1 3

I 0 20 es (deg.) 30

Fig. 3. Staical Stability Curve (Full load)

Table 2. Rolling Period (sec)

IC

.25 .55

TYPE I k IF 17F rn-MARK J _{HALF LOAD} FULL LOAD ----Z < --o z 5: -0 15 0.8 MARK HALF LOAD FULL LOAD ----... .. - . ...

Fig. 4. Pitching Angle for Drifting Model in Waves (Original type) WAVE PERIOD 1.2 Tw (sec.) 1.4 18 15 -12 ^ 15 .1 WAVE PER,100 0.8 1.0 1.2 Tw (sec.) 1.4 ---,BANGLE OF WAVE SLOPE (ew) ANGLE OF .,,,WAVE SLOPE (8.)

Fig. 5. Pitching Angle for Drifting Model in Waves (Horizontal type)

tl 8H

TYPE llc 111 CF

-

TYPE 111c1-1 CF MBH MARK HALF LOAD FULL LOAD -Z o 0.8 1.0 WAVE PERIOD 1.2 Tw (sec.) 1.4

Fig. 7. Force by Wind and Water

18 .12 Is

12ANGLE OF WAVE SLOPE

Fig. 6. Pitching Angle for Drifting Model in Waves (Vertical type)

c) Wind and Water Resistance Test

As shown in Fig. 7 and Table 3, external forces and moment of

forces induced by wind and waves were loaded on the model. Fig. 8 shows the drag coefficient for upper part CD to the raft pitching angle

Os. The curve of CD is almost symmetrical to the axis of 6.9=0. When

Os exceeds about fourteen degrees, the underside of the floor begins to

come up to the water surface, and CD becomes greater. Fig. 9 shows the wind moment lever above water line h. The value shows negative,

0

--J

in wind D=CDpU2Al2, L=CLpU2Aw12, Mo=CmpU2A112, h=MolD=(Cm1CD)1, U: wind velocity FULL LOAD -15 Table 3. Definitions D'=Co'p'172A'12, L'=CL'p'172Aw/2, Mo'=Cm'p'V2A'112, h'=Mo'ID'=(Cm'ICD')1,

V: water flow velocity

CD

I.0

0.5

in water

-10

Fig. 9. Wind Moment Lever above Water Line

U 14 m/s

Re .6x10

.155 m2

es (deg.)

-15 - 10 -45 0 5 (0 15

Fig. 8. Drag Coefficient for Upper Part

U= 14 m/s _{10 FULL LOAD} R = .6 x 106 0.1 A = .155m2 = . 73 m es _(deg.) -10 -5 5 10 I 5 -0.1 P: specific gravity of fluid

A: projected area of raft

Aw: water plan area of raft

length of raft D: drag force L: lift force Mo: moment of force h: moment lever

displacement of raft

A =

_2.5

CD

-15 -10 -5 0 5 1 0 15

Fig. 10. Drag Coefficient for Lower Part

Fr = 0.23 0 0.7 -0.1 50 30 -10 es (deg) FULL LOAD 10 es (cleg.) 15

Fig. 11. Water Moment Lever below Water Line 0.6 0.5 Re=0.5x 106 V =0.6 m/s = 0.055m2 =0.73m *, I IC _2.0 ° "IC Fr 0.23

FULL LOAD Re= 0.5 x106

V 0.6 mIs

<1)

a).

0

0

DIRECTION

_OF _W IND eu,

0 5 it0 u (rn/s) 15

Fig. 12, Pitching Angle 1:11! °Wind and Waves

4'0 U(m/s) 15

1(ect-eu)/2

I (eu+ed)/2

which Will be attributed to the conical shape of the canopy. According. ly, it is necessary to consider the vertical component of wind force to

know the exact values of drafts, forces and moments of forces. Fig. 10

shows the drag coefficient for lower part CD'. C,' for type I is almost

symmetrical to the axis of Os=0 and, with the rise in the absolute value

of O. C1,' increases as the true projected area of the lower part of a raft

increases. C,' for type II or III is larger than that of type I because of

the increase of water resistance by the skirt. CD^ for type I or II is not

symmetrical to the axis of 8.9=0, and it is a function of the pitching

angle, size and rigidity of the skirt, and relative speed of the raft. Fig.. 11 shows the water moment lever below water line h". When the abso-lute value of Os is small, the action point P' of horizontal component of water resistance force in Fig. 7 is below the lowermost point of the raft.. It is considered that this is due to the same reason as that for h.

From the foregoing, it has been found that the imaginary horizontal

components of wind and water force to the raft. act at the extremely low point, unlike the eases of ships% In general,, the point of action of

FULL LOAD Tw ew Tw ley/ 83 5.9 1.18 8.5

I

I'

horizontal wind force is in the water, not on the water, and the

hori-zontal water resitance acts at the point in the water lower than the

lowermost point of the raft. CD, CD', h and h' obtained in the wind tun-nel test for the prototype showed similar values as those obtained in the above mentioned tank test.

d) Drifting Test in Wind and Waves

The pitching angles of models in wind and waves are shown in Fig. 12. The neutral angle of pitching was about 0-2 degrees in wind of 15 m/s, and the average angle of pitching amplitude is about 4-9 degrees

in waves of Tw= 1.18, 6w=8.5. The drifting speed of models is shown in

Fig. 13, and it becomes greater almost linearly as the wind velocity in-creases.

FULL LOAD

a

Fig. 13. Drift in Wind and Waves

e) Test in Strong Wind for Real Raft

Before the test, the wind velocity was 0-7 m/s, the height of the

wave was 0-10 cm and the pitching angle of a raft was 1-4 degrees. A

horizontal wind of 15-25 m/s was exerted on the raft both in light and

half load conditions. However, the increased pitching angle was only 0-2 degrees.

Then the floor boards, which were equivalent to the weights of CO,

cylinders and equipments, were removed and the tests were continued.

5 10 U (m/sec) 15

Photo. 4. Tests in Strong Wind for a Real Raft

Now, the type I and the type IV of real rafts were overturned as shown in Photo. 4, but there was no sign of overturning for the type III with the vertical skirt.

A= 5 0.031 m2

1.

xRIGHT CONE CANOPY

.---.DEFORMED CONE CANOPY

-15 -10 -5 0 5 es 10 (deg.)15

Fig. 14. Drag Coefficient for Upper Part (Wind tunnel test)

f) Additional Wind Tunnel Test of Upper Part of Raft

The value of C, for right cone canopy in Fig. 14 is somewhat dif-ferent from that in Fig. 8, because of the difference of the boundary conditions of tests. When the raft heels to leeward, C for the deformed

cone canopy is larger than that for the right cone canopy. But when

the rafts heels to the windward, the values of CD for two types are

al-most the same and they are largest at five degrees of heeling angle.

The coefficients of lift forces CL for two types are nearly equal as shown

in Fig. 15, and the positive values mean that the lift forces are exerted

11=0.3 m

A=0.031 m?

-J -.-5

1

-Fig. 16, Coefficient of Wind Moment (Wind. tunn'et test)

HIS -110 -5 ₅ es tO (deg.)115

1-0.3

Fig. 17: Wind Moment Lever ;above Water Line (Wind tunnel test)

on the rafts by wind. Fig. 16 shows coefficient of wind moment CM of models. The wind moment lever for the right cone canopy in Fig. 17 is almost the same as that in Fig. 9 and it is a negative quantity, but the value for the deformed cone canopy is positive and it is the largest at

five degrees Of heeling angle to the leeward

-=0.5 0=0063 m2" CL -06 1=0.3th A=0.031 eti2 h/1 sr -= _ es 10 (deg.) I5 1 -15 -10 01 es 10 (deg.) 1.5

Fig.. 16.. _{Coefficient of Lift Force (Wind tunnel test)}

-15

-0.5

0.5

-5 5

4. DISCUSSIONS

The discussion on the stability of rafts should take into considera-tion of not only the shapes of the main tubes, the canopy and the skirt

but also the rigidity, the amount of air entrapped under the floor, Reynolds

number, Froude number etc. However, considering the various test results above mentioned, the pitching angle of the real raft is about 2 degrees when the half loaded raft is drifting at a steady wind velocity of about

15 m/s, so that overturning will not be experienced even if there are sub-stantial waves.

Accordingly, though it would be necessary to examine the sea and weather conditions, and the load conditions of the rafts at the time of overturning of the real rafts, the following could be considered as the

causes of overturning.

Rigidity of Material for Raft

The real raft is made of soft materials such as a rubberized nylon cloth and so the floor is flexed downwardly when persons ride on the raft. Therefore, GZ and the limit angle Oi of the raft are decreased. Further, when the inner pressures of main tubes are low and the ex-ternal forces are concentrated, there may be a possibility that the tubes are buckled.

Arrangement of Persons on Board

The weig-th of the raft is very light, i.e. less than that of one person

on board. Accordingly, if an external force is applied in the condition where the weight of the persons is unbalanced to one side of the raft,

the heeling angle would exceed the limit angle O. Thereby the underside of the floor of the raft is exposed to the windward. and GZ of the raft is decreased. Moreover the wind moment is increased.

Wind and Waves

The raft might be capsized easier when the raft is affected by irreg-ular waves of short wavelengths and simultaneously exposed to a gust. By attaching the skirts to the prototype, the limit angle could be at least

twice as large as that for the prototype.

The effects of the horizontal skirt and the vertical skirt to increase

the stability of the rafts are substantially the same. However, the

follow-ing problems are to be considered from the design standpoint. Although

the vertical skirt is preferred for the reliability for increasing the

stabil-ity, the symmetry of the shape of a raft to the floor plane, which is the

unique feature of the B-class raft, is lost. Moreover, when extending the

required in order to evacuate the air from the space surrounded by the

skirt and the underside of the floor. On the other hand, the horizontal skirt does not require such evacuation and the symmetry is maintained.

But air tubes should be attached to the skirt in order to fully extend

the skirt to horizontal directions. And considerations such as size,

dis-position, and flexural rigidity of air tubes are required to prevent the

skirt from being blown up or folded in any conditions of wind and waves.

Although the inflatable life raft with skirts have a few problems

from the design standpoint, it is not considered that they are difficult to

solve. Life raft are fluid dynamically complicated in shape. Moreover,

deformation by external force must be considered. This report presented

some basic data which clarified the direction to be taken in the future

study. That is, studies are required on the cases (i) where no canopy

is used or the shape and the rigidity of the raft are different, (ii) where persons on board are unbalanced to one side and (iii) where external forces induced by irregular waves and wind gust are exerted. We are now planning to study these subjects.

ACKNOWLEDGMENT

The authors would like to thank Mr. N. Mori, Ship Dynamics Division, who gave us valuable advice during the initial stage of the tests, and thank the Maritime Safety Agency who cooperated with us in the sea

tests for real life rafts.

REFERENCES

1) T. Tsuji, Y. Takaishi, M. Kan and T. Sato: Model Test about Wind Forces Acting on the Ships, Report of Ship Research Institute, Vol. 7, No. 5, 1970.

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No. 51 On the Effect of the Forward Velocity on the Roll Damping Moment, by Iwao

Watanabe, February 1977.

No. 52 The Added Mass Coefficient of a Cylinder Oscillating in Shallow Water in the Limit K-0 and K-00, by Makoto Kan, May 1977.

No. 53 Wave Generation and Absorption by Means of Completely Submerged Horizontal

Circular Cylinder Moving in a Circular OrbitFundamental Study on Wave Energy Extraction, by Takeshi Fuwa, October 1978.

No. 54 Wave-power Absorption by Asymmetric Bodies, by Makoto Kan, February 1979.

No. 55 Measurement of Pressures on a Blade of a Propeller Model, by Yukio Takei,

Koichi Koyama and Yuzo Kurobe, March 1979.

In addition to the above-mentioned reports, the Ship Research Institute has another series of reports, entitled "Report of Ship Research Institute". The "Report" is