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Wave Devouring Propulsion Sea Trial Y. Terao (Tokai University, lapan) H. Isshiki (Hitachi Zosen Corporation, Japan) ABSTRACT The development of Wave Devouring Propulsion System and results of it's sea trial are presented in this paper. WDP system is an idea not only for the ship propulsion system which con- verts wave energy directly into thrust but also the ship motion reduc- tion system. This system consist of a ship hull and a hydrofoil installed at the bow. Improvement of the propulsive efficiency in waves and high seawor- thiness is measured during the sea trial. NOMENCLATURE . a B g Hw K L Ra Ra ~R T T Tw 1/3 1.INTRODUCTION It is well known that aquatic mammals, dolphins or whales, which propel themselves with their lunatic tails have high propulsive efficiency. Theoretical or experimental studies have already- succeeded in explaining this oscillating hydrofoil propulsion system. The thrust generating mechanism of these propulsion system is quite simple. With forward speed and the com- bined motion of heaving and pitching of the fin Produces relative fluid veloci- in the fin. Due to the ift theorem, lift perpendicularly to the flow. The mechanism of thrust generation is shown schematically in Fig.1 . t.y whic h brings about an apparent clined flow against Kut t a- Joukowsky l force is generated wave ampl itude . ship breadth. 2 2 C-Qg Hw B /L Ship speed gravitational acceleration. \` wave double amp] itude. \ I Hw=2a \ I signi f icant wave height . \ I 1 wave numbe r . sh ~ p l ength . added resistance in waves. nond imens i anal added res i stance . Ra =Ra/C res i stance _ _ AR =Ra-T nondimens tonal coef f ic lent of resistance increase. R =dR/C thrust of foil. nondimens tonal coefficient. T =T/C signif leant wave period. mass dens it: of water . pitch amplitude. nondimensional pitch amplitude. 6~=9p/(K.a) wave length. ncr`?~!:;~ 1 n WaV`~ . thrust Thrusts ~ Damping ~ oP ~ ng Point force Lift A_ Fig. 1 Thrust generation of a fin in waves. I f we put a horizontal hydrofoil in the oscillating flow field, such as in waves, the relative flow acting on the hydrofoil causes thrust. The wave devouring propulsion system (WDPS) consists of a ship hull and hydrofoil, which acts as a direct Y. Terao: Dept . Naval Archi lecture, 1'okai Un i v.; 3-2()-l, Orido Shi mi Mu, Shi z~toka 424, JAPAN H. Isshiki :Hi tachi Rosen Corp.; 1 Sakura; ima, Konoharla-ku,Osaka 554,JAPAN 287

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wave energy to the thrust converter using these mechanisms. The hull is a collector of wave energy, and the motion of hydrofoil generates the thrust. The hydrofoil is installed in front of the ship's bow and the pitch motion of the hydrofoil is controlled under the sea. The flow against the hydrofoil in waves is (U + u , v). U is the ship's forward speed. u, v is the wave's vertical and horizontal velocity compo- nent plus relative hydrofoil motion due to ship motion and hydrofoil pitch motion in the waves. Thrust is gener- ated as a horizontal component of lift(L), the magnitude of which is periodically increased or decreased but the thrust has a negative direction of U . 'moreover, we must note the verti- cal direction of the lift which has an opposite direction to inflow velocity v. This means that the lift force creates pitch canceling moment, and thus, decreases the ship's pitch mo- tion. Now, the object of this study will be discuss. Our concern is the perform- ance of the WDPS ship in waves. There- fore, two important subjects were ex- pected. First, the improvement of the propulsive efficiency of the ship using an oscillating hydrofoil, because the hydrofoil converts wave energy directly into thrust. - Second, the reduction of the ship motion in waves is expected. Moreover, it is needed for WDPS ship production that the force acting on the hull and hydrofoil quantitative- ly through the sea trial. Resistance increase of the ship in waves is well known phenomena for naval architects. This problem was studied theoretically or experimentally by many researchers. From the momentum theory, the resistance increase in waves has a physical explanation that the work done by the relative hull motion against the wave. Linear damping component is concerned with the wave making resist- ance which takes away the energy in the form of progressive waves. If the reduction of motion in waves was possi- ble, we could expect the reduction of "resistance increase in waves". But if we consider it from such a view point that the reduction in motion reduces the increment of added resistance in waves, there are few studies. It seems that there is a field left not yet studied . It may be true that there is a limitation of the improvement in the total ship resistance in waves by normal hull design. Furthermore, we can expect some thrust increase in waves due to the hydrofoil effect. For example, the ship proceeding in the North Pacific sea in the winter season was studied. It was observed that the wave length of about 100m has excessive wave power in this area, therefore, a ship with a length of less than 80m is desired for WDPS system. From the feasibility study of the thrust generation and the reduction of resistance increase in waves, we can expect the WDPS ship running in waves at 8 kt without any energy supply. Our approach, using a hydrofoil in the wave, might be the answer to improve the propulsive effi- ciency and advance seaworthiness of the ship. Surprisingly enough, in 1985, H. Linden(1) already field a British pat- ent. He really built a 13-ft boat, named "Autonaut", equipped with two elastic fins both at bow and stern. According to the contemporary report, she could travel herself against the wind and waves at a speed from 3 to 4 Kt. Regarding recent experimental studied of WDPS, which are concerned with direct wave energy to the thrust conversion, only Jakobsen(2) and the author(3),(~) have carried out experi- ments. Abkowitz(5) reported on an anti- pitching fin using a model test. He used a pair of hydrofoils, total pro- jected area of the hydrofoils is less than 7% of the ship waterline area, and the effectiveness of antipitching fin was confirmed. He also mentioned a significant improvement in speed in waves. He was concerned with the advancement of the lateral motion of the ship, therefore, did not discussed the trust generation of the fin. T.Y. Wu(6) first showed theoreti- cally that wave energy can be convert- ed into thrust and propulsive efficien- cy becomes more than one or even minus. He studied the two dimensional oscil- lating hydrofoil in waves. Bessho(7) studied the restriction of the lateral ship motion using two fins one at the bow and stern. His group showed the possibility of ship with less heaving and pitching in waves. Naitou and the author(8) succeeded in calculating the motion of a WDPS ship and the propulsive efficiency, especially the reduction in resistance increase in waves using OSM and the steady wing theory. The results will be discussed later. 288

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9.wnP~ ~F.A TnTAI. PRo.TF.cT WDPS sea trial project was planned using a 20-ton fishing vessel. Princi- pal dimensions of the test ship, the foil dimensions and foil section are listed below. To know the thrust in- crease in waves, a larger hydrofoil area was selected compared with Abko- witz's experiment. The projected area of hydrofoil is 7~4/O of the ship's waterline area. 2.1 TEST SHIP PRINCIPAL DIMENSIONS Lpp B d Displacement: Speed Max : 10 kt ( 2000 rpm) Service: 7. 4kt ( 1500 rpm) : 15.7 m : 3.8 m 1.1 m 19.9 ton Hydrofoil Dimensions Cord Length : 1.05 m 3.8 m 1.65 m Span Depth Section : NACA0015 Center of the wing pitch motion is length aft w . _ located at a quarter cord from the wing leading edge. General arrangement of WDPS ship, side view and top view are shown in Figs. 2 and ~ 2 2 THEORETICAL AND MODEL TIONS - Model testing and calculat ions are performed actual sea trial. A 3.5 m model was tested in a tank in 1987, i ~ e . the hydrofoil cord length was Lpp/15, span is same as the ship breadth. The pivoting point of the hydrofoil is located at the quarter cord length of the hydrofoil span which is identical to the center of the steady 1 i f t f orce . The thrust generation and resist- ance increase is calculated theoreti- cally. The coefficients of ship motion was calculated by OSM, the hydrodynam- ics force acting on the hydrofoil is calculated using the quasi-steady theory including the three-dimensional effect. The resistance increase in wares is calculated using a simplified version of the Gerritsuma(9) formula. INVESTIGA theoretical before the Fig. 2 Side view of WDPS ship. ~ ' Fig. 3 Top view of WDPS ship. 289

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Theoretical prediction and the results of model testing of the pitch motion, with and without hydrofoils shown in Fig.4. 2. HI theory exp. Without foil O O ~ with foil -a- 1 . 0 L 0 An,- t /.e al.. , a, . . . . 1.0 2.0 And L Fig. 4 Theoretical and model testing results of nondimensional pitch ampl. itude at a head sea condi t ion ( Fn=0 . 2 5 ) . Up to 30% of the pitch reduction compared to the original ship is seen at the wave length to ship length ratio is greater than 1.6, so we can expect a pretty good pitch reduction effect, but compared to the reduction of model experiments, we had a much higher pitch reduction efficiency. Figure 5 shows the theoretical results of the resistance increase in the head sea condition at Fn=0.25. Ra~ l 1. 01 I 0 ~ ' Or without foi 1 O ~1. smith foi 1 R~ 166F - - - 10. th - 0. 4 - Q-~0 Ia _ ~. I,, ~ ~=_1 1.0 2.0 3.0 A/J L Fig. 5 Theoretical calculation of the resistance increase in head sea condition(Fn=0.25). rt This Figure shows the comparison of the coefficients of resistance in- crease with and without a hydrofoil at sea. Figure 6 shows the thrust increase in waves. We can see that the reduction in the resistance increase in waves is due to the additive effects obtained by the direct effect and indirect ef feet . The direct ef feet is thrust generation due to the hydrofoil and the indirect effect is dependent on the reduction in ship motion which is also effectively affected by the existence of the hydro- foil. TO 2 OF 3~0 I_ 1^0E I At' I,~ 1.0 2.0 3.0 A L Fig. 6 Calculation of the thrust generation of a fin in head sea condition (Fn=0.25). .3 A, theory exp. Without foi 1 - O O ~ smith foil ,^i -at- ~ ,, ~ ~i ~: _ .~-. 1 , , - - i i, I 0 1.0 2.0 A / L ~ _ -0.4 _ 290 3.0 Fig. 7 The theoretical calculation of WDPS strip is resistance increase in head sea condition.(Fn=0,25)

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Figure 7 shows the theoretical calculation of resistance increase in head seas. We can see a minus resist- ance increase at the results of WDPS ship in the wave length to ship length ratio greater than 1.1. This is total thrust gain from wave energy. If the bull resistance during a calm sea is less than the thrust gain from the waves, the WDPS ship can sails against waves without the need to use fuel for power. The results of 3.5m model towing test during a head sea condition is also plotted. Towing speed was Fn=0.249. The difference between with hydrofoil (WDPS) and without hydrofoil is shown drastically. Frictional cor- rec t ion i s not taken account. Theoreti- cal prediction and tested results are in fairly good agreement with this f igure . WDPS ships at sea i s shown in Fig.8 and 9. It is observed in Fig.8 that the rather t iger splash in f rant of the strut . This f igure shows speed trial test using engine power and a foil. In FIg.9, we can see a foil and strut configurations which WDPS ship was equipped with. 3 . WDPS SYSTEM DESIGN Before the actual sea trial, a WDPS structural analysis and design are needed. Selected design wave height to length ratio is 1/30 and design wave length is 1.6 Lpp of the ship. Taking into the consideration of the actual sea trial using a rather small Vessel, structural design was decided carefully to have enough strength. At the same time, reinforce- ment of hull structure are investigat- ed. During the tank test in 1987, we used the two bow hydrofoil system, left and right, which was moved independent- ly using two servo motor. But in this actual sea trial, we selected one hydrofoil and the passive control system to simplify the experiment and keep safety of the mechanism at sea. WDPS hydrofoil supporting system consists of two struts, a hydrofoil pitch spring system, and the struts up-down mechanism. The necessity to change the spring constant easily on board, variable spring constant system consists of a hydraulic pressure cylin- der and an accumulator was selected. Changing the nitrogen gas pressure of the accumulator, we were able to use aide pitch spring constant. Loading and equipment of the system, hull reinforcement of the testing ship were accomplished at Kanasashi Ship Yard. The ship used in the test was made of FRP and aged. The structure of the ship is of a monocoque type so that the fore deck shell scarcely supports the external force or weight of the testing apparatus. There- fore extra reinforcement of the support structures are needed in the bow sec- tion. Fig. 8 WDPS ship speed trial (1500 rpm. at a heading sea condition). Fig. 9 WDPS ship raising a foil. 291

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Support structures and equipment weighting up to about 6 tons, were installed at the bow, and more weight (3 ton) is needed at the stern to keep the even trim and to have enough GM height. During the experiments, we had a hard situation that the blue sea hit the hydrofoil when hydrofoil was hauled from sea but we had no damage to the hull, struts, hydrofoil or any of the instruments. So, we consider that the reinforcement and structural design were a success. . RESULTS OF SEA TRIAL The period of experiment was from December 12th,1988 to January 13th, 1989. The measurement and testing plan is given below. (1) Efficiency of WDPS using as a sub- propulsi on s ys tem. (2) Motion reduction effect of WDPS at sea. (3) Self propulsion test of WDP5 in waves. (4) Force measurements acting on the hydrofoil and struts. (1) and (2) measured the differ- ence of the ship speed and motion, with and without a hydrofoil in the same sea condition at the same rotation of the propeller. Tidal or ship wake effects are canceled using a relative flow speed-meter. - (3) measured the forward speed of the ship using a WDPS without engine power. (4) measured the force acting on the straits using strain gauges. 4.1 APPARATUS OF SEA TRIAL Suruga Bay ,offshore of Miho Kunou was used as the test area. items and apparatus used in the are given below. 4 . 1 . 1 TEST I NG APPARATUS Relative Ship Speed : Relative Flow Meter Propeller Rotation : Optical Rotating Meter Ship Motion(6 degree of Freedom) : Rate Gyro Wave Height : Wave Prove (Drop type) Wind Direction and Speed : Wind Meter Stress of the WDPS arm : Strain Meter Pitch Angle of Foil : Linear Potentio Meter and The test An on board microcomputer was used for data sampling, and at the same time, data were recorded on a data re- corder. Sampling program and analysis program were made for this fast sam- pling and analysis. 4.2 SPEED TRIAL IN WAVES A calibration test of the relative flow meter was carried out using the mile post of the Miho beach. The output of this flow speed data was adopted as the standard of the ship log speed. To know the basic propulsive performance of the ship, original condition without a fin, a speed trial was carried out on a day in which the sea was calm. Results of speed trials are shown in Fig.10. This figure shows the speed reductions in a rather slow speed range, less than 1 Cod rpm' due to the frictional resistance increase by the existence of the hydrofoil and struts. However the high speed range (2000 rpm) there was no discrepancy between with and without hydrofoil results because the wave making resistance dominated at the higher speed range for the total resistance. _ 7.5 _ 5.0 _ 2.5 _ 0 :,,-- . / ~' O Fo i 1 out of water --I-- Foi 1 in water 506 1 000 1 500 2000 Prop. rev. (rpm) Fig.10 Ship speed trial in calm water. It was normally observed that the operators of such small ships drive them at high cruising speeds. It may be considered that the weakness of the WDPS at rather the low speed range in a calm sea is not so serious. Figure 11 and Table 1 show the results of the wave data. A significant wave height and mean wave period can be seen in Figs 12 and 13. 292

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Table 1. Wave Statics Data (01/12/1989) E lapsed time (min.) 30 75 EMS (m) 0.192 0.192 . 120 0.196 _ T Max (sec. ) 11.00 11.00 10.20 T 1/3 (sec. ) 5.77 6.18 _ 6.83 _ Along the coast of Hiho and Kunou, it is known that the wave condition is not so sever even in the winter season' Also it is a one reason why we selected the WDPS test f ield, but during abnor- mall: hot weather in the winter sea- son, we scarcely had a proper wind and wale condition. You can see in Figs 12 and l3 that the wave is a wind wave, because the wave period is shorter and the length of waves are less than the ship length. 3. RB _ 2. R ~ n E-BS ~ see_ - Pover Density T mean (s ec. ) 3.41 3.37 ~ en Hw sax (m) 1.20 1.22 ~.vv 1.23 Hw 1/3 (m) D.71 0.73 _ .0.70 . . Hw mean (m) 0.43 0.40 _ 0.38 January 12th, 1989 we had a good wave condition. The direction of the incident waves and the swell were different all day but in the after- noon, wave height decreased and the swell subsided. Table l and Fig. 11 shows changes in the wave height. Hw 1/3 (m) 2.0 _ 1.0 _ .~ - _~ 25 .5 t 11z. ] .75 1.8 1.25 BITT i m e ( H our ) Fig.ll Example of wave power spectrum (01/12/1989). 293 To mean(sec. ) 4.0 3.0 2.8 1.0 ~ -~. N_ 1 , 1 12121 12/22 12/26 1/10 1111 1112 1/13 Day Fig. 12 Significant wave height. \ . _..___ ~ I N- 12/21 12122 12/26 1110 1111 1112 1/13 Day Fig.13 Mean wave period.

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On this day, we had a speed kt in the head sea condition using engine power. By the Froude, this speed corresponds kt with a ship length of 80 of 2.5 without law of to 5.6 m. In season, the than this It might be possible that the ship of a length of 80 m could e art a speed 8 kt North Pacific in the winter wave condition was better case. WDPS cruis The steering speed of this WDPS ship is 2.5 kt. Also it was observed in the unidirectional incident wave, the COPS ship turns her bow to the incident wave s . With a hydrofoil in waves, the speed increase in the head sea is observed during the certain wave condi- tions. Each speed, with or without a hydrofoil in waves, are shown in Figs 13 and 14. The speed increase in waves, especially in the head sea, is thought as a propulsive efficiency increase in waves. We can see the wave length affecting the ship's advance speed in Fig. 7 and model testing results are also shown in Fig. 8. The wave condi- tion strongly influenced the ship's speed and significant wave height (Hw l/3) and mean wave period (Tw mean) are shown in wave tables in Table 2 and 3. Table 2. Wave Data (01/12/1989 Heading Sea) Prop. (rpm) to 1000 20 00 Hwl/3 (m) 0.965 0.965 0.955 _ 0.965 _ _ Tw mean (sec.) 3. 12 3.12 3.06 3.12 Table 3. Wave Data (01/12/1989 Following Sea) Prop. (rpm) to 1000 1500 2000 Hwl/3 (m) 0.965 0.965 0.955 _ 0.955 Tw mean (sec.) 3. 12 3. 12 3.06 3.06 Sh i p speed (kt) 10.0 . 5.0 . t.0. 0 5.0 0.0 (S~ 294 Fol lowing without . . . _ _ . ~Fnl lowin .. . ~ 0 ff ea il ,;' W~ ,f,^' " , ~ \ Ca l m sea w sea with oi l . th foil 1 000 1 500 2000 Prop. rev. (rpm) Fig.14 Results of speed trial in head sea condition. Sh i p speed (kt) ~Ca l m~ sea ~w _ -_ _ 7 _ _ _ _ _; ~ Head sea . ith foi l i,,,.: Head sea with foi l me, ., ,:: . shout to i l 1 000 1 500 2000 Prop. rev. (rpm) Fig.15 Results of speed trial in a following sea condition.

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Sailing with a propeller rotation speed 1500 rpm, against the swell, a 7.7 kt forward speed could be obtained. This speed is equal to a calm sea condition without a hydrofoil. This data shows the possibility that the WDPS is useful as a sub-propulsor for the ship. 4.3 SEAWORTHINESS OF WDPS SHIP The motion reduction effect, espe- cially pitch motion, is discussed here. From the theoretical analysis, it is expected that the pitch reduction effect is superior. Figure 15 shows the rate of the significant pitch ampli- tude, with hydrofoil data are divided by the without hydrofoil data. If the pitch motion of WDPS is less than the ordinary ship, then the plotted values become less than one. In this case, we have a 20 to 35% pitch reduction at the mean wave period is 1.75 to 2.7. Ratio of pitch decrease .0 _ 0.5 .0 _ at' 0 1.0 2.0 3.0 4.0 Wave period To mean(sec.) Fig.16 Results of pitch motion in a head sea condition. Not only can we see the pitch reduction effect from this figure, but also the crews of the ship mentioned that she has less pitching motion in the rough sea than they had experi enced. At first, the crew would not sail WDPS ship during the rough sea condi- t ion even the bay area because they have no significance. However after the e f f ec t ivene s s o f the hydrofoil was c on- f irmed, they willingly tested the WDPS ship in heavy sea conditions where they had never sailed ordinary hydro- foil-less ship at the test speed range. We had a chance to compar e the sat 1 ing test of her sister ship in the same rough sea at the same time. The sister ship could hardly sail together with the WDPS ship dale to hard bow slamming of the waves. 1 ILog data . ~ l 1 0 ~Course trajectory 150~ , ~./' "W., ~I, I \\ \\ i/ ~ Hi j 1 ~ \~ -ad,. _ ~ Sampling Data 1200 1~0m without foil L 15 | 2 Fig.17 Turning trajectory of ship (without foil 1600rpm). Log data ~ - _ - - - - -~ ~ - - - 1 (min.) Course trajectory 1504 a r 0 1 1 1 295 r W` . ~N. i' /.' ~,,. 170m with foil L 15 ~. a, SAmpling data 1200 / ,- I Fig.17 Turning trajectory of ship (with foil 1500 rpm).

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It was not observed during the experiment which Abkowitz stated the horizontal hull vibration of the anti- pitching fin due to the hydrofoil impact. The author consider that it is associated with the depth and projected area of the foil, which we adopted was deeper and larger area than the anti- pitching fin. The larger hydrofoil restricted the lateral motion of the ship fairly well and the phenomena of the foil penetration of the surface did not occur because of the deeper hydro- foil position. 4.4 TURNING ABILITY OF WDPS SHIP Figure 17 and 18 show the turning trajectory of the ship with a hydrofoil and without a hydrofoil in waves. It is apparent that the turning radius in- creased about 15% compared with the ordinary ship, but a lesser heeling angle was observed* This tendency was the same as a the calm sea turning Lest. Considering the demerit of the in- creased turning radius, the merit of the increase of safety due to a lesser heeling angle especially in waves is more attractive in this ship. _. CONCLUS I ONS From our experiment of the wave devouring propulsion system at sea, following results were obtained a) Improvement of the ship propul- sive performance in waves were ob- ser`-ed. b) Reduction in motion, especial- ly the pitch motion was observed. c) Vertical hull vibration due to the foil was not observed. 6. ACKNOWLEDGEMENT 1 This work has been supported by the Japan Shipbuilding Industry Founda- t~on. The authors would like to thank the crew of Hokuto for their support during the experiment at sea. REFERENCES (l)The Naval Architect(1973,Nov.)pp 239 (2)Jakobsen,E.,2nd Int. Symp. on Waves & Tidal Energy , BHRA Fluid Engineering ,1981, pp.363-368 (3)Terao,Y.,"A Floating Structure Which Moves Toward the Waves,"Journal of The Kansai Society of Naval Architects, No.184, Sept.,1984,pp.~1-54. (4)Isshiki,H., Murakami,K., Terao,Y., "Utilization of Wave Energy into Pro- p~lsion of Ships (Wave Devouring Pro- pulsion)," lath Symp. Naval Hydrodynam- ics, 1985, pp.539-552 (5)Abkowitz,M.A., "The Effect of Anti- pitching Fins on Ship Motions", Trans. of SNAME, vol.67, 1959, pp.210-252 (6)Wu,T.Y.,"Extraction of Flow Energy by- Wing Oscillating in Waves", Journal of Ship Research (1972,Mar.) pp.66-78 (73Bessho,M., Kyo~uka,Y., "On the Ship Motion Reduction by Antipitching Fins in Head Seas", 15th Symp. Naval Hydro- dynamics, 1985, pp.l09-118 (8)Naitou,S., Isshiki,H., Fujimoto,K., "Thrust Gene rat ion of a Fin Attached to Ship in Waves", Journal of The Kansai Society of Naval Architects, No. 202, Sep. 1986, pp. 23-28. ( 9 ) J . Ge r r i t sma, W . Ben ke 1 man, " Anal y s i s of the Resi stance Increase in Waves of a First Cargo Ship", International Shipbuilding Progress, Vol . 19, No. 217, Sept., 1972 , pp. 285-293. 296