trends in drogue design

Trends in Drogue DesignAuthor(s): Edward C. Monahan and Elizabeth A. MonahanSource: Limnology and Oceanography, Vol. 18, No. 6 (Nov., 1973), pp. 981-985Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2834592 .

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NOTES 981

1972. Carbon limitation in sewage lagoons, p. 98-105. In G. E. Likens [ed.], Nutrients and eutrophication. Amer. Soc. Limnol. Oceanogr. Spec. Symp. 1.

MONOD, J. 1949. The growth of bacterial cul- tures. Annu. Rev. Microbiol. 3: 371-388.

PARK, K. 1969. Oceanic CO2 system: An evalu- ation of ten methods of investigation. Limnol. Oceanogr. 14: 179-186. - D. W. HOOD, AND H. T. ODUM. 1958.

Diurnal pH variation in Texas bays, and its

application to primary production estimation. Publ. Inst. Mar. Sci. (Texas) 5: 47-64.

VERDUIN, J. 1951. Photosynthesis in naturally reared aquatic communities. Plant Physiol. 26: 45-49.

1957. Daytime variation in phyto- plankton photosynthesis. Limnol. Oceanogr. 2: 333-336.

Submitted: 20 March 1973 Accepted: 17 August 1973

Trends in drogue design"2 ABSTRACT

Drogues in use today in the Lagrangian measurement of currents are in general inferior in behavior to those in use a century or more ago. This results from the historical tendency toward smaller drogues. For op- timum measurements, one should select a drogue design that maximizes ease in handling and then scale it up to the largest size practicable.

In conducting a general review of devices used in the Lagrangian measurement of current, we considered in particular that category of drogue, drag, or sea anchor whose motion is determined by tracking a surface buoy from which the submerged device is suspended at a selected depth. The various types of drogue are illustrated in Fig. 1.3 Each design is assigned to one of four classes based on geometrical con- siderations. Certain trends in drogue usage can be seen that limit the quality of the measurements obtained with the modern drogues.

BASIC DROGUE MECHANICS

A drogue-buoy pair undergoes only very slow changes in speed or direction of motion, i.e. very slight accelerations. It is

' Study supported in part by the University of Michigan Sea Grant Program, maintained by NOAA, U.S. Dept. of Commerce.

2 Publication No. 213 from the Department of Atmospheric and Oceanic Science, The University of Michigan. ' A copy of this figure, with a complete list of bibliographic citations, is available from the Uni- versity of Michigan Sea Grant Program (Monahan and Monahan, in prep.).

thus appropriate to assume at any instant that a drogue-buoy system has no net external force acting on it. In the unique instance where the horizontal flows at the surface and at the depth of the drogue are the same, then the drogue-buoy system has no horizontal forces acting on it, and the drogue and the buoy, taken individually, have no horizontal external forces acting on them. (In this example, and in the one to follow, it is assumed that the force exerted by the wind on the exposed portions of the buoy can be neglected. This is a reasonable assumption if the buoy is almost awash and has only a thin radio antenna or a small radar transponder element pro- truding upward, but if the buoy has con- siderable freeboard, a flag, a radar reflector, or several such features, then for winds greater than a few meters per second, the wind force will be significant and must be included as an external force on the drogue- buoy system.)

In the situation typically encountered, where the surface current is greater than the current at the depth of the drogue, the drogue-buoy system has several external forces acting on it which sum to zero. In those cases where the wire by which the drogue is suspended from the buoy is not of excessive length, these horizontal external forces are two in number and equal in magnitude: the drag force resulting from the motion of the surface water relative to the buoy and the drag force due to the motion of the drogue relative to the current at its depth. The drag force on the buoy is essentially proportional to the square of



982 NOTES

Sall 1576 Logrand311 L)ATE ~~~~~~~~~~~~~~ ~Suspended Bucket 166 1600( u Line

Drogue Soil 1675

1 700 -! _______________________ BucketWith Canon Ball1683 Log Boord Hung FromUmbrello 78

(Bouguer) 17737(~~ 147 Bouguers I j\

178Kiee Weighted Kite6 Cone l7B6

> <Log

1800

Linen l85~~~ ~ Iron PitchPot 1854 F7 Linen 18 _ <

Patterson 184 ?Copper ~~ W*(Jjg~~~6 Globe Chollenger CWvg Mi8thll R859 9 25 m7: Convos 873

Calovs M18e5f FS550 ,,a 6r; T~~~~~rse Mit~ cheil Tube Logs,

Looded Block of Wood 1855 m 1 Sheorw1te Hem1 ( 2 i. Pommeronia X6 ( Lines 1877

Gozelle SailcloCan /

Board With Short ~~~~~~olvon ized Cross 186B X Borrel LeadI R Boaxrd WithiShort | t i Gozeile Sh eet- 1,7' 8177 Cylinder r

1900 Tmn DeepFan Sock Ful Iof Rocks 1900 ~~~~~~~~~Oiled Canvas On 1904jj Wooden Frame Cross _1

?Smoal Cross 1920 1D;I

1924 ~~1905 X27 Iron So I mSrae

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Buoys

3m' Spee

T Sheet Iron

B rrels 1908 1907 1884

1951

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asm~~~~~Mea VjCors X,a el

Muslin Cone 1952

Wooden Braces 2aQl

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1950 1951 2a G.6CInn m Drill P le1950 0 (4 19350 1 {3ippldneol)'J Metal Cross ! - I

Netting 119541 ~ 951- 5~ -

Netting (1954) H > 195 C WjBiplane 19 linm Pole Log 19523 n idle Spreod

Sm Drift Pole 1954(3 1R95 [ r~ fI rlTl956 Current Pole 956 7m Parachutes 196 .... 1964 PARALLEL 13 > I954 W ether illle J 30m Cargo Chute, 1963 (P-, 1) PARALLEL L-~~~~~~~~ W r 1100 ~~~~~ 1967 TO FLOW | Masonite Hinged 0 Pyod 200 m Radar I)1956 Maonte Q wodRope Reflector 1965

0 Aluminum ~~~~~~Metal Cross I981 Aluminum (R2S Michigan 9

10 I\ X21 X23 1 Alumnum

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PLANE PERPENDIC- CRUCIFORM CIRCULAR PARACHUTE ULAR TO FLOW C C

Fig. 1. Evolution of drogue designs. Dates given indicate first use of the design, when known, otherwise they represent the date of publication. Citation for the designs are given in text. A complete list of bibliographic citations for all designs represented in this figure can be found in Monahan and Monahan (in prep.).



NOTES 983

the slip velocity between the buoy and the surface water and to the projected area of the buoy exposed to the surface current. Likewise, the drag force on the drogue is approximately proportional to the square of the slip velocity between the drogue and the water at its depth and to the cross- sectional area of the drogue at right angles to this slip velocity. Since the two drag forces will always be equal in magnitude, the only possible way to minimize the slip velocity between the drogue and the water at its depth is to maximize the ratio of the drogue area exposed to the current at its depth to the buoy area exposed to the surface current. Ways of calculating this drogue slip velocity from simple fornulas, thus correcting the results obtained using drogue-buoy pairs for the effect of the surface currents on the buoys, are described by Terhune (1968). A more exact approach, using plots of drag force versus relative velocity obtained by towing drogues and buoys individually through a long tank with an instrumented carriage, has been set forth by Monahan et al. (1973).

Recently current measurements have been made in a bay of Lake Michigan using "vee" drogues of 33,200 cm2 projected area (item X21 on Fig. 1) in conjunction with disk-shaped flag-buoys of 228 cm2 submerged projected area (Monahan et al. 1973). Use of these drogue-buoy pairs, where the ratio of areas is 146:1, in a situation where the surface current is 15 cm sec-1 and the current at a drogue depth of 35 m is 5 cm sec-1, results in a slip velocity of 0.65 cm sec-1. Thus, if the assumption were made that the common motion of the drogue and buoy can be taken to be the motion of the water at drogue depth, a 13% error would be incurred. It is clear from this example that even with large ratios of projected areas, the cor- rection for the drogue slip velocity should be applied when there is significant vertical shear in the current structure. If the ad- ditional information needed to determine the drogue slip velocity, i.e. the surface current velocity and the drogue and buoy drag coefficients (or their drag force calibration

curves) are not obtained, then it is im- perative that the largest possible ratio of projected areas be incorporated in the selection of a drogue-buoy pair. This ad- monition holds in particular for the many jury-rigged deep drogue devices where such basic information as the draft of the surface buoy usually goes unrecorded.

MINIATURIZATION

The demise of the use of sail led to a reduction in the size of ships' crews and to the disappearance of various pieces of tackle that had once been standard. The general purpose research vessel of today is often less suited for the launching and re- trieval of large current drogues than a vessel of 100 years ago. Even those who have the use of specially designed research vessels often find themselves aboard the smallest ship feasible for their particular assignment, and hence are discouraged from using large, cumbersome drogues. Thus the tendency to use small, easily handled drogues is understandable.

The significance of this trend becomes apparent if one considers the design criteria for drogue-buoy pairs.

A well-designed drogue-buoy pair in- cludes a buoy which is just large enough to support a flagpole, radar reflector mast, or radio antenna, and to provide sufficient excess buoyancy to offset the negative buoyancy of the drogue proper. The drogue itself should, in comparison to the buoy, present a very large cross-sectional area to the flow and have sufficient negative buoyancy to keep the buoy mast upright and to keep the connecting wire essentially vertical. This last consideration is important because it is assumed that the drogue is at a depth equivalent to the wire length and because certain drogue designs show a tendency for unstable oscillation when they are tilted from the intended vertical orientation.

It is impossible to take such a well- designed drogue-buoy pair and scale it down in size without sacrificing perfor- mance. If such a pair is scaled down by a factor R in linear dimension, the vertical



984 NOTES

component of the tension in the wire is reduced by a factor of R3, since the vertical component of wire tension is simply equal to the negative buoyancy of the drogue which is proportional to the drogue's volume. This same scaling will result in a reduction in the horizontal component of wire tension by a factor of only R2, since it is equal to the net drag force on the drogue proper or on the buoy (they are equal in magnitude), each of which is proportional to the respective projected area. Since the tangent of the wire angle is equal to the ratio of the drogue drag force to the drogue's negative buoyancy, the suggested reduction in size by a factor of R results in an increase by a factor of R in the tangent of the wire angle, and hence an increased wire angle. To overcome the increase in wire angle one can add ballast to the drogue, but this will cause the surface buoy to float lower in the water or, if it had negligible freeboard to begin with, it will necessitate the use of a new, larger buoy. In either case, it will result in a pernicious reduction in the ratio of the drogue's projected area to the buoy's projected area exposed to the current.

It is true that not all the early current measurements involved the use of well- designed drogue-buoy pairs, but it is clear that the trend toward smaller drogues has often led to intrinsically inferior current measurement procedures. The sail drogue in use 400 years ago (F2 on Fig. 1, Gilbert 1598) was thus conceptually preferable to the "window-shade" drogue in use today (F12, Terhune 1968; F13, Monahan et al. 1973). Likewise, few of the present day current-crosses are a match for the one used on the Challenger expedition (X4, Tizard et al. 1885).

Considering those drogues that are circular in horizontal cross section, the recently popular cylindrical drogues (1R22, Scott and Landsberg 1969; R23, Gannon and Bru- baker 1969) do not measure up to the U.S. Coast and Geodetic Survey's barrel drogue of 100 years ago (R8, Marindin 1877).

The use of small pilot parachutes (P9, Miller et al. 1972) in place of the much

larger personnel parachutes (P2, Volkman et al. 1956) is a step backward, and a reg- ular umbrella (PIO, Palmer 1972) is a poorer drogue than Benjamin Franklin's "umbrella swimming anchor" (P1, Franklin 1785).

OTHER TRENDS

The use of a drogue-buoy pair where the drogue's effective cross-sectional area to the flow is of the same order of magnitude as the submerged cross-sectional area of the buoy as a device to measure surface currents has a long history (e.g. R12, Richard 1907). Such a device is essentially a flexible version of the pole log (R7, Mitchell 1859), somewhat inferior to the pole log in the manner in which it "aver- ages" the currents at the several depths in the surface layer. Recent attempts to use such systems to measure deep currents (e.g. R23, Gannon and Brubaker 1969) represent doubtful extensions of the role of such devices.

Although most users of parachute drogues recognize the need to use spreaders (e.g. P2, Volkman et al. 1956; P8, Hamblin and Rodgers 1967) to keep the canopies open at the low relative velocities encountered by properly functioning drogues, some users do not. Those who use pilot parachutes invariably rely on the coil spring contained within the fabric to keep the canopy open (P9, Miller et al. 1972), yet when a pilot parachute is drawn through the water at a few centimeters per second, it hangs limply beneath its buoy.

When it comes to building drogues, there is a marked tendency to "reinvent the wheel." A recent news item (Amer. Geophys. Union 1972) describing how the personnel aboard the NOAA ship Mt. Mitchell de- vised a drogue out of two oil drums (R24) to tag a Gulf Stream eddy makes interesting reading when juxtaposed with a description of the much older drogue made of two wooden barrels (R13, Thoulet 1908).

CONCLUSIONS

Clearly to obtain the best current measurements in any given case, the largest drogues possible should be used.



NOTES 985

From a consideration of the hydrody- namics of such low velocity flows as that of the water relative to a drogue, it is apparent that the particular geometry of the drogue is not very important. The drag force on a "window-shade" drogue is essentially the same as that on a "vee drogue" (X21, Hamblin and Rodgers 1967) or a current cross of the same projected area, for example.

One should therefore select a drogue design that maximizes ease in handling, and then scale it up to as large a drogue as practicable. A very large sail drogue, for instance, can be deployed and retrieved from a surprisingly small ship.

EDWARD C. MONAHAN ELIZABETH A. MONAHAN

Department of Atmospheric and Oceanic Science The University of Michigan Ann Arbor 48104

REFERENCES

AMERICAN GEOPHYSICAL UNION. 1972. News. Trans. Amer. Geophys. Union 53: 875.

FRANKLIN, B. 1785. Sundry marine observations. Trans. Amer. Phil. Soc., Ser. 1 2: 294-329.

GANNON, J. E., AND D. C. BRUBAKER. 1969. Sub-surface circulation in South Fishtail Bay, Douglas Lake, Cheboygan County, Michigan. Mich. Acad. 2(2): 19-35.

GILBERT, H. 1598. A discourse to prove a pas- sage by the Northwest to Cataya and the East Indies, p. 40-49. In R. Hakluyt [ed.], The principal navigations, voyages, traffiques and discoveries of the English nation, v. 12. Hakluyt Soc. Mem. Ed. 1889. Goldsmid.

HAMBLIN, P. F., AND G. K. RODGERS. 1967. The currents in the Toronto region of Lake On-

tario. Great Lakes Inst. Univ. Toronto Progr. Rep. 29. 143 p.

MARINDIN, H. L. 1877. Description of apparatus for observing currents, p. 104-107. In U.S. Coast and Geodetic Survey Report, 1877.

MILLER, R. L., C. S. ALBRO, J. M. COHEN, AND J. F. O'SULLIVAN. 1972. A preliminary study of tidal erosion in Great Harbor at Woods Hole, Massachusetts. Woods Hole Oceanogr. Inst. Ref. 72412. 28 p.

MITCHELL, H. 1859. Report on the physical surveys of New York Harbor, p. 211-317. In U.S. Coast and Geodetic Survey Report, 1859.

MONAHAN, E. C., G. T. KAYE, AND E. D. MICHE- LENA. 1973. Drogue measurements of the circulation in Grand Traverse Bay, Lake Michigan. Univ. Mich. Sea Grant Tech. Rep. 35. 35 p.

PALMER, M. D. 1972. Measurement of currents in the Great Lakes. Int. Field Year Great Lakes Tech. Manual Ser. 3. Information Canada, Ottawa. 32 p.

RICHARD, J. 1907. L'Oceanographie. Vuibert and Nony.

SCOTT, J. T., AND D. R. LANDSBERG. 1969. July currents near the south shore of Lake Ontario. Proc. 12th Conf. Great Lakes Res. (Int. Ass. Great Lakes Res.) 1969: 705-722.

TERHUNE, L. D. B. 1968. Free-floating current followers. Fish. Res. Bd. Can. Tech. Rep. 85. 33 p.

THOULET, J. 1908. Instruments et operations d' oceanographie pratique. Chapolet et Cie.

TIZARD, T. H., H. N. MOSLEY, J. Y. BUCHANAN, AND J. MURRAY. 1885. Narrative of the cruise of H.M.S. Challenger with a general account of the scientific results of the expedition. Rep. Sci. Results HMS Challenger 1873-76.

VOLXMAN, G., J. KNAUSS, AND A. VINE. 1956. The use of parachute drogues in the measurement of subsurface ocean currents. Trans. Amer. Geophys. Union 37: 573-577.

Submitted: 2 April 1973 Accepted: 27 July 1973



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