The Marsh Screw Amphibian

Journal of Terramechanics, 1965, Vol. 2, No. 4, pp. 83 to 88. Pergamon Press Ltd. Printed in Great Britain.

THE MARSH SCREW AMPHIBIAN M. J. N~UMEYel~ and B. D. JONES* CONCEPT MAN la3SSESSES a remarkably adaptable locomotion system and has learned to extend the capabilities of the basic system by using such mechanical devices as snowshoes, bogshoes, swimming flippers and ice axes and pitons. The last century or two have seen a remarkable extension of man's speed and carrying ability by application of power to several types of automotive vehicles. Although the efficiency of these vehicles has altered our entire way of life, it must be pointed out that little has been accomplished in relating mud-and-water environments to vehicle performance. Unfortunately, it seems that we must often fight in such environments. The inability of military vehicles to operate effectively in the marshes and rice paddies in the delta country of Southeast Asia has underscored the need for mobility studies directed toward providing equipment to travel in such marginal terrain. Conventional wheeled and tracked vehicles depend on soil bearing strength for support, and on frictional and cohesive soil shear strength for propulsion. The soil strength available for proper function of these mechanisms deteriorates rapidly with water content. Reflection on this problem led Chrysler to the conclusion that an entirely different system of terrain.vehicle interface mechanics was essential for truly effective performance in mud-and-water media, and the Marsh Screw Amphibian was born of this conclusion. In this concept, the support function is fulfilled by bouyant flotation, rather than by intrinsic soil strength. Propulsion is accomplished by viscous shear and reaction to mass movement of the medium, rather than by friction and cohesion in the soil mass. The screw is hardly new, and application to semifluid media dates to 250 B.C. and Archimedes' invention of a screw-type water pump. Thomas J. Wells patented the "Buoyant Spiral Propeller" in 1841 to provide "improvement in the manner of constructing and of propelling steamboats" [1]. This attempt, and others to follow, was a somewhat crude approach tailored to specialized requirements. Amphibious operation was notably missing from a survey of these requirements. Dr. B. N. Cole initiated the first known serious investigation into screw-propelled amphibians in 1960. Published results [2] were inadequate for guidance of Chrysler developments. The key element that makes the Marsh Screw Amphibian different from previous screw-propulsion vehicles is the buoyant rotor principle. This concept is ascribed to Dr. D. P. W. Lett of Chrysler Defense, who theorized that the answer to efficient performance in semifluid media lay in drag reduction, which in turn implied skimming on the surface. The hardware embodiment of the concept has rotors with a volume 175 per cent of the displacement required for flotation. The hull is *ChryslerDefense Engineering,Detroit, Michigan,U.S.A. Communicated by D. R. Freitag. 83

84

M.J. NEUMEYER and B. D. JONES

4 in. clear of the surface in static condition, and rises farther at full speed in the water. RESEARCH Initial design studies were tailored to a payload of ½ ton, or 6 men, which was an existing military requirement. A parametric study of rotor size, power requirement and performance was made, which resulted in selection of rational components and dimensions. Performance could not be calculated with assured precision using known techniques, so a decision was made to conduct powered model tests. The initial model performed in general accord with theoretical predictions and tests on water, sand, snow, mud, grass and hard surfaces indicated a promising performance level. The model was demonstrated to personnel of the Advanced Research Projects Agency of the U.S. Department of Defense and of the Bureau of Ships (BuShips) of the U.S. Navy. A contract was awarded for additional research and construction of a test rig. A 1/5-scale model was built (Fig. 1), with provision for fitting six

FIG. 1. A 1/5-scale model of the Marsh Screw Amphibian. different pairs of rotors. These rotors included three ratios for length to diameter and two helix angles, which were used to establish an optimum compromise of performance in the various media. Model test results for water were scaled up using the following law :

THE MARSH SCREW AMPHIBIAN L = IX

85 (1)

(Froude's Numbers)

(2)

=vX ~/~

(3)

V / ~/ (Lg) = V~ ~/ (lg) V=v

(4)

HP= FV

550 F = fX3

(5)

H P = ].3"3 " vX'/2

(6)

550 H e = hpA s'5

(7)

where (capital letters indicating full scale and lower case letters reduced scale): L = Some important length, ft. V = Velocity. ft/sec.

F = Force, lb. l i P = Horsepower

X = Scale factor PreAictions for full.scale performance in mud were not made from the model tests, since the dimensional analysis indicated that soil scaring would be required for precise indications. Since a programme of this type did not appear compatible with funding limits, it was decided to accept the performance available from the compromise rotor. Model tests implied that this would be acceptable for the test rig. Tests of the full-size vehicle (Fig. 2) in water indicated acceptable correlation with model test results. Performance in other environments has also been about as expected, DESIGN The results of the model test programme were applied directly to establishment of power train characteristics. A commercial gasoline engine provides adequate power, and overall ratio was predicated on operation in frictional sand. A commercial three-speed automatic transmission provides sufficient ratio spread for high-speed operation on low-friction surfaces, and second gear is near optimum for water operation. TESTING Initial vehicle tests included 30 hr in water, 20 hr on sand, 20 hr in marshes, 5 hr on a hard surface and 15 hr of bank and ditch operation. The tests revealed the Marsh Screw Amphibian as a new "high" in mud and swamp mobility. Preliminary evidence of practicability was favourable. Further detail is availeble in Chrysler's test report [3]. To compare the Marsh Screw Amphibian's performance with that of other vehicles under similar terrain conditions, BuShips requested the U.S. Army Engineer

86

M.J. NEUMEYER and B. D. JONES

FxG.2. The Marsh Screw Amphibian.

Waterways Experiment Station (WES) to undertake a programme of "trafficability tests." WES has developed and refined testing procedures and methods of soil measurements for classifying the performance level of vehicles for relative and independent measures of performance. A programme was established and tests were conducted on soil conditions near Vicksburg, Miss. The results of this testing programme also were encouraging [4]. At a conference at the U.S. Army Mobility Command on 16 April 1964, representatives of the Office of the Chief of Research and Development, Test and Evaluation Committee, U.S. Transportation Board, Combat Developments Command, and Army Materiel Command concluded that the Army, rather than the Navy, should have prime evaluation interest. A military potential test was conducted running the vehicle in environments of the Appommattox River, Fort Lee Airstrip, Messik Point, Va.; Fort Lee, Va.; Chickahominy River, Va.; Norco, La,; and Houma, La. The test results have just been published by the General Equipment Test Activity of the Test and Evaluation Command [5]. The U.S. Army Tank-Automotive Command conducted tests at Houghton, Mich., last winter. Vehicle speeds of 20-25 m.p.h, were recorded in deep snow; and, in drawbar-pull tests, the vehicle repeatedly produced values averaging 54 per cent of its own weight. Typical values for tracked vehicles in this same snow run about

THE MARSH SCREW AMPHIBIAN

87

37 per cent. The vehicle was demonstrated to members of the International Society for Terrain-Vehicle Systems during the course of this test programme. DESCRIPTION The overall vehicle configuration is illustrated in Fig. 3, which also includes performance data. The Marsh Screw Amphibian rotors have rounded ends with helical blades attached to the outer surface. The rotors are counter rotated for balanced thrust. With a fully loaded vehicle, an average ground pressure of 0-90 lb/in. 2 is achieved at only 2½ in. of surface penetration. At a penetration of 13 in. (half rotor cylinder diameter), the average unit pressure is only 0.54 Ib/in. 2 The rotors are constructed of 6061-T6 aluminum and are filled with rigid, closed-cell polyurethane foam to keep the vehicle afloat if the rotors are punctured.

GENERAL WlBel4T ¢N[W (me ~ktv~)

2,338 178 LbL

FUIrLand OIL

178 LINk

PASSENeER$ (I)er CAReO vt.H:u~ m , a 4 T

1.080 L.b& 3.T~5 Lt~

p u L r r l u ~ m ee ~?~S L ~ wt. FUrL CAPACITY

PERFO~ANCE (U,:. ~ ,

45 eollon|

wlmie Umk~)

MUD WATER DEEP SNOW

IO to 2o Mira 7"6 Mph zz Mplt

ENGINE MAKE:CHRYSLER MOOEL~RIIfllm~WITYPE:SFerk ign, Slent6, Wi~r ¢ N k d 0tI¥ 8OAl[end STIqOICE 3,4Ox4-Zll OHIPLACEMENT 226 CeLlo Imkee 80VERNED Sfg.lO 3100 Rim NET HORSEPOWER il8 HPot ]HIOORpm POWER TRAIN TRANSMI8841ON: Ctrlaler Terqtmfllte Ided411A-')~, S" ePee4 tmmndmlee tdtk ekeatt '~uml Hu'aq~ e lUl~n ~ , 4nl~ emlmthm~ml d ~ . retie E-liStt ELECTRICAL SYSTEM IE VOLT w/AI.TEflN&TOR

FIG. 3.

Specification of the Marsh Screw Amphibian.

The hull has concave cylindrical sides to conform to the configuration of the rotors. A sloping bow, a short fore deck, and vertical transom characterize the

88

M.J. NEUMEYER and B. D. JONES

shape of the hull. A windshield is mounted on the fore deck. Lifting-towing eyes are centrally located fore and aft on the body, and four corners of the body are equipped with eyes for helicopter lift. The engine is amidship and drives through the automatic transmission to a twostage chain drive at the stern. The final drive divides power between the rotors through electric clutch-brake units controlled by an automotive steering wheel. The driver sits in the centre of the 3-man front seat, which is atop the fuel tank. Additional seats for four men are placed behind the engine. DISCUSSION It is now recognized in mobility research circles that exceptional performance in very difficult environments requires specialized design. The Marsh Screw Amphibian is specialized, and the spectrum of performance is almost exactly the reverse of conventional vehicles. It is at its best in really soupy media, but can move only with difficulty on highways. It is significant that performance is not degraded by secondary aspects of the environment. In operations in swampy lands in Louisiana, Mississippi, Virginia, and Michigan the rotors were found to be unaffected by vegetation, including lily pads, subsurface grass, high grass, cane, and weeds. Rarely can a type of mud be found that will adhere to the rotors to the extent that it will build up, cover the blades, and immobilize the vehicle. This happened on occasion in a mud that was sufficiently firm to be walked on, was very sticky and had low water content, i.e. 10 to 15 per cent moisture content. Operation at Dauphin Isle, Ala., showed very good capabilities in 3 ft surf and good stability even when sideways to the surf. It must be remembered that the current Marsh Screw Amphibian is a test rig, built to prove a principle of locomotion mechanics. It has done its job, and attention can now turn to more flexible and utilitarian variations. REFERENCES [1] T.J. WELLS. Buoyant Spiral Propeller. Patent No. 2400 (1841). [2] B. N. COLE. lnqui~ into Amphibious Screw Traction. Applied Mechanics Gr., The Institution of Mechanical Engineers. June 1961. London, [3] Test Report--Marsh Screw Amphibian--Contract No. NObs 4558. [4] Traflicability Tests with the Marsh Screw Amphibian on Course-Grained and FineGrained Soils--U.S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Mississippi. [5] Final Report of Military Potential Test of Marsh Screw Amphibian--USATECOM Project No. 7-5-0524-01-9.