US3606317A - Rowing simulator - Google Patents

Abstract

A SIMULATOR FOR ROWING INCLUDES A RAIL-SUPPORTED TRAIN OF ROLLING DOLLIES EACH HAVING A SUSPENDED CRADLE MOUNTING A SLIDING SEAT, A FOOTBOARD AND A RIGGER. THE RAILS STAND ON A PIER BETWEEN A PAIR OF FLUMES FOR PUMPED CIRCULATING WATER IN WHICH THE OARS ARE SWEPT. THE DRAG OF WATER FRICTION ON THE HULL OF A SHELL IS SIMULATED BY THE FORCE OF THE FLOWING WATER ON VANES SUPPORTED ON A YOKE THAT IS CARRIED BY THE TRAIN. THE VANE POSITIONS ARE ADJUSTABLE TO COMPENSATE FOR ROLLING FRICTION AND ARE AUTOMATICALLY RESPONSIVE TO CONTROL THE MEAN LONGITUDINAL POSITION OF THE TRAIN IN RELATION TO THE PIER. ALTERNATIVELY OR ADDITIONALLY, A SERVO CONTROL AUTOMATICALLY VARIES THE PUMP SPEED AND THEREBY THE WATER SPEED TO CONTROL THE TRAIN''S POSITION.

Description

Sept. 20, 1971 J. H. FRAILEY ROWING SIMULATOR 3 Sheets-Sheet 1 Filed Oct. 13, 1969 INVENTOR JACK H. IFRAILEY BY Wk? ATTORNEYS Sept. 20, 1971 J. H. FRAILEY ROWING SIMULATOR 3 Sheets-Sheet 2 Filed Oct. 13, 1969 INVENTOR JACK H. FRAILEY BY ATTORNEYS Sept. 20, 1971 FRAlLEY 3,506,317

ROWING SIMULATOR Filed 001;. 13, 1969 3 Sheets-Sheet 5 I24 FIG. 5 W

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CQT'"L 7 IOO/ IOQA I34 SERVO TO w AMP M MOTOR THROTTLE 204 OF ENGINE 34 FIG. 7

ATTORNEYS United States Patent O 3,606,317 ROWING SIMULATOR Jack H. Frailey, 9 Marthas Point Road, Concord, Mass. 01742 Filed Oct. 13, 1969, Ser. No. 865,687 Int. Cl. A63b 69/ 06' U.S. Cl. 272-72 19 Claims ABSTRACT OF THE DISCLOSURE A simulator for rowing includes a rail-supported train of rolling dollies each having a suspended cradle mounting a sliding seat, a footboard and a rigger. The rails stand on a pier between a pair of flumes for pumped circulating water in which the oars are swept. The drag of water friction on the hull of a shell is simulated by the force of the flowing water on vanes supported on a yoke that is carried by the train. The vane positions are adjustable to compensate for rolling friction and are automatically responsive to control the mean longitudinal position of the train in relation to the pier. Alternatively or additionally, a servo control automatically varies the pump speed and thereby the water speed to control the trains position.

BACKGROUND OF THE INVENTION The field of this invention relates generally to rowing as a sport and as a means for scientific physiological experimentation and evaluation. More particularly, it relates to a rowing simulator comprising a shell simulating structure for supporting one or more oarsmen, and being recipFocable about a more or less fixed longitudinal position in a manner simulating the surge and retardation of an actual shell with respect to its average velocity in the water.

There have been many attempts to devise a rowing simulator, but so far none is known to have attained an exact simulation of the conditions affecting a shell being propelled through the water. The exact equations of motion are complex and non-linear, thereby complicating the problems of simulation. As used herein the term shell refers to a hull equipped with sweeps, that is oars used with two hands, and also wherever the context permits it includes sculls, that is hulls equipped with oars used with one hand, a pair to an oarsman.

There are two basic conditions to be simulated, namely roll or tilt instability caused by the usual location of the metacenter of a shell below the center of gravity when the oarsmen are in position, and the frictional drag of the water on the hull which varies with its surge and retardation and, of course, with its average velocity during the stroke and recovery of the oars.

The simulators heretofore employed have been generally relatively crude devices. In some the footboard and rigger are fixed and the oar is dipped into a stationary body of water. In many, the roll instability is not simulated, nor is account taken of the fore and aft drag response to the catch and recovery of the oar. Devices of this kind are essentially individual trainers with no capacity for simulating the composite or resultant effort of a crew.

The United States patent to Cunningham No. 927,833 dated July 13, 1909, discloses a simulator with forced flow in sweep ways or flumes. There is a channel filled with water for floating a practice boat or shell, but the frictional drag of the water in this channel does not simulate the drag at normal water velocity. This patent, as well as others showing a reciprocating shell simulating structure, discloses restraints to prevent the boat from ice hitting the ends of the structure and to urge it toward a mean position. Such means include spring action having a force varying linearly with the position of the shell simulating structure with reference to the mean position, but such a force has no counterpart under actual rowing conditions.

A French Pat. No. 528,380 to Saint-Fe published Nov. 10, 1921, disclose a shell simulating carriage on inclined rails and urged downstream by a counterweight. The constant counterweight force does not accurately simulate actual rowing conditions. This patent does not simulate such factors as water speed, wheel friction, and roll instability.

SUMMARY OF THE INVENTION This invention provides not only a close simulation of rowing conditions in open water, but also provides adjustments and controls to vary the operation and characteristics of the apparatus for training or experimental purposes. Simulation of roll instability and variations in the drag on the hull are both provided.

The invention is embodied in a tank having two sluices, troughs, sweep ways or fiumes separated by a pier. A shell simulating train consisting of one or more dollies is supported for translational reciprocation on the pier, each dolly having a pivotal seat, rigger and footboard similar to the corresponding fittings on an actual shell or scull. Means are connected to the dolly for reciprocation therewith and include a yoke that supports vanes immersed in the water.

An important characteristic of the invention is the provision of means to vary the drag exerted by the flowing water on the vanes, and the controls associated therewith. These controls have multiple uses, depending upon the particular purposes in operation. For example, where the dolly or dollies are mounted upon wheels that introduce rolling friction which necessarily reverses upon each change in direction, the drag on the vanes is changed on the stroke and recovery of the oars by an increment suflicient to render the net drag independent of friction. In addition, the drag is further automatically adjusted as a function of the excursion limits of the shell simulating structure to maintain it within operative limits on the pier and without sacrificing the conditions for close simulation.

The apparatus is adapted for joint or independent control of a number of operative conditions including controlling the drag by changing either the water speed or the vane positions in the water, or both, and changing the pivotal supports on the dollies to vary the roll instability, thereby taking into account the number of the crew, its strength and frequency of stroking and the types of oars in use, for example. Moreover, various forms of measuring apparatus may be associated with the mechanisms to provide data for measuring performance and evaluating training, diet and fatigue eflFects, as well as experimenting with blade shapes, rigging distances and dilferences in rowing styles and techniques.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary plan view of a tank for a rowing simulator embodying the invention.

FIG. 2 is an overall plan view of the rowing simulator tank illustrating the controls for water recirculation.

FIG. 3 is an elevation in lateral section taken on line 33 of FIG. 1, illustrating parts of the means for controlling the drag on the vanes.

FIG. 4 is a side elevation taken on line 4-4 of FIG. 1, illustrating a control dolly with the reference and response means operatively associated with the vane controls.

FIG. 5 is a plan view of a dolly for an oarsman.

FIG. 6 is a side elevation of the dolly shown in FIG. 5. FIG. 7 is a schematic and partially diagrammatic representation of controls for varying the water speed.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 shows a portion of the floor plan of a building enclosing the rowing simulator, the simulator being designated generally at 12. A reinforced poured concrete foundation 14 is formed with a pair of troughs or fiumes 16 and 18, of which the profiles in cross section are best illustrated in FIG. 3. The fiumes are separated by an elongated horizontal pier designated generally at 20 and having sloping side walls 22 and an elongated open central well 24, preferably somewhat longer than the length of the cockpit of an eight-man shell. The flumes are connected at their ends by header spaces 26 and 28, the latter being connected with a water duct 30 installed beneath the pier. A pump propeller 32 is installed in the duct and connected through bevel gearing with the drive shaft of an internal combustion engine 34. This engine is equipped with a throttle by which its speed and power output may be varied as hereinafter described. In operation, the pump forces the water in the direction indicated by the arrows in. FIGS. 1 and 2. In the illustrated embodiment the circulating pressure is provided solely by the pump, but if desired the header 28 may have a head tank constructed above it and the pump may be used to maintain a predetermined static head at the entrances to the flames.

The shell simulating structure comprises a train 36 of wheeled dollies rolling on a pair of U-shaped longitudinal channel irons 38 hereinafter referred to as rails. The rails are supported on uprights 40 (FIG. 3) rigidly spaced and bolted together with braces 42.

The dollies comprise modules of which variable numbers may be connected together, depending upon the number of oarsmen. In the illustrated embodiment the train is made up to simulate a shell having a plurality of sweeps. For this purpose the modules have riggers on alternate sides, there being one rigger per module. It will be obvious that for simulating a scull each module will have an identical rigger on each side. In FIG. 1 typical dollies for individual oarsmen are shown at 44 and 46, and other dollies 48, 50, 52 and 54 are of identical structure but are shown in fragmentary form to avoid unnecessary complication of the drawing. The dollies are connected in a manner hereinafter described and reciprocate together on the tracks to simulate the surge and retardation of a shell. The structure of a typical dolly 48 is shown in more detailed form in FIGS. and 6 and is described below.

At the bow end of the shell simulator is fastened a restraining cable or rope 56 passing over a fixed pulley 58 and secured to an end of a compression spring 60 in a fixed cylinder 62. The cable 56 restrains the simulator near the permissible limit of its aftward drift, but under normal operating conditions it is slack throughout the full length of both the stroke and the recovery of the oars, the shell simulator being at all times further upstream than the position at which the spring 60 begins to compress.

In addition to the dollies for oarsmen there is a drag control dolly 64 secured at the stern of the shell simulator in approximately the same position relative to the crew as that occupied by the coxsvvain in a shell. The structure of this dolly is further described in detail below with reference to FIG. 4. This structure supports a transverse beam, frame or yoke 66 extending over the flumes 16 and 18 and supporting frames 68 and 70 (FIG. 3) in which control vanes 72 and 74 are pivotally mounted. The vanes are preferably flat rectangular sheets of metal or plastic attached to control pulleys 76 and having pivotal attachments to the frames. Coil springs 77 have ends attached to the arms of the yoke (FIG. '1) and opposite ends attached to cables 78. The cables pass over pulleys "79 on the yoke, around the control pulleys 76 (FIG. 3), and over pulleys 80 and 82 on the yoke to a pivotal cam follower arm 84- (FIGS. 3 and 4) to which the cable ends are attached. This arm may be moved up or down as viewed in FIG. 3 to change the attitude of the vanes 72 and 74- in relation to the streamlines. The springs 77 maintain tension on the cables 78 and tend to rotate the vanes toward positions projecting maximum areas in the direction of the streamlines. These positions are adjustable as described below. The yoke 66- is sufiiciently rigid to prevent deflection of the vane frames 68 and 70 in directions lateral to the streamlines.

In operation, the oarsmen sit in the dollies and row in the same manner as in an actual shell against the drag of the water on the vanes 72 and 74, this drag being transferred to the shell simulator by the yoke 66. This results in the reciprocation of the shell simulator between fore and aft limit positions in response to the net forces exerted on it by the effort of the crew, the water drag on the vanes and wheel friction that reverses with each change in the direction of movement. In the illustrated embodiment wheel friction is made relatively small compared to drag by using ball or roller bearings on the Wheels. If it is desired to eliminate rolling friction, other suspension means for the train may be employed but generally at greater expense, such means including, for example, compressed air or hydraulic cushions and magnetic fields. The illustrated mechanical embodiment is preferred for its simplicity and because the vane control mechanism hereinafter described is adapted to change the drag on each stroke and recovery to balance out the forces of rolling friction on the train.

FIGS. 5 and 6 show details of the structure of the dolly 48. p

A transverse bar 86 is secured to a pair of wheel support bars 88 to form a rigid structure for pivotal support of the forward wheels of the dolly. There are two wheels 90 with horizontal axles on one side and a single wheel 92 with a horizontal axle on the other side. On the latter side there are also two wheels 94 with vertical axles, these wheels having an outer diameter somewhat smaller than the inside clearance between the upstanding flanges of the channel tracks 38. In the form shown with a single rigger and sweep, the wheels 94 bear on the inboard track flange to counteract a twisting torque on the dolly about a vertical axis, such torque being applied at the catch of the sweep in the water. The bars 86 and 88 together with the wheels attached thereto is referred to below as a carriage and designated 96 in the drawing. An identical carriage 98 with a transverse bar 99 is placed with wheels reversed on the tracks and forms a part of the adjacent dolly 46.

A cradle designated generally at 100 is pivotally suspended between the carriages 96 and 98 on link bars 102 swinging from bolts 103 passing through the bars 86 and 99. A bar 104 is fastened between the links .102 at a predetermined distance below the bolts 103, this distance being adjustable as hereinafter further described. A similar bar 106 on the dolly 46' has an end close to but preferably not touching the end of the bar 104 as shown in FIG. 6. Preferably, the ends of the bars have partial cylindrical concavities to conform to the shank of a bolt such as 108 shown at the opposite end of the bar 104, this bolt passing through a clevis clamp 110 or a similar device effectively joining the bars 104 and 106 for swinging in unison, if desired.

Bars 112 are attached to the bar 104 of the cradle and support slides 114 for a seat 116, the slides and seat being in structure substantially identical to those usually employed in a shell. The bars 112 also support a rigger 118 comprising uprights 119, a saxboard or gunwale 120 and arms 122 supporting an oar lock 124 of conventional form. A footboard 126 with shoes 128 is secured to the bar 104, and is adjustable fore and aft.

It will therefore be seen that the dolly 48 comprises the carriage 96 and the cradle 100 and is so constructed as to simulate and adjust roll instability. To this end the axis of the bolts 103 from which the rockers are suspended simulates the metacenter of a floating shell. The center of gravity of the dolly with the oarsman and oar in place, which is adjustable at the points of attachment of the bar 104 to the links 102, is located somewhat above the bolts 103, for example two to four inches approximately, to simulate the instability in a floating shell in which its center of gravity is located substantially the same disance above the metacenter.

The degree of instability may be increased or decreased in relation to the value found in an actual shell for training purposes by attaching the bar 104 higher or lower on the links 102. For a novice crew attachment is made to a lower position, and if desired the rolling action may be temporarily eliminated by clamping the links 102 to the bars 86 and 99 in any conventional manner.

Tension springs 130 are fastened between studs 132 on the bars 86 and 99 and holes 134 located near the bottoms of the links 102, these springs providing means for resiliently urging the cradle 100 to a level position. The springs 130 may be eliminated, if desired. If used, they may be adapted by conventional means to provide more or less restoring force to the cradle, hence providing another mode of varying the effective rolling instability. As previously indicated, the roll characteristics of each dolly may be completely independent of any other, or the bars 104 and 106 of adjacent dollies may be connected by clamps such as 110 to cause the rockers to pivot in unison. Thereby, a shell or scull having a crew of any number may be simulated.

FIG. 4 shows details of the drag control dolly 64. This dolly has a pair of carriages 136 and 138 similar to the carriages 96 and 98 (FIG. and including transverse bars 140 and 142 secured to wheel support bars 144- in which wheels 146 are pivotal. The bars 140 and 142 are bolted to angle irons welded to a platform 148 (FIGS. 1 and 4) to which the yoke 66 is bolted by means of bolts 150. Also supported on the platform are a directional switch 152, a solenoid 154 and a battery 156. The solenoid plunger 158 has a transverse hole receiving one end of a ratchet bar 160 pivotal at 162 on the platform. Upon energization of the solenoid 154 the bar 160 pivots to engage a plate 164 attached to the cam follower arm 84, the latter being pivoted on the platform 148 at a pin 166.

The arm 84 carries a stud 168 in position to engage a flange on a reference member or cam 170, the latter consisting of a sloping angle iron bolted by bolts 172 to uprights 174 secured to a plate 176 fixed on the end of the pier 20. The uprights 174 have slots to permit variation of the slope of the cam reference member 170 as hereinafter described.

As previously stated, the cables 78 are fastened to the arm 84. Assuming that the dolly 64 is moving aft toward the cam 170 (toward the left as viewed in FIG. 4) with the solenoid 154 deenergized, the stud 168 eventually engages the cam 170, and as the movement continues the arm 84 is turned counterclockwise as viewed in FIG. 4, pulling down on the cables 78 and deflecting the vanes 72 and 74 (FIG. 3) toward positions having progressively smaller projected areas in the direction of the streamlines.

When the dolly 64 stops and reverses direction a feeler spring 178 frictionally bearing on a wheel 146 is deflected to depress the plunger of the switch 152, closing a circuit through wires 180 connecting the battery to the solenoid and energizing the latter. The plunger 158 is shifted, pivoting the ratchet arm 160 and causing it to engage with the plate 164 to lock and hold the arm 84 in its downwardly deflected position as the dolly 64 moves forward out of contact with the cam 170. The vanes 72 and 74 remain in their deflected positions throughout the ensuing stroke and until the dolly again reverses direction at the opposite end of the pier 20. The latter reversal of direction causes the feeler 178 to be oppositely deflected by the wheel 146, releasing the plunger on the switch 152 and opening the circuit to the solenoid 154. When this occurs the arm 84 moves upwardly or clockwise as viewed in FIG. 4 under the action of the springs 77 (FIG. 1) until the arm abuts an adjustable stop sleeve 182 on a rod 184 secured to the platform 148. This restores the vanes 72 and 74 to a selected maximum drag position. Thus a greater drag of selected magnitude is exerted upon the shell simulator during the recovery of the oars.

The adjustment of the slope of the cam or reference member and the adjustment of the stop sleeve 182 comprise means for closely simulating actual rowing conditions; or if desired, these adjustments may be intentionally varied from actual conditions for training or other purposes.

For accurate simulation, during the recovery portion of the stroke as the shell simulator moves in the direction of the water, the vanes 72 and 74 are located by adjustment of the sleeve 182 in positions producing a drag exceeding the mean drag on an actual shell during recovery by the total mean rolling friction force opposing the motion. The net force acting on the shell simulator is then the same as the mean drag on the shell.

The aft limit of the dolly 64 during recovery is a variable depending upon many factors, as previously noted. Conveniently, this position may be'considered with reference to a predetermined normal position, which may be taken for example as that illustrated in FIG. 4. When the arm 84 is depressed to the extent indicated at the normal position, the vanes are rotated to positions producing a drag during the stroke which is less than the mean drag on an actual shell during a stroke by the total mean rolling friction force which now augments the drag. Thus the net force acting on the shell simulator continues to equal the mean drag on the shell.

If, now, the aft limit position of the shell simulator at the catch of the oars varies from the indicated normal position, a corresponding adjustment occurs in the drag forces operative during the ensuing stroke. For example, as the crew tires the aft limit of the shell simulator drifts aft of the normal position, the arm 84 is further depressed and the drag on the vanes is correspondingly reduced during the next stroke. Therefore, the simulator surges correspondingly further ahead for the same crew effort and the net effect is to maintain the shell simulator within the operative limits of the pier, while at the same time continuing to approximate closely the conditions in an actual shell.

FIG. 7 illustrates a further control over the drag on the vanes that is carried out either independently of or in conjunction with the changes in the vane positions described above. The FIG. 7 control operates to vary the speed of the engine 34 and thereby the speed of the water and its drag upon the vanes. A cable 186 is fastened to the yoke 66, and extends aft over a control pulley 188 about which it is looped, to a reel 190. The reel is equipped with a coil spring to keep the cable under constant tension. The pulley is fastened to a brush 192 of a potentiometer 194 having its ends connected to terminals of opposite polarity of batteries 196 having a grounded common connection. The switch 152 is connected to wires 198 which, like the wires of FIG. 4, are connected together when the dolly 64 stops in its aft limit position and reverses direction, thereby connecting the brush 192 with a sample-and-hold circuit designated generally at 200. The operation of circuits of this type is well understood, and it is sufficient for present purposes to summarize it as follows. The time constant of the circuit comprising a resistor 202 and a capacitor 204 is selected to produce an exponentially diminishing pulse of which the maximum amplitude is determined by the displacement of the dolly 64 from the above-described normal position. In that position the brush 192 is preferably at its mid position on the potentiometer and at ground potential. The pulse is connected through an amplifier 206 to a 7 servomotor 208 having its rotor mechanically coupled to the throttle of the engine 34. The pulse produces a discrete displaceemnt in the throttle position, the magnitude of the displacement increasing with increasing displacement of the dolly 64 from the normal position. This produces a decrease in Water speed and drag on the vanes of corresponding magnitude as the limit position of the dolly 64 moves further aft of the normal position.

It will be understood that while the illustrated embodiment herein described embodies the preferred form of this invention, numerous modifications may be made in the structure as herein above indicated, and as Will be further evident to one skilled in the art. For example, the variable drag means including the cam follower arm 84, the ratchet 160, the solenoid 154 and related parts may be replaced by other known structures having an equivalent function. Also, the speed of the water in the flumes may be varied as a function of the forward or aft limit of motion of the shell simulator, or as the average or some other function of the forward and aft limit positions, and the controls for these purposes may be accomplished either automatically or by visual observation. Further, the drag control dolly may be adapted to cooperate with an additional cam like the cam 170 at the bow end of the pier to adjust the drag on recovery of the oars in the same manner as that described above for the stroke. Still further, force measuring devices of known construction may be installed at each rigger and footboard to measure the propulsive efficiency of each oarsman. Other modifications, structural additions and arrangements of the parts will also be evident from a reading of this specification, and may be employed without departing from the spirit or scope of this invention.

What is claimed is:

1. A rowing simulator having a tank including a pair of water flumes and a pier separating the flumes,

means to circulate water in the flumes,

a dolly supported for translational reciprocation on the pier and bearing a seat and an oarlock,

means connected to the dolly for reciprocation therewith and including a yoke,

a vane supported on the yoke and immersed in the water, and variable drag means including a control adapted to move the vane in relation to the water to vary the drag thereof on the dolly, reference means supported in predetermined position relative to the pier, and response means on the dolly responsive to its position relative to the reference means to operate the control.

2. The combination according to claim 1, with means to connect plural dollies to form a train having as many seats as a shell to be simulated.

3. The combination according to claim 1, in which the dolly is supported on wheels.

4. The combination according to claim 1, in which the yoke supports a vane in each flume.

5. The combination according to claim 1, in which the dolly comprises a carriage and a cradle, said cradle sup porting said seat and oarlock and being supported on the carriage pivotally about an axis longitudinally oriented to the direction of reciprocation.

6. The combination according to claim 5, with plural dollies to form a train and means for interconnecting the cradles on a pair of adjacent dollies to cause them to pivot together.

7. The combination according to claim 5, with restoring means resiliently urging the cradle to a predetermined balanced position.

8. The combination according to claim 5, in which the seat is slidably supported in the cradle.

9. The combination according to claim 5, in which cradle has a rigger supporting the oarlock.

10. The combination according to claim 1, with a pair of rails on the pier, the dolly having wheels supported on the rails and means laterally engaging the rails to transfer a torque to the dolly in reaction to that applied thereto by a sweep oar catching in the water.

11. The combination according to claim 1, in which the response means are adapted to cause the control to be operated when the dolly reverses its direction.

12. The combination according to claim 1, in which the reference and response means are adapted to cause a progressive change in the drag as the limit position of the dolly progressively approaches an end of the pier.

13. The combination according to claim 12, in which the reference means comprise a cam mounted on the pier and the response means comprise a cam follower and mechanism connecting the cam follower with the control.

14. The combination according to claim 12, in which the response means are adapted to cause the control to be operated when the dolly reverses its direction.

15. The combination according to claim 12, in which the response means are adapted to cause the control to be operated when the dolly reverses its direction and held in fixed position until it again reverses its direction.

16. The combination according to claim 15, in which the control is operated when the dolly reaches its downstream limit position and restores the vane to a predetermined position when the dolly reaches its upstream limit position.

17. A rowing simulator having a tank including a pair of water flumes and a pier separating the flumes,

a pump to circulate water in the flumes,

a dolly supported for translational reciprocation on the pier and bearing a seat and an oarlock,

means connected to the dolly for reciprocation therewith and including a yoke,

a vane supported on the yoke and immersed in the water, and

variable drag means including a control adapted to vary the speed of the pump as a function of the position of the dolly relative to the pier.

18. The combination according to claim 17, in which the variable drag means include means for producing a control signal varying with a limit position of the dolly on the pier, and means responsive to the control signal for varying the speed of the pump.

19. The combination according to claim 17, in which the variable drag means include means for generating a voltage varying in magnitude with the displacement of a limit position of the dolly relative to a predetermined position on the pier, and means responsive to said voltage for varying the speed of the pump.

References Cited FOREIGN PATENTS 528,380 11/1921 France 27272 ANTON O. OECHSLE, Primary Examiner M. SISKIND, Assistant Examiner U.S. Cl. X.R.

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