PROPELLER FIELD OF THE INVENTION The present invention relates generally to a can propellant system and more particularly to a propellant which is intended to be used in conjunction with a can propellant system. BACKGROUND OF THE INVENTION U.S. Patent No. 4,566,855 discloses a propeller comprising a propeller unit having a plurality of helical vanes which serve to propel the vessel by pushing back the water, a control unit driven by a engine, and a rubber sleeve located between the control unit and the propulsion unit. The connecting portions of the propelling unit, the sleeve and the control unit are provided, respectively, with several elongated projections or retention grooves, which enable the output torque of the motor to cause the control unit to rotate through the motor. the control unit and the sleeve, in such a way that the backward thrust of the water is generated to drive the boat. The elastic sleeve is able to partially absorb the shock force generated instantaneously by the helicoidal blades when a foreign object present in the water hits them, thus mitigating the destructive effect of the impact force on the blades and the propeller shaft of the engine. The prior art propeller described above has design flaws while the elastic sleeve is rather expensive, and while it is difficult to achieve a high quality elastic sleeve and, moreover, as the shock absorbing effectiveness of the elastic sleeve depends to a large extent of the rigidity of the sleeve. For example, if the sleeve is insufficiently rigid, the propelling unit can not be effectively operated to rotate. On the contrary, if the sleeve is excessively rigid, the shock absorbing capacity of the sleeve is greatly reduced. Another U.S. Patent No. 7,826,404 has a propeller consisting of a propelling unit and the control unit, which are provided, respectively, with a plurality of elastic members in the form of rods capable of mitigating a force of impact exerted on them. It is capable of being destroyed to cause the propulsive unit and the control unit to operate at low power, in such a way as to protect the propeller blades and the motor that drives the propeller blades. However, the destruction of the elastic members can result in the incapacitation of the boat. Moreover, the job of replacing the destroyed elastic member can not be done by an amateur without professional help. SUMMARY OF THE INVENTION It is therefore a principal object of the present invention to provide an improved propeller having a transmission unit capable of absorbing or diminishing a shock force exerted by a foreign object on the moving propeller, in such a way that protect the structural integrity of the propeller blades without causing engine power interruption. It is another object of the present invention to provide a propeller with a control unit capable of absorbing a shock force exerted by a foreign object on the propeller in changing form. According to the principle of the present invention, the above objects of the present invention are achieved by a propeller consisting of a propelling unit, a control unit, and transmission units. The propulsion unit and the control unit are provided at the inner and outer ends of the adjoining areas thereof, with several retaining slots for retaining the transmission units capable of transmitting the output torque of the motor from the control unit to the control unit. propulsive unit. The transmission units are tubular in shape and capable of being deformed to absorb a shock force. The objects, features, functions and advantages of the present invention will be more readily understood after a deliberative deliberation of the following detailed description of the embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a perspective view of a first embodiment of the present invention. FIG. 2 shows an exploded view of the first embodiment of the present invention. FIG. 3 shows a sectional view of the first embodiment of the present invention. FIG. 4 shows a sectional view of a portion taken in the direction indicated by a line 4-4 as shown in FIG. 3. FIG. 5 shows a schematic sectional view of the present invention in operation. FIG. 6 shows a sectional view of a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION As shown in FIGS. 1-5, a propeller 10 incorporated in the present invention is composed of a propelling unit 12, a control unit 14, and several transmission units 16 located between the propelling unit 12 and the control unit 14. The propelling unit 12 has an axial portion 20 and three helical blades 26 constructed integrally with the axial portion 20, such that the blades 26 are separated from one another in an equiangular manner. The axial portion 20 has an outer bushing 21 of cylindrical construction for mounting the blades 26, and an inner bushing of cylindrical construction and located in the outer bushing 21. The inner bushing 22 has a first axial hole 23 coaxial with the outer bushing. 21. The first axial hole 23 is provided on the inner wall thereof with eight first retention grooves 24 extending along the direction of the longitudinal axis of the first axial hole 23, such that they are equiangularly spaced from each other . Located between the outer wall of the inner bushing 22 and the inner wall of the outer bushing 21 are three reinforcing ribs 25 that are equiangularly spaced apart from one another. The control unit 14 is tubular in shape and is located in the inner bushing 22. The control unit 14 has a second axial hole 40 which is provided in the inner wall thereof with a plurality of frames 42 extending in the direction of the longitudinal axis of the second axial bore 40. The second axial bore 40 is further provided in the outer wall thereof with the second eight retaining grooves 44 extending in the direction of the longitudinal axis of the second axial bore 40, in such a way that the second retention grooves 44 are located opposite one another. The second retention slots 44 are similar in cross-sectional shape to the first retention slots 24. The cross-sectional shape may be semicircular, arcuate or rectangular. The transmission units 16 are tubular in shape and are integrally manufactured with an expandable metallic material having adequate stiffness. The transmission units 16 are circular in cross section. The transmission units 16 can be fabricated with an elongated metallic material having a round or rectangular cross-section through a helical extension along the longitudinal axis direction of the material at a relatively small pitch. The control unit 14 is first attached to a drive shaft 17 of a motor (not shown in the drawing) and then mounted to the inner bushing 22 of the drive unit 14. Thereafter, four drive units are located. transmission 16 between the first retention grooves 24 and the second retention grooves 44. The inner bushing 22 and the transmission units 16 are pressed with a stop piece 18 and fastened with a nut 15 which engages the drive shaft 17. The control unit 14 is driven by the drive shaft 17 to rotate at high speed, in such a way as to force the propelling unit 12 to rotate and generate the backward thrust of water to drive the can. As shown in FIG. 5, each of the transmission units 16 is exerted by a shearing force at the moment when the blades 26 of the propellant 10 collide with a hard object present in the water in which the propeller 10 operates. The shearing force it can be so destructive that it causes the transmission units 16 to buckle. Such paralysis of the transmission units 16 serves to safeguard the structural integrity of the blades 26 and the drive shaft 17. The transmission units 16 are capable of such a protective event as described above in view of the fact that they are tubular in construction. capable of absorbing a shock force at the moment of the destruction of the tubular construction by the impact force. Indeed, the transmission units 16 have functional advantages insofar as they are expandable, and as long as they are deformed by the impact force exerted on them without separating immediately, and also in as much the original opposite arrangement of the first retention grooves 24 and the second retaining grooves 44 is modified slightly to an alternative arrangement during the course of the collision, thereby causing the deformed transmission units 16 to remain in the retaining grooves 24 and 44, so as to enable normal operation of the propelling unit 12 and the control unit 14. If the transmission units 16 are helical tubular construction, their diameter is shortened first when they are deformed by a force exerted on them in a direction perpendicular to their axis. However, the helical and tubular transmission units 16 are able to recover their original shape after being relieved of a force exerted thereon, provided that the force in question is not so excessive as to cause the permanent deformation of the units. of transmission 16. As a result, the propellant 10 of the present invention is relatively less vulnerable to destruction. Moreover, the helical transmission units 16 of the present invention can be provided, respectively, with a rod-shaped reinforcement unit 19, which is inserted thereto to facilitate the implantation of the transmission units 16 in the first slots of 24 and in the second retention grooves 44. As shown in FIG 6, the control unit 14 has an outer diameter smaller than the inner diameter of the first axial hole 23. For this reason, a gap is formed in the form of ring 46 between the surface of the outer wall of the control unit 14 and the surface of the inner wall of the first axial hole 23 when the control unit 14 is located in the center of the inner bushing 22. The ring-shaped gap 46 serves to provide the transmission units 16 with a space that may be required to accommodate the deformation of the transmission units 16 to improve the shock absorbing effect of the e the transmission units 16.