ELECTRICAL CABLE FOR SHIPS TECHNICAL FIELD [1] The present application generally relates to an electrical cable, and more particularly to an electrical cable for ships. BACKGROUND [2] Electrical cable for ships is an important component of the yachts, ships and offshore operation etc. Firstly, the carrying capacity of the electrical cable for ships is not enough, which affects the transmission of electric energy, and the equipment with the electrical cable of the prior art is unable to operate normally so as to bring short life service. Secondly, the conductor of the electrical cable of the prior art has a large DC resistance and large heating loss, so that the use of the electricity power is in effective. The electrical cable for ships in the prior art usually includes only one insulating layer, it is easily to be damaged during the process of repeated bending or rub, bringing unnecessary danger. [3] There is an urgent need to invent a new electrical cable for ships. SUMMARY [4] According to the present application, an electrical cable with long service life, effective electric energy transmission and safe inflaming retarding is provided to overcome the shortcomings of the prior art. [5] To achieve the above objectives, one particular embodiment of the invention pertains to an electrical cable for ships. The electrical cable for ships comprises a conductor; an insulating layer surrounded around the conductors; a sheath coated around periphery of the insulating layer; the conductor is concentrically stranded by inner conductors and outer 1 conductors, the outer conductors are stranded around the peripheral of the inner conductors, the inner conductor is made of copper alloy, the outer conductor is made of copper. [6] Furtherly, the inner conductor and the outer conductor have the same size, the shape of the conductive part thereof in cross-sectional view is circular, the diameter of the inner conductor and the outer conductors are both 2.03mm. [7] Furtherly, at least three stranded layers of the outer conductors are defined around the periphery of the inner conductors, the most inner stranded layer is composed of seven concentric stranded cores, the most outer stranded layer is composed of eighteen concentric stranded cores. [8] Furtherly, the inner conductors consist of seven concentric stranded cores, concentric stranding nineteen cores, thirty-seven cores stranded together. [9] Furtherly, the insulating layer and the sheath have the same thickness of 1.6mm, the insulating layer and the sheath are both made of polyvinyl chloride. [10] The conductor of the invention has the following advantages. The conductor of the present invention includes inner conductors and outer conductors. As the inner conductors are made of copper alloy, the outer conductors are made of copper, DC resistance of the conductors is reduced, carrying capacity is improved, heating loss is decreased, the service life is extended in the end. Compared with the electrical cables in prior art, the electric energy transmission efficiency of the present invention is higher, the electric energy transmission is ensured, and good security is assured. Meanwhile, the insulating layer provides flame 2 retardant, waterproof and protection of the conductors. Furtherly, safety and service life are increased as the sheath is coated around the outside of the insulating layer. [11] Other objects, advantages and novel features of the invention will become readily apparent from the following detailed description when taken in conjunction with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWINGS [12] Fig. 1 is a cross-sectional view of the electrical cable for ships according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [13] The accompanying drawings are provided together with the following description of the embodiment for a better comprehension of the present invention. The drawings and the embodiments are illustrative of the present invention, but are not intended to limit the scope of the present invention. The present invention will now be described in detail with reference to an embodiment thereof as illustrated in the accompanying drawings. [14] In order to better appreciate and understand the present invention, reference is first made to Fig. 1 Fig. 1 shows an electrical cable for ships, the electrical cable includes conductors, an insulating layer 3 and a sheath, the insulating layer 3 is surrounded around the conductors, the sheath 4 is coated around periphery of the insulating layer3; the conductor is stranded by an inner conductors 1 and a plurality of outer conductors 2, the outer conductors are concentrically stranded around the peripheral of the inner conductors 1, the inner conductor 1 is made of copper alloy, the outer conductor 2 is made of copper. The electrical cable of the present 3 invention includes inner conductors 1 and the outer conductors 2, the conductor is stranded by inner conductors 1 and outer conductors 2. As the inner conductors 1 are made of copper alloy, the outer conductors 2 are made of copper, DC resistance of the conductors is reduced, carrying capacity is improved, heating loss is decreased, the service life is extended in the end. Compared with the electrical cables of the prior art, the electric energy transmission efficiency of the present invention is higher, the electric energy transmission is ensured, and good security is assured. Meanwhile, the insulating layer 3 and the sheath 4 are made of polyvinyl chloride (PVC) with elastics, tensile properties and high modulus. The insulating layer 3 provides flame retardant, waterproof and protection of the conductors. Furtherly, safety and service life are increased as the sheath 4 is coated around the outside of the insulating layer 3. [15] Furtherly, the inner conductor 1 and the outer conductor 2 have the same size, the shape of the inner conductor 1 and the outer conductor 2 both in cross-sectional view are circular. The insulating layer 3 and the sheath 4 have the same thickness. In the embodiment of the present invention, the diameter of the inner conductor and the outer conductors are both 2.03mm. The insulating layer 3 and the sheath 4 have the same thickness of 1.6mm. The electrical cable for ships provides effective electrical transmission, stable support and flexible bending, which is easily to manufacture in the condition of same size, also has reasonable cost and good practicability. [16] Still further, as shown in Fig. 1, at least three stranded layers of the outer conductors are defined around the periphery of the inner conductors, the most inner stranded layer is composed of seven 4 concentric stranded cores, the most outer stranded layer is composed of eighteen concentric stranded cores. The electrical cables of the present invention have reasonable and stable structure to withstand high oscillation, repeated drag and stretching on the condition of same size. It is worth mentioned that the layers of the cable are various, such as one layer, two layers, four layer or five layers etc. Alternatively, the inner conductors 1 in the other embodiment consist of seven concentric stranded cores, concentric stranding nineteen cores, thirty-seven cores stranded together. [17] It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 5