PROBLEM TO BE SOLVED: To provide an underwater propulsion device of waterborne vehicles with reduced load on the motor.SOLUTION: An underwater propulsion device 20 of waterborne vehicles with an on-water floating part 2 that a user boards is connected to the on-water floating part 2 via a prop 3 and extends in the propulsion direction, and comprises a hollow main body part 21 the inside of which is partitioned into a first chamber 27 of the bow side and a second chamber 28 of the stern side, a motor 22 housed in the first chamber 27, a propeller 23 housed in the second chamber 28, and a power transmission shaft 24 extending in the propulsion direction and connecting the propeller 23 with the motor 22. The first chamber 27 is waterproofed, the second chamber 28 has a water-conveyance hole 29 formed closer to the bow side than the propeller 23 and formed in the circumferential direction of the power transmission shaft 24, and a jet flow hole 30 formed at the edge of the stern side, and the propeller 23 is formed such that its outer diameter is smaller than the inside diameter of the first chamber 27.SELECTED DRAWING: Figure 7
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