A combined propeller cap (50, 60, 110, 130) for reducing rotational flow and hub vortex and improving propulsive efficiency, the combined propeller cap (50, 60, 110, 130) has: - a contractive section (53, 63, 113, 133) having a diameter that decreases as it goes away from a propeller; and - a second diffusive section (54, 64, 114, 134) extending from the contractive section (53, 63, 113, 133) and having a diameter that increases as it goes away from the propeller, characterized in that, the combined propeller cap (50, 60, 110, 130) further comprises: - a first diffusive section (51, 61, 111, 131) having a diameter that increases from an end of the propeller as it goes away from the propeller; and - a straight section (52, 62, 112, 132) horizontally extending from the first diffusive section (51, 61, 111, 131); the contractive section (53, 63, 113, 133) extending from the straight section (52, 62, 112, 132) and having the diameter that decreases as it goes away from the propeller; and the second diffusive section (54, 64, 114, 134) extending from the contractive section (53, 63, 113, 133) and having the diameter that increases as it goes away from the propeller, wherein the combined propeller cap (50, 60, 110, 130) is formed at a front portion by the first diffusive section (51, 61, 111, 131) having the diameter that increases from the end of the propeller as it goes away from the propeller, so pressure at a pressure side of the propeller increases and propulsive efficiency is improved, pressure of a flow passing the propeller cap (50, 60, 110, 130) from the propeller is recovered by the contractive section (53, 63, 113, 133), so the propulsive efficiency is improved, and rotational flow (vortex) of a flow from the propeller is weakened by the second diffusive section (54, 64, 114, 134), so hub vortex cavitation is reduced. - a contractive section (53, 63, 113, 133) having a diameter that decreases as it goes away from a propeller; and - a second diffusive section (54, 64, 114, 134) extending from the contractive section (53, 63, 113, 133) and having a diameter that increases as it goes away from the propeller, - a first diffusive section (51, 61, 111, 131) having a diameter that increases from an end of the propeller as it goes away from the propeller; and - a straight section (52, 62, 112, 132) horizontally extending from the first diffusive section (51, 61, 111, 131); the contractive section (53, 63, 113, 133) extending from the straight section (52, 62, 112, 132) and having the diameter that decreases as it goes away from the propeller; and the second diffusive section (54, 64, 114, 134) extending from the contractive section (53, 63, 113, 133) and having the diameter that increases as it goes away from the propeller, wherein the combined propeller cap (50, 60, 110, 130) is formed at a front portion by the first diffusive section (51, 61, 111, 131) having the diameter that increases from the end of the propeller as it goes away from the propeller, so pressure at a pressure side of the propeller increases and propulsive efficiency is improved, pressure of a flow passing the propeller cap (50, 60, 110, 130) from the propeller is recovered by the contractive section (53, 63, 113, 133), so the propulsive efficiency is improved, and rotational flow (vortex) of a flow from the propeller is weakened by the second diffusive section (54, 64, 114, 134), so hub vortex cavitation is reduced.