A fender device for boats, comprising: - a container with a door, said container being configured to be positioned inside a boat and communicating with the outside through a passage opening in a boat side closed by the door, - a winch inside the container, comprising a winch drum and a winch shaft containing inside a compressed air fitting, - a compressed air generator connected to said compressed air fitting, - a fender assembly suitable to be wound on the winch drum and comprising an air chamber made of extensible material in order to be inflated and deflated, an external containment bag for the air chamber and a pressure hose having a first end fastened to the air chamber and a second end fastened to the compressed air fitting so that the air chamber is inflated by said compressed air generator, the fender assembly in deflated condition being adapted to be wound on the drum when the winch turns in a direction and to be unwound from the drum when the winch turns in the opposite direction, for passing through said opening passage in the boat side, characterized in that - the door, suitable to open and close said passage opening, is joined laterally to a pair of racks, the door being provided inferiorly with a sealing sheath, - the fender device further comprising a frame surrounding said opening passage in the boat side, the container being configured to be connected to the boat side by said frame, and the container being further provided with side walls (500, 501) including sliding guides, perpendicular to the frame for the sliding of said racks, as well as a gasket interposed between the door and the frame, - the fender device further comprises a door opening and closing mechanism including a first motor that drives said racks in such a way as to open and close the door, - the fender device further comprises a second motor, the winch being driven by the second motor for unwinding the fender assembly from the winch drum and winding up it on the winch drum, and - the fender device further comprises means adapted to facilitate the unwinding and winding of the fender assembly comprising a plurality of upper rollers and a plurality of lower rollers both rubberized and grooved, supported on said side walls (500, 501) of the container, a third motor driving through a wheel and a plurality of gears conjugated in series, said plurality of upper rollers and said plurality of lower rollers making easy the unwinding and winding of the fender assembly in a deflated condition on the winch drum, the door opening and closing mechanism, the winch for unwinding the fender assembly from the winch drum and winding up it on the winch drum, and the means adapted to facilitate the unwinding and winding of the fender assembly being adjusted for deploying the fender assembly on the boat side and for withdrawing it from the boat side. - a container with a door, said container being configured to be positioned inside a boat and communicating with the outside through a passage opening in a boat side closed by the door, - a winch inside the container, comprising a winch drum and a winch shaft containing inside a compressed air fitting, - a compressed air generator connected to said compressed air fitting, - a fender assembly suitable to be wound on the winch drum and comprising an air chamber made of extensible material in order to be inflated and deflated, an external containment bag for the air chamber and a pressure hose having a first end fastened to the air chamber and a second end fastened to the compressed air fitting so that the air chamber is inflated by said compressed air generator, the fender assembly in deflated condition being adapted to be wound on the drum when the winch turns in a direction and to be unwound from the drum when the winch turns in the opposite direction, for passing through said opening passage in the boat side, characterized in that - the door, suitable to open and close said passage opening, is joined laterally to a pair of racks, the door being provided inferiorly with a sealing sheath, - the fender device further comprising a frame surrounding said opening passage in the boat side, the container being configured to be connected to the boat side by said frame, and the container being further provided with side walls (500, 501) including sliding guides, perpendicular to the frame for the sliding of said racks, as well as a gasket interposed between the door and the frame, - the fender device further comprises a door opening and closing mechanism including a first motor that drives said racks in such a way as to open and close the door, - the fender device further comprises a second motor, the winch being driven by the second motor for unwinding the fender assembly from the winch drum and winding up it on the winch drum, and - the fender device further comprises means adapted to facilitate the unwinding and winding of the fender assembly comprising a plurality of upper rollers and a plurality of lower rollers both rubberized and grooved, supported on said side walls (500, 501) of the container, a third motor driving through a wheel and a plurality of gears conjugated in series, said plurality of upper rollers and said plurality of lower rollers making easy the unwinding and winding of the fender assembly in a deflated condition on the winch drum, the door opening and closing mechanism, the winch for unwinding the fender assembly from the winch drum and winding up it on the winch drum, and the means adapted to facilitate the unwinding and winding of the fender assembly being adjusted for deploying the fender assembly on the boat side and for withdrawing it from the boat side.