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A boat can have a propulsion unit connected to a rear surface of a transom plate of a hull. The propulsion unit can have a propeller for producing thrust, a power device for driving the propeller, and a power transmission mechanism for transmitting power from the power device to the propeller. The p

A boat can have a propulsion unit connected to a rear surface of a transom plate of a hull. The propulsion unit can have a propeller for producing thrust, a power device for driving the propeller, and a power transmission mechanism for transmitting power from the power device to the propeller. The propulsion unit can also include a braking device configured to exert a generally vertical upward force on the propulsion unit during a braking operation.

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What is claimed is: 1. A propulsion unit configured to be connected to a transom plate of a hull, the propulsion unit comprising a stern drive or an outboard motor configured to be supported by a transom plate of a boat, the propulsion unit also including a propeller configured to produce thrust, a

What is claimed is: 1. A propulsion unit configured to be connected to a transom plate of a hull, the propulsion unit comprising a stern drive or an outboard motor configured to be supported by a transom plate of a boat, the propulsion unit also including a propeller configured to produce thrust, a power device configured to drive the propeller, a power transmission mechanism configured to transmit power from the power device to the propeller, a cavitation plate disposed above the propeller and comprising at least a rearward part, and a braking device configured to exert a generally upward force on the propulsion unit, wherein the braking device comprises a resistance member forming part of the cavitation plate provided on the propulsion unit forward of the rearward part and extending along left and right sides of the propulsion unit, a resistance member opening/closing mechanism configured to open and close the resistance member such that, in the open position, the resistance member acts directly on the water to guide the water to flow downwardly relative to a flow of water past the propulsion unit during forward movement through water, and a brake operation device configured to operate the resistance member opening/closing mechanism to open the resistance member during cruising, wherein the rearward part of the cavitation plate comprises a forward edge, the resistance member comprising a rearward edge, the rearward part and the resistance member being arranged such that the rearward and forward edges are aligned when the resistance member is in the closed position. 2. The propulsion unit according to claim 1, wherein the resistance member, which forms a part of the cavitation plate, is pivotally connected to a lower case of the propulsion unit, and wherein the opening/closing mechanism comprises a cylinder and rod arrangement configured to move the resistance member between the opened and closed positions. 3. The propulsion unit according to claim 1, wherein the brake operation device comprises a brake pedal hydraulically connected to the opening/closing mechanism. 4. The propulsion unit according to claim 3 additionally comprising a spring configured to return the brake pedal to a resting position, against forces of the hydraulic connection between the brake pedal and the opening/closing mechanism. 5. The propulsion unit according to claim 1, wherein rearward part of the cavitation plate comprises a downwardly inclined edge and the resistance member comprising an upwardly inclined edge configured to engage with the downwardly inclined edge. 6. The propulsion unit according to claim 1, wherein rearward part of the cavitation plate comprises a stop configured to prevent the resistance member comprising from moving past the rearward part. 7. The propulsion unit according to claim 1, wherein rearward part of the cavitation plate comprises a stop configured to prevent the resistance member comprising from rotating upwardly past the rearward part. 8. A propulsion unit configured to be connected to a transom plate of a hull, the propulsion unit comprising a propeller configured to produce thrust, a power device configured to drive the propeller, a power transmission mechanism configured to transmit power from the power device to the propeller, and a braking device configured to exert a generally upward force on the propulsion unit, the propulsion unit further comprising a first drive device comprising a tilt drive device and a trim drive device configured to rotate the propulsion unit about a tilt shaft over a tilt range of movement and a trim range of movement, respectively the braking device having a second drive device configured to rotate the propulsion unit about the tilt shaft further toward a rear surface of the transom plate beyond the tilt and trim rangers of movement. 9. The propulsion unit according to claim 8, wherein the braking device further comprises a resistance member provided on the main propulsion unit, a resistance member opening/closing mechanism configured to open and close the resistance member such that, in the open position, water is guided to flow downwardly, in a direction generally perpendicular relative to a flow of water past the propulsion unit during forward movement through water, and a brake operation device configured to operate the resistance member opening/closing mechanism to open the resistance member during cruising. 10. The propulsion unit according to claim 9, wherein the resistance member is disposed below a cavitation plate on the propulsion unit. 11. The propulsion unit according to claim 8 additionally comprising a bucket supported at the lower part of the propulsion unit, the bucket including a water-flow relief opening. 12. The propulsion unit according to claim 8, wherein output from the power device is decreased in association with the brake operation. 13. The propulsion unit according to claim 12, in combination with a boat having the hull and the transom plate disposed at a rear end of the hull, the propulsion unit being supported by the transom plate. 14. The propulsion unit according to claim 12, wherein the power device is an engine. 15. The propulsion unit according to claim 12, wherein the braking device is configured to cause the generally upward force to be sufficiently large to cause the bow of the hull to fall and to thereby make more contact with a body of water in which the hull is operating to thereby enhance a braking operation, when the braking device is activated. 16. The propulsion unit according to claim 8 additionally comprising a controller configured to control a power output of the power device, the controller configured to decrease a power output of the power device when the braking device is activated. 17. The propulsion unit according to claim 9, wherein the resistance member is spaced from the propeller such that the resistance member is not impacted by water discharged by the propeller. 18. The propulsion unit according to claim 8, wherein the braking device comprises a brake operation device configured to allow an operator to activate the second drive device. 19. The propulsion unit according to claim 18, wherein the second drive device comprises a cylinder, a piston moveably mounted in the cylinder, and a rod connected to the piston, the piston dividing the cylinder into a first chamber and a second chamber. 20. The propulsion unit according to claim 19, wherein the cylinder is pivotally supported by the propulsion unit with a support shaft. 21. The propulsion unit according to claim 19, wherein the brake operation device comprises a brake pedal, a hydraulic pressure feeder device, and at least a first brake hose connecting the hydraulic feeder device with the second chamber, wherein the hydraulic feeder device is configured to feed hydraulic fluid to the second chamber when the brake pedal is activated. 22. The propulsion unit according to claim 19, wherein the brake operation device comprises a brake pedal, a hydraulic pressure feeder device, and at least a first brake hose connecting the hydraulic feeder device with the second chamber and a second brake hose connecting the hydraulic feeder device with the fist chamber, wherein the hydraulic feeder device is configured to feed hydraulic fluid to the first chamber through the second hose when the brake pedal is released which causes hydraulic fluid to move from the second chamber to the hydraulic feeder device through the first hose. 23. The propulsion unit according to claim 18, wherein the brake operation device operates with hydraulic pressure. 24. The propulsion unit according to claim 18, wherein the brake operation device comprises a brake pedal and a hydraulic pressure feeder device configured to direct hydraulic pressure to the second drive device when the brake pedal is activated. 25. The propulsion unit according to claim 18 additionally comprising a braking display device configured to display a braking condition of the braking device, the braking condition display device including a brake operation sensor, a controller, and a light-emitting display, the brake operation sensor configured to detect operation of the brake operation device and to send a brake operation detection signal to the controller, the controller being configured to activate the light-emitting display based on the signal. 26. The propulsion unit according to claim 25, wherein the light-emitting display is disposed on a rear side of the propulsion unit. 27. The propulsion unit according to claim 25, wherein the light-emitting display is disposed on a rear side of the hull. 28. The propulsion unit according to claim 8 additionally comprising a braking display device configured to display a braking condition of the braking device. 29. A propulsion unit configured to be connected to a transom plate of a hull, the propulsion unit comprising a stem drive or an outboard motor comprising a housing configured to be supported by a transom plate of a boat, the propulsion unit also including a propeller configured to produce thrust, a power device configured to drive the propeller, a power transmission mechanism configured to transmit power from the power device to the propeller, a cavitation plate assembly disposed above the propeller and extending along left and right sides of the propulsion unit, the cavitation plate assembly comprising a first portion fixed relative to the housing and a second deployable portion disposed forward from the first portion, and an actuator configured to move the second deployable portion between a retracted position and a deployed position, wherein the second deployable portion is aligned with the first portion when in the retracted position and is spaced downwardly away from the first portion when in the deployed position. 30. The propulsion unit according to claim 29, wherein the second deployable position is pivotally connected to the housing, such that a rear edge of the second deployable portion moves downwardly away from a forward edge of the first portion when the second deployable portion moves from the retracted position to the deployed position. 31. The propulsion unit according to claim 30, wherein the rear edge of the second deployable position is pivotally connected to the housing, such that a rear edge of the second deployable portion moves downwardly away from a forward edge of the first portion when the second deployable portion moves from the retracted position to the deployed position.