Rapid shaft stop devices and transmissions are described that utilize permanent magnets for coupling and/or braking and are useful for electronic propeller guards and other equipment. In an embodiment, one or more capacitive discharge pulses are used to rapidly stop a shaft. A magnetic transmission is provided having axially oriented magnets on each side of an air space junction that transmit torque across the junction with a torque/speed profile that particularly suits boat propellers. The junction may include a bearing and allows slippage when the propeller resistance exceeds a given value. This slippage acts as a variable gear reduction. One or more electromagnets may be energized and thereby add to or subtract from one or more magnetic fields and provide electronic control of torque and of gear reduction ratio for devices such as watercraft drive systems.