Hydraulic type gearboxes do  not have the ability to be locked when the motor is not running so the shaft is always free to turn when sailing. These use a small pump that is driven off the grearbox on the motor side  only when running to provide oil pressure to energise the clutches that then engage either Ahead or Astern.

Kiwiprops current design offers a solution to this problem with unique W shaped foils at the rear of each blade ensuring stability of the blades when sailing.

This obviates the need to lock the unit in gear and leaves the shaft stationary when sailing.

The type of gearbox you have is information that is required on the the Order Form. Hydraulic type gearboxes are typically found on older model engines   ... eg Perkins 4.108's and on modern higher powered engines.

They are generally of a higher commercial duty rating not seen on pleasure vessels due to their higher costs.

Modern gearboxes and Saildrives with dog, cone or clutch pack  type clutches are engaged mechanically so can be used to lock the shaft when sailing even when the engine is stopped.