The energy losses in a reaction turbine may be very simply described as the so-called shock loss at entrance to the runner if the relative velocity of the water leaving the guide vanes is abruptly changed in either magnitude or direction or bolt when it enters the runner; fluid friction in the casing, through the guide-vane passages and through the runner passages; kinetic energy loss due to the absolute velocity head of the water at discharge from the runner, of which up to 80 percent might be regained in the most.