text: Expressed in metric units, the "specific speed" is n = 0.2626 n /h
  * where n is the wheel speed in rpm
  * P is the power in kilowatts
  * h is the water head in meters
  The factor 0.2626 is only required when the specific speed is to be adjusted to English units. In countries which use the metric system, the factor is omitted, and quoted specific speeds are correspondingly larger.