Conventional ship propulsion systems and hydro jets differ in many ways. However, they are both responsible for sailing and manoeuvring of ships in a forward or reverse direction. The hydrodynamic jet is what they have in common, but the characteristics of the jets differ. For example, the outflow velocity of hydro jets is much higher than the one of conventional jets. Another very relevant difference is the jet direction towards the river or canal bed when the jet is in the reverse mode. Formulas are presented to estimate the flow velocities for two types of hydro jets and compared with formulas for a propeller jet and a free jet. The high flow velocities at the bed in reverse mode can result in scour, therefore, also formulas are presented to estimate the scour depth.