By Anoop Thomas
To start of, let us first understand the term Transverse Thrust. What is Transverse. On a ship, anything (motion, bulkhead, force) that acts in the athwartship direction is called Transverse. Now let’s define Thrust. Trust as we know in Physics, is the force exerted per unit area. That means if I load 100 tons of weight spread evenly over a steel plate measuring 1m x 1m, then the thrust on the plate is 100 tons per square meter (100 tons/m2)
Now that we’ve understood what the term means, let us move further, and find out what Transverse Thrust means with respect to ship maneuvering.
Consider a Single Screw ship (ship with only one propeller) with a Right Hand Fixed pitch propeller. For better understanding, let us just say a Right hand propeller has the blades so aligned that, when the propeller turns clockwise (as seen from the aft) the vessel will move forward. A fixed pitch means the angle of the propeller blades cannot be changed. (The technical definition of a pitch is the distance moved ahead during 1 rotation of the propeller.) When such a vessel is at rest, and then starts her engines to move ahead, the propellers will start turning clockwise. Although the propeller is located on the centre line of the ship and with the rudder at mid-ship, you will observe that the vessel, instead of moving ahead on the same course, will drift slightly to the port side. In other words, you will observe that the vessel is turning to port. This turning / swinging is called Transverse Thrust.
Now let us understand the physics behind this apparently abnormal behavior of the ship.
We know that the pressure of the water increases as the depth increases. So it may be said that there is a pressure gradient between the top end of the propeller and the bottom end of the propeller. When the propeller turns, it draws water from forward of the propeller and pushes it aft, and it is this pushing force that is transferred onto the ship. But that is not all the propeller does. As the blades of the propeller are at an angle (like in the case of a ceiling fan – not horizontal and not vertical, but at an angle to the rotor), part of the energy is transmitted abeam of the vessel.
In the illustration, when the blades of the top of the propeller turns clockwise, it not only pushes the water astern but also to the Stbd Side. Similarly, the blades at the bottom, push the water to the port side. If the pressure of the water was equal at both Top and Bottom, then the force to stbd and port would have been equal and opposite and hence would have canceled each other. However, since there is a pressure gradient, between the top and the bottom, with the pressure at the bottom being greater than that at the top, the pushing force exerted to the Port (due to the bottom blades) would be greater than the force to starboard. Therefore, the resultant is a pushing force to the Port side. This will cause the stern of the vessel to swing to Stbd (as the propeller is attached to the stern of the vessel). As a result, the bow will swing to Port. (See illustration)
This phenomenon happens only when the vessel begins to move. Subsequently, as the vessel gathers speed, the Port and Stbd forces equalize themselves and nullify this Transverse Thrust.