Thrust alludes to the force that pushes or spurs a body to move in a particular direction. Regarding ships, Thrust can be defined as the propulsive force that drives the vessel through the water against the resistive forces, mostly hydrodynamic.
How Does It Work?
Again, in simple words, the Thrust is derived from the propeller’s action that is caused due to the rotational motion of the blades, which is translated into linear motion.
More specifically, the torque stemming from the rotational motion of the blades is converted into Thrust that produces a large fraction of dynamic force to the surrounding water medium, which in turn, impels the vessel due to the resultant reaction (Newton’s third law).
Returning to the basics of propulsion, it is important to know that the overall Thrust produced due to the propeller action on water is multi-directional in nature, like most other physical forces caused due to external means. However, the axial component of force is the predominant component that triggers the vessel to move ahead or astern, making the net effect of motion in a linear sense.
The smaller component of this Thrust, which is the one acting in the direction perpendicular to the vessel’s motion (the y-component when perceived in a conventional reference coordinate system), is what we call the transverse Thrust.
The effect of this component is not very significant in the resultant motion of the vessel but can be somewhat found from the initial tendencies of motion of the vessel.
About Transverse Thrust
Let us look at this interesting phenomenon in detail. For our better understanding, we take the simplest case of a conventional single-screw vessel with a right-handed propeller, that is, the clockwise motion of the blades when viewed from the aft, causes the vessel to move in a forward direction, that is, in the direction of its bow. Conversely, from the same position of reference, a left-hand motion or a counterclockwise motion of the same set of blades causes the astern Thrust or the tendency of the vessel to move aftward. Now, about transverse Thrust, two cases arise:
- The vessel is in forward motion
When the vessel moves in a forward direction, which is the most common case, the propeller taken for consideration moves in a clockwise or a righthand sense when viewed from behind.
Due to the action of the blade forces in the slipstream of the propeller, there is a high degree of pressure on the starboard side (as the principal action of the propeller blades is towards the right or clockwise direction).
Furthermore, during the initial stages, when the engine power is high, but the resultant speed of the vessel is low, that is, the vessel is gradually accelerating, and the axial Thrust is still not very high enough, the transverse component of the Thrust is more pronounced.
This induces the stern side to turn towards the starboard. This means the bow now turns in the anti-clockwise direction, which is towards the port. This entire couple takes place at the pivot point of the vessel at that time.
Now, as we know, for all practical purposes, for conventional vessels moving ahead, the pivot point is located towards the bow (1/3rd to 1/4th distance of length from the bow) and vice-versa. When a ship is at rest, the pivot point is more or less centered towards the midships.
Look at the below figure. The moment arm or the linear distance between the point of action and the pivot point is way larger in this case concerning the stern as compared to the bow.
Hence, as per the coupling equation for balance:
Fb X Db + Fs X Ds = M
Where the suffixes b and s stand for the bow and stern, respectively. Fb essentially means the force component or the transverse Thrust acting on the front end or bow.
Similarly, Fs stands for the thrust component acting on the aft end or the stern. Db and Ds are the distances from the point of action from the pivot point for the bow and stern, respectively. M is the resultant or net unbalanced moment.
- The vessel is in a backward motion
Now, what happens in the reverse case? As expected, the opposite phenomenon. For the same propeller, the flow dynamics and pressure patterns are reversed, and the transverse Thrust is created at the stern in the opposite direction, that is, port in our case.
Critically, for reverse motions, the pivot point of the vessel is now centred close to the stern region. Again, refer to the below figure. For an initial transverse thrust directed towards the port at the stern for our right-handed propeller, the bow now tends to turn in the clockwise sense, that is, towards the starboard to complete the moment couple. The position of the point of action of this couple, that is, the pivot point, is now very crucial. Now, Db > Ds, which means the moment arm from the bow is greater as compared to from the stern, thanks to the location of the pivot point.
So, taking individual moments, the product Fb X Db is higher at the bow in this case.
Furthermore, the rudder action is not fully effective in this scenario; thus, the effects are increased further. In other words, the steering effects of the rudder are not sufficient enough to suppress the turning action of the transverse thrust component arising from the interaction of the propeller-induced flow and the hull.
Also, due to the hydrodynamics of the flow and the propeller, during an astern move, the pressure build-up on the starboard aft ward part of the hull is quite large. Hence, the transverse Thrust is quite large towards the port at the stern, and so is the moment produced.
As Fb=Fs, the transverse Thrust at the bow is also higher proportionally during an astern move. This high value of Thrust, coupled with the larger moment arm or lever, creates a significantly high value of turning moment at the bow region (towards the starboard in our case).
Henceforth, for all practical purposes, when a vessel is going astern, the effects of transverse Thrust are higher. Thus, there is a significant tendency for the vessel’s heading to turn or drift sideways (towards starboard for a conventional right-handed propeller).
Factors Affecting
Other than the two important cases described above, other factors affect the value or magnitude of the transverse Thrust irrespective of the direction of motion.
As already mentioned above, the value of the transverse Thrust is the highest when a vessel is at low speeds or starting from rest.
This is because, during these periods, the torque produced from the propeller action is more significantly expended in the transverse component of the Thrust as compared to the axial component, as the heading of the vessel is still at lesser speeds.
Thus, at slow speeds, there is a higher tendency of the vessel to turn or change its heading as compared to steady higher speeds when there is a continuously high value of axial Thrust to move the vessel ahead or astern, overcoming the visible effects of transverse Thrust.
Therefore, the highest pronounced effects of the transverse Thrust are when the vessel is moving astern at low speeds.
For all practical purposes, for a vessel moving astern, the average propulsive power of the propeller consumed in the transverse Thrust for a conventional, sea-going commercial ship varies between 10-15 %.
The depth of the water also plays a crucial role. The effects of heading or turning due to transverse Thrust are more pronounced in shallow water than in deeper waters due to hydrodynamic effects on the propeller.
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Source: MarineInsight
