Ahead Movement of the Propeller
The effect of transverse thrust whilst making an ahead movement is arguably less worrying than that of an astern movement, perhaps because the result is less noticeable. It is sufficient to summarize the main factors which are evident with an ahead movement of a right handed propeller.
(a) The helical discharge from the propeller creates a larger pressure on the port side of the rudder.
(b) A slight upward flow from the hull into the propeller area puts slightly more pressure onto the down sweeping propeller blades.
(c) It was evident during the tests that the speed or flow of water into the propeller area is uneven in velocity.
The net result is a tendency for a right handed propeller to give a small swing to port when running ahead. Whilst this may be noticeable in calm and near perfect conditions it is easily influenced by other likely factors such as wind, current, shallow water, tugs, rudder errors, and so on.
Astern Movement of the Propeller
The importance of transverse thrust when using an astern movement is of much greater significance to the ship handler. The helical discharge, or flow, from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so it behaves quite differently.
On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force in tonnes that is capable of swinging the stern to port giving the classic 'Kick Round' or 'Cut' of the bow to starboard.
Force in Tonnes
Mainly a function of water flow, the transverse thrust can be increased or decreased by varying propeller rpm. This in turn varies the magnitude of the force in tonnes applied to the quarter and it can be viewed clinically as one of the forces available to the ship handler in much the same manner as rudder, tug or bow thruster forces. It is, however, a weak force and can be roughly calculated if the shp of a particular ship is known.
For example let us take a ship of 80,000 dwt with a full ahead of 20,000 shp. If full astern is only 50% of this then it only has a maximum of 10,000 shp astern.
For practical purposes it can be taken as a rough guide that transverse thrust is only 5 to 10% of the applied stern power therefore in this case at best a force of 1,000 shp or 10 tonnes. (100 shp appx 1 tonne)
Whilst shaft horsepower is an important factor in determining the magnitude of transverse thrust and how much a ship will cut when going astern a further consideration must be the position of the pivot point.
Pivot Point and Transverse Thrust
Vessel Making Headway
Looking at another ship, this time of 26,000 dwt with a maximum of 6,000 shp astern, it can be seen that shp relates to approximately 6 tonnes of force on the starboard quarter. When the ship is making slow enough headway for the propeller wash to reach the hull, it is acting upon a pivot point that is forward and thus a turning lever of 110 metres. This creates a substantial turning moment of 660 tonne-metres.
The forward speed of the ship must be considered because at higher speeds the full force of propeller wash will not be striking the quarter. As the ship progressively comes down to lower speeds and with the pivot point still forward, the magnitude of transverse thrust will slowly increase reaching its peak just prior to the ship being completely stopped. It is an unfortunate fact of life that at the slower speeds approaching a berth, if stern power is applied, transverse thrust is likely to be at its maximum.
Vessel Making Sternway
With the same ship making sternway the pivot point will now move to a new position somewhere aft of amidships. With the propeller working astern the flow of water on to the starboard quarter is still maintaining its magnitude as a force of 6 tonnes but is now applied to a reduced turning lever of 40 meters. Unlike the situation with headway, we now have a reduced turning moment of 240 tonne-meters with sternway.
In the first instance, this may not seem strikingly important. It must be remembered, however, that transverse thrust may be a poor force in comparison to other forces such as wind and tide. With the example of sternway, a wind acting forward of the pivot force may be strong enough to overcome that of transverse thrust.
It is sometimes apparent that a ship when using stern power in the close proximity of solid jetties, banks or shallow water will "cut" the wrong way. There are two possible causes for this occurrence and only a pilot's local knowledge is likely to pinpoint them.
The first is a phenomenon known as the "Wedge Effect". This occurs when the ship with a fixed pitch right-handed propeller has a solid jetty or other vertical obstruction close to its starboard side. If excessive stern power is used, the wash created is forced forward between the ship and the obstruction.
If we look at the above figure it can be seen that if the flow of water is restricted then a force is exerted on the ship forward of the pivot point. This is particularly apparent when the ship is stopped or making sternway. The
Alternative Design Features
Throughout these examples, we have, for practical purposes, adopted a simplistic approach by only considering a fixed-pitch right-handed propeller. There are of course ships with fixed pitch left-handed propellers, propeller tunnels, and controllable pitch propellers, the latter becoming increasingly more common.
Left Handed Propeller
With a left-handed propeller it is simply a case of remembering that the results of the transverse thrust are the opposite in so much that the flow of water from the propeller working astern is up and on to the port quarter and not the starboard quarter. In basic terms, the "cut" of the bow is therefore to port when working the propeller astern.
Controllable Pitch Propeller
The controllable pitch propeller rotates constantly in the same direction no matter what movement is demanded of it. Viewed from astern, a clockwise rotating propeller is still rotating clockwise with stern power, only the pitch angle of the blades has changed. This gives the same effect as a conventional fixed pitch left-handed propeller, which is also rotating clockwise when going astern so the bow will swing to port.
Similarly, if a variable pitch propeller constantly rotates counterclockwise when viewed from astern, this will be the same as a fixed-pitch right-handed propeller which is also rotating counter-clockwise during an astern movement, the bow will thus swing to starboard.
For economical purposes, propellers in shrouds or tunnels are growing in number, even on large VLCCs. This ultimately has some bearing upon transverse thrust because they alter significantly the flow of water exiting the propeller area. It may be more concentrated and is likely to impose an equal thrust upon both sides of the hull thus resulting in little or no transverse thrust.
Finally, hull design features may also play a significant part in altering this simplistic and traditional concept of transverse thrust. It is possible, for example, because of a different hull shape or length to breadth ratio, for the point of impact of water flow to be much closer to the position of the pivot point when going astern. In such a case, transverse thrust, although relatively pronounced with headway, maybe surprisingly weak with sternway, to the extent that the bow may literally fall off either way, particularly if influenced by wind or shallow water.