Learning by turning
This article focuses on a very specific issue, which has had not much attention up till now but may play a role during the use of a turning circle or similar situations. It may not appear very urgent outside certain specific situations, but it is interesting, and bears further investigation.
During a simulator training course for pilots, attendees noticed that although the turning circle or turning space on the river appeared to be large enough, the pilots often had to give astern on the engine because they ran out of space. How was that possible? The pilots and the instructor did not understand why this was happening.
The reasons why this could happen will be addressed below so that other pilots either can learn from it or can confirm it from their own experience.
The more basic requirements for turning a ship in a turning circle will be addressed first.
Turning with no current
When entering a turning circle, it is best to have the ship stopped in the centre of the circle. If the ship still has some ahead or astern speed when it starts turning, the turning space may be too small and engine power and/or tug power is then needed to take speed off the ship.
Turning on a river with current
When turning on a river with current, it is important that the ship is stopped in the water, which means speed through the water is zero, before the ship starts the turn. If the ship still has some headway or sternway, the available turning space might be too small, unless engine and/or tugs are used to halt the forward or astern movement of the ship. Starting the turn when the ship is stopped in the water is always safest.
Ship getting ahead speed when turning
Turning with one bow tug
Now we turn to the specific issue mentioned in the introduction. The graphic above right, taken from Tug Use in Port (4th edition, page 133), shows a ship gathering speed while turning (Figure 1). The figure shows a tug fastened to the bow of a tanker of 200,000 tons displacement dead in the water, pulling constantly with 50 tons perpendicular to the bow. There is no wind and no current and the ship is in deep water. This simulation shows that while turning, the ship gets a speed of up to as much as 2.5 knots. How is it possible for a ship to gather speed while there seems to be no forward directed force?
In figure 2 a container ship is pulled around by a tug, in the same way as the tanker in figure 1. The red dot is the pivot point of the ship and the yellow dot the centre of gravity. What you see is that the centre of gravity is trying to follow an arc.
It is like a car driving through a bend in the road. Because of the friction between tyres and road, which is the centripetal force, the car can take
the bend with speed. On a slippery day that friction is gone and the car flies out of control by the centrifugal force. This is the same force that is driving the ship forward.
The centrifugal force is equal to the centripetal force and can be calculated with the formula: Fc = mv2/r
Where m is the mass of the vessel, v the speed in m/sec along the arc and r the radius of the arc in metres; the distance between the yellow and red dot. This force drives the ship forward. Both the actual position and the forward speed are shown in figure 2.
The larger the mass or the higher the rotation speed, the larger the forward driving force. On the other hand, the larger the mass the more force is needed to drive the ship forward. As the ship gathers forward speed, the centre of hydrodynamic forces will move forward, resulting in the pivot point moving forward too. The tug has then to overcome a higher force. As the power of the tug is limited to 50 tons, the rate of turn will decrease, and consequently so will the forward driving force. The decreasing rate of turn and forward moving pivot point can be seen when studying figure 1. However, for some reason the speed increases there instead of decreasing.
This is admittedly a situation which seldom occurs; it is rare for just one tug to pull a large ship around. It could happen, for instance, if the aft tug has a break down and just one tug is then left.
Nevertheless, we will have a look at the consequences for the required turning area. Two aspects play a role: The pivot point and the forward movement of the ship.
In a more usual situation with one tug forward and one aft, both of equal force, the ship will pivot round a point in the middle of the
ship. The minimum turning space needed is then equal to one ship
length, not counting the room needed for the tugs (see circle 1, below). With just one tug forward, the situation is different. The ship will then pivot around a point further aft (see the red dot in figure 2). This has the consequence that more room is needed for turning the ship, in this case up to turning circle 2 (see figure 3).
As the ship also moves forward, yet more additional room is needed, even beyond circle 3 in figure 3, unless engine or tug actions are taken to stop the forward movement of the ship.
Circle 2 is the turning circle recommended by PIANC (Permanent
International Association of Navigation Congresses), being two times the ship length. As we have shown here, for a ship with just one tug forward, even that rather large turning circle is still too small. The ship needs at least the space given in turning circle 3, if no engine actions
are taken when turning.
Three tugs
Now we will look at a scenario very similar to that observed by the pilots mentioned in the introduction. Here, a containership is being turned by three tugs of 50 tons power; two forward and one aft (figure 4).
The tug is pulling constantly perpendicular to the ship.
The red dot is the pivot point and the yellow dot is the centre of gravity of the ship.
Figure 4 Containership, length 300 m, displacement 142,000
tons, pulled around by three tugs. Plots each two minutes.
Note: Tugs are added afterwards.
It can be seen that once the ship has been turned about 120 degrees, less space is required for turning than with just one tug forward. Note, however, that it will still require more space than if we were using two tugs with the same power of 50 tons forward and aft, because the ship still has ahead speed (again, not counting manoeuvring room for the tugs). This is because the addition of the two tugs, one forward and one aft, does not create a forward moving force on the ship. They just turn the ship around the pivot point. The forward moving force is still only created by the original 50 tons tug forward, although the forward moving force decreases, as explained previously. What the two added tugs do is increase the turning speed.
Consequently, the ship has less time to move forward than with one tug at the bow, but it is still moving forward. Note, for instance, the position of the ship when it has turned about 170 degrees. This forward movement can become a problem if the pilot has not noticed that it is happening in time to counter it.
Practical testing
Some simulator institutes were generous enough to carry out some
simulations to verify whether the turning effect on ship speed described above, and as set out in Tug Use in Port, is correct. One result corresponded rather well with that described in Figure 1. All outcomes showed a difference in speed and ship track after the ship had turned 90-120 degrees. Most attention has therefore been paid to the turning phase up to about 120 degrees. One simulator manager remarked: ‘I don’t know how realistic it [the simulation] is.’ It is indeed questionable to what extent the software used in simulators can present realistic ship behaviour with respect to the centrifugal force generated while turning and how the tugs operate in practice. The change of the centre of hydrodynamic force towards a more forward lying position on the ship due to the gradual increase in ship’s speed, causing a delay in the rate of turn, also requires careful attention. So, too, does the effect of a small under keel clearance. This notwithstanding, the simulations had one thing in common. All of them showed a ship gathering speed while being turned by a force at the bow, in particular during the first part of the turn. Now the tugs. In practice, the forces at the bow and stern will be generated both by the tug(s) and by the vessel’s own bow and stern thruster, if one or both side thrusters are available. It can be a problem for the tugs to remain at an angle of exactly 90 degrees to the moving ship at all times. As the ship gathers speed, the tugs will try to stay in a good safe position to pull. This means the tugs will try to keep pace with the ship by staying in a somewhat more forward lying position, as shown in figure 4. This has the consequence that the tugs can easily increase ship’s speed due to the tug force vector created in the ship’s longitudinal direction. The speed shown in figure 4, when the ship has turned about 45-120 degrees, might therefore easily be higher than expected. This results in the ship crossing the PIANC turning circle line and so running out of space. This might explain what happened during the pilot simulation training mentioned in the introduction, if it has all been well simulated.
The tugs could also stop the forward movement. However, they should then be told to do so by the pilot. If the pilots had the opportunity to check the ship’s speed regularly, they could have ordered astern on the engine to stop the forward movement.
There is another effect of the tugs which hardly can be represented by a simulator. In the situation shown in figure 4, the propeller wash of the tugs may hit the ship’s hull. Depending on the angle of inflow, this may either help or counteract the turning process. An effect to be
aware of!
It has been observed that tankers dead in the water will start moving forward due to the propeller wash of pulling tugs directed at the bow. It is doubtful whether this effect will be experienced with more slender ships such as container vessels.
Conclusion and recommendation
During a simulator training course pilots experienced a problem when turning a ship on the river. They ran out of space, even though the available turning room appeared to be large enough according to expectations. There is a possibility that this is caused by the following reasons:
If a ship dead in the water is turned by tug(s) and/or bow thruster forward and tug(s) aft, all pulling perpendicular to the ship, and total power forward is significantly larger than total power aft, the ship will start to move forward due to a centrifugal force. When the ship is gathering forward speed, the tugs will try to keep pace with the ship by pulling from a somewhat more forward lying position, with the consequence that they are imposing a further speed-increasing force on the ship.
The aft lying pivot point on the ship, the centrifugal force and the speed-increasing force created by the forward tugs may all lead to a situation where the turning ship runs out of turning space.
The propeller wash of tugs hitting the ship’s hull may also affect the turning of the ship or ship’s speed. More research is needed to obtain a better insight into ship’s behaviour when turned by more (tug) power forward than aft, or when the vessel is being turned by just one tug forward or bow thruster only. This should also include the effect of tug propeller wash hitting ship’s hull, under keel clearance and ship size. Simulation of all the factors that play a role during the turning is a real challenge. However, the ‘hidden force’ has been clarified. It is now clear that in order to neutralize that hidden force, as far as possible equal power forward and aft should be used for turning a ship. If pilots are able to note the data of a turn in the situation described above, such as sideways speed aft and forward, rate of turn and forward speed, it would certainly give better insight into the process.
Special thanks to:
Captain Bo Caspersen, General Manager, KASI Center for Maritime Simulation & Innovation, Malaysia
Professor Paul Brandner and Damien Freeman of the University
of Tasmania for their generous support
Earlier published in Seaways, January 2025