INTERACTION

            Figure: Pressure field in headway (Click picture to enlarge)

Figure: Interaction while overtaking (Click on image to enlarge)

Figure: Interaction (Click on image to enlarge)

a) Prior to the maneuver each ship remains in the centre of the channel for as long as possible. Failure to do so, could expose either ship to bank effect, leading to a sheer across the path of the oncoming ship or grounding.
b) Speed should be low to reduce the interactive forces. There is then, plenty of reserve power for corrective 'kicks ahead'.
c) If the ships pass from deep to shallow water, at any time during the maneuver, the forces will increase drastically and extreme caution should be exercised.
d) The smaller of two ships and tugs, are likely to be the most seriously affected. Large ships should be aware of this and adjust their speed accordingly.
e) Figures above illustrate the anticipated sheers that may develop throughout each maneuver and the maximum corrective helm that may
be required, in this case 35°.
f) The engines should be brought to dead slow ahead for the maneuver, particularly turbine or fixed pitch propeller ships, so that power is
instantly available to control the ship with 'kicks ahead'.
g) On completion of the maneuver each ship should regain the centre of the channel as quickly as possible to avoid any furtherance of bank effect.

It should be remembered that the strength of interaction zones and the distance they extend out from the ship can increase dramatically, not only with a small increase in ship speed but also if the ship passes into shallow water and the pressure zones are restricted.

When a tug is working its way in towards the ship's forebody, with the intention of passing a line forward, it may pass through one or more of these important areas (see following figure) and experience adverse handling characteristics.

In position 1 for example, and similarly all the way down the side of the ship, if the tug is allowed to get in too close, it might, despite all the efforts to prevent it, be bodily and inexplicably sucked into the ship's side. This might occur unintentionally in strong winds, when a tug is in the lee of a large ship which is drifting down upon it. Once trapped alongside it can be extremely difficult to get off again, unless the ship's speed is substantially reduced thereby relaxing the strength of the suction area. For the unfortunate tug master, this can be the start of a chain of handling difficulties which can accumulate and end in disaster.

In position 2 the tug is again working in close to the ship's side and passing through an area where it is half in and half out of the respective pressure and suction zones. A positive force is pushing the bow out from the ship, while another force is pulling the
stern into the ship. This combined turning couple will create a strong shear away from the ship which will require rapid and bold use of both helm and power to correct it.

In position 3 when working close in under the bows, the tug may have run slightly ahead of the ship's bow pressure zone and consequently find a very strong positive force being exerted on the stern and rudder. This will give a similar effect to that of
putting the helm hard over towards the bow of the ship and the tug could sheer rapidly across its path. Bold corrective counter rudder with power will be needed instantly, but even then may be ineffective against a force which can be very strong.
If the ship's speed is too high and the interaction forces correspondingly severe, or if the tug master fails to keep control, the tug can find itself in position 4 with alarming and fatal rapidity. The consequences may be flooded decks and serious collision damage, particularly from underwater contact with the ship's bulbous bow, with the possibility of capsize and loss of life. A sudden and catastrophic loss of stability is the most likely cause of a capsize and this can occur even with a very slight collision. Tugs, it should be noted, roll over and flood extremely quickly, thus affording little time for the crew to escape!

When a tug is approaching to pass a line aft it is also likely to feel the effect of interaction and may, similar to the forward tug, experience some handling difficulties. This will be particularly evident if the ship's speed has not been sufficiently reduced.
The resultant interaction forces may be too strong, causing vigorous suction, or low pressure area, around the after body of the ship (see following figure). This is compounded by the more obvious and widely recognized risk that is associated with working under the stern, in close proximity of the ship's propeller.

When a tug makes its approach and is in, for example, position 1, it will be influenced by this suction and may start to take a sheer towards the ship's stern. As this maybe a low pressure area, the tug will have less water resistance ahead of it and may also experience an unexpected increase in speed. Unless quick action is taken, with counter rudder and appropriate power, the tug will be drawn unwittingly into the stern of the ship and become stuck somewhere alongside in the region of position 2.

Extreme cases are possible, when the forces are so strong that the tug fails to respond to full rudder or power and may inadvertently land heavily alongside. If the ship is in ballast, partly loaded or has a large overhanging stern the tug could be drawn into position 3, with the possibility of serious structural damage to the tug's superstructure and upperworks. The danger from the propeller is a more obvious threat and, naturally, care should be exercised whenever a tug is working close under the stern.