Automatic speed control

The integrated navigation equipment often includes the automatic speed control of the vessel. There are usually two automatic speed control modes in integrated navigation equipment. The most common control mode is the setting of the standard speed, and it can e.g. be called Set Speed mode, which refers to the speed set by the user. When the speed control is activated, the automatic system chooses the vessel’s instantaneous speed as the set value. In addition to this, the integrated navigation system can follow e.g. the speed profile saved in the pre-programmed route plan.

The own control systems of the propeller equipment and the main engines partly affect the changes in the speed. When the speed is increased on modern vessels, the main engines’ own load control automation delays the increase of the propeller pitch and the number of revolutions. In this way the user can usually safely ask for the propeller performance to be increased without this leading to an overload of the main engines. This also applies to the automatic speed control. In a speed profile saved in the route plan, the increase of speed can vary e.g. from twelve knots to twenty knots when one point of definition is used (Figure 71). In an acceleration situation the flow rate of water over the rudder increases, and this has a positive effect on the manoeuvrability of the vessel. Thus it is safe to increase speed if it only is possible with respect to the fairway area and traffic situation.

S1/2004M b Practices in Pilotage – Past, Present and Future

Figure 71. The speed setting in the waypoint is 20 knots. It is possible to increase speed from twelve knots to for example twenty knots by one change.

The situation is different when the speed is reduced. Because decreasing propeller performance does not cause overload problems to the main engines, reduction of the propeller pitch and number of revolutions only depends on the response of the control mechanics. When the speed is reduced, the propeller thrust can drop to zero in a couple of seconds. The manoeuvrability of the vessel must thus always be taken into consideration when reducing speed and speed must be reduced carefully. This factor must be taken into consideration also when defining the speed profile in the route plan. If the automatic speed control of the route plan does not include protection logic to change the track speed, the speed must be reduced gradually along the track (Figure 72). This can happen by adding into the route plan several successive points of definition, in which the speed is changed by small steps. It is safest to determine the change points of speed on the straight sections of the route. The system can also include a parameter, which is set separately. It determines the rate of reduction of propeller performance. One single waypoint is enough to change speed in that case, and the automatic system takes care of reducing the speed of advance gradually in such a way that the vessel does not lose its manoeuvrability.

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Practices in Pilotage – Past, Present and Future

Figure 72. The speed of the vessel is reduced in the route plan first by e.g. half a knot and then with the intervals of one knot. The distance between the point of definition is e.g. on a ro-ro vessel approximately one vessel length. The distances defined for tankers are longer.

All the levers of the engine order telegraph located on the bridge usually follow the changes in setting of the active manoeuvring place or the automatic speed control. In that case it is possible to follow the functioning of the automatic system easily by monitoring the movement of the levers of the engine order telegraph. In that case it is easy to switch off from automatic control, because the switch should never generate a change from one speed mode to another. The control mode which controls the speed according to the schedule is called e.g. Arrival Mode. The desired time of arrival to the port of destination is fed into the system, and the programme calculates the target speeds for different track legs by taking the speed restrictions into account. If the time of arrival is set too early, the programme calculates the schedule at the maximum allowed speed profile, and tells the user how much the vessel will be late. The automatic system thus always follows the speed restrictions which are saved in the route plan despite the schedule.

It is usually possible to set the limiting values to the automatic speed regulator; the regulator functions within the limiting values. The appropriate regulating limit is ± 0.2 knots in sheltered fairways and ± 2.0 knots at open sea. It is also possible to set the maximum value of speed restrictions. Moreover, it is useful to set the alarm limit of low speed, because it gives a pre-warning of too low a speed, which would cause the loss of manoeuvrability.

It is difficult to follow the changing speed restrictions of a fairway in the archipelago, because the changes in the propulsion performance slowly affect the vessel’s speed of advance. The major advantage of the automatic speed

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control system is how easy it is to follow the speed limits. The automatic speed control allows the person providing pilotage to concentrate on keeping the course and monitoring position determination. Quick reduction of speed in special situations must always be done by using manual steering.