22152637 - Notes - on - Ship - Handling

22152637 - Notes - on - Ship - Handling

(Parte 1 de 6)

1THE PIVOT POINT 1 to 6
2SLOW SPEED CONTROL 1 to 14
3TURNING 1 to 12
4THE EFFECTS OF WIND 1 to 17
5ANCHOR WORK 1 to20
6INTERACTION 1 to 14
7THE USE OF TUGS 1 to 42
8THE USE OF BOW THRUSTERS 1 to 2
9SPECIAL PROPELLERS AND RUDDERS 1 to 15
10TWIN SCREW WORK 1 to 14
1SHIP TO SHIP OPERATIONS 1 to 6

Chapter Pages Introduction i to i

APPENDICES 1. SBM OPERATIONS 1 to 20

Unless stated otherwise each example assumes a ship on even keel in calm conditions and still water. In this situation no forces are involved and the ship has its centre of gravity approximately amidships.

Figure 1-1 Pivot point - stopped

Making Headway

Two forces now come into play, firstly the forward momentum of the ship and secondly, longitudinal resistance to forward momentum, created by the water ahead of the ship. These two forces must ultimately strike a balance and the pivot point moves forward. As a rough guide it can be assumed that 25% of the ships forward momentum, at constant speed, is spent in overcoming longitudinal resistance and the pivot point will be approximately 1/4L from forward.

Figure 1-2 Pivot point - making headway

Making Sternway

The situation is now totally reversed, the momentum of sternway must balance longitudinal resistance, this time created by the water astern of the ship. The pivot point moves aft and establishes itself approximately 1/4L from the stern.

Although not intended, some publications may give the impression that the pivot point moves right aft with sternway. This is clearly not correct and can sometimes be misleading. It should also be stressed that other factors such as acceleration, shape of hull and speed may all affect the position of the pivot point. The arbitrary figures quoted here however, are perfectly adequate for a simple and practical working knowledge of the subject.

Figure 1-3 Pivot point - making sternway Notes on Shiphandling 3 Pivot Point

Turning Levers and Moments

More important perhaps, than the position of the pivot point, is the effect its shifting nature has upon the many turning forces that can influence a ship. These are rudder force, transverse thrust, bow thrust, tug force, interactive forces and the forces of wind and tide.

Vessel Stopped

If we look at the ship used in our example, we can see that it has a length overall of 160 metres. It is stopped in the water and two tugs are secure fore and aft, on long lines, through centre leads.

If the tugs apply the same bollard pull of say 15 tonnes each, it is to a position 80m fore and aft of the pivot point.

Thus two equal turning levers and moments of 80m x 15t (1200tm) are created resulting in even lateral motion and no rate of turn.

Figure 1-4 Turning levers - vessel stopped Pivot Point 4 Notes on Shiphandling

Making Headway

With the ship making steady headway, however, the pivot point has shifted to a position 40m from the bow. The forward tug is now working on a very poor turning lever of 40m x 15t(600tm), whilst the after tug is working on an extremely good turning lever of 120m x 15 t (1800tm).

This results in a swing of the bow to starboard.

Figure 1-5 Turning Levers: Making Headway. Notes on Shiphandling 5 Pivot Point

Making Sternway

The efficiency of the tugs will change totally when the ship by contrast makes sternway. Now the pivot point has moved aft to a position 40m from the stern. The forward tug is working on an excellent turning lever of 120m x 15t(1800tm) whilst the after tug has lost its efficiency to a reduced turning lever of 40m x 15t(600tm).

This now results in a swing of the bow to port.

Figure 1-6 Turning Levers : Making Sternway

This simple method can also be used to obtain a basic knowledge of rudder, propeller and thruster efficiency, effect of wind, trim, interaction and tug positioning. In each Session that discusses those particular subject areas and in practical exercises in the manned models, it is the basis of all analysis!

Pivot Point 6 Notes on Shiphandling

General

The estimation of speed and knowing when to reduce speed when approaching a berth is not always easy and confidence can only come with experience. On very large ships, such as VLCCs, some guidance may be available from reliable doppler logs, but on many ships a doppler log is not available. In any case, total reliance upon instrumentation is not wise and is no substitute for experience. A pilot jumping from one ship to another, sometimes several during one duty period, has to develop a "feel" for the type of ship he boards and con "by the seat of his pants."

Speed

Many casualties are proven to occur as a direct result of excessive speed. Its effect can be insidious and a Master may find that he cannot keep up with events, which are happening too quickly. Effective control of the ship is slowly but inexorably lost. Against this are commercial pressures, on Masters and Pilots alike, for expedient passages and turn-round times. Whilst there are arguments either way, they are clearly not compatible and experience has shown that a fast pilot is not necessarily a good pilot - just lucky!

It is therefore desirable to balance a safe and effective speed of approach, against a realistic time scale. It would be unwise for example, to conduct a three mile run-in, at a speed of one knot. Three hours would stretch anyone's patience!

It is, of course, impossible to give exact figures, the requirement is dictated to a large degree by variable factors such as type of ship, tonnage, draft, shaft horse-power, wind and tide. Generally speaking, ships of less than 40,0 dwt are inclined to run their way off relatively quickly when engine speed is reduced, whereas larger ships carry their way for much larger distances. Speed must be brought firmly under control at greater distances from the berth.

It is usually obvious when the speed of a ship is too slow and can be easily overcome with a small increase in revolution; it is not always obvious when the speed is too high. The speed of a large ship, during an approach to a berth, particularly without tugs, can increase in an insidious manner and it is invariably difficult to reduce that speed in a short distance and keep control of the ship.

Slow Speed Control 2 Notes on Shiphandling

Loss of Control

If we look at Figure 2-1 we may illustrate some important points. In this example we have a medium size ship of 60,0 dwt, which we will assume is diesel powered with a single, right handed, fixed pitch propeller and single conventional rudder.

At one mile from the berth and running at an approach speed of 6 knots, it is well in excess of a dead slow speed of 3 knots. As the ship approaches the 1/2 mile mark, speed is still over 3 knots, despite a rapid reduction in rpm. It is now necessary to stop the engine and thence sustain a prolonged period of increasing stern power in order to stop the ship in time.

During this substantial time interval the ship is at the whim of transverse thrust, wind, tide, bank effect or shallow water effect. It is effectively "out of control" in so much that we can only stand back and hope that it will do what we want. This is literally hit or miss stuff and the more we can reduce this prolonged period of increasing stern power and thus retain control, so much the better!

Notes on Shiphandling 3 Slow Speed Control

1 mile from berth speed over 3 knots !

Panamax

60,0 dwt Loaded

Control lost for a very long period

Figure 2-1 Loss of Slow Speed Control Slow Speed Control 4 Notes on Sniphandling

Slow Speed Control

In the Figure 2-2 we see the same ship, again one mile from a berth but this time at its dead slow speed of 3 knots or less. Before it approaches the 1/2 mile mark it may also be necessary to stop the engine to further reduce excessive headway and allow plenty of time to adjust the ship's approach and positioning for the berth. Now the biggest worry is the loss of rudder effectiveness at very slow speeds, particularly without any tug assistance and the fear that we cannot keep control of the ship's head. For a variety of reasons such as poor steering, wind, shallow water or directional instability, the bow may well begin to develop an unwanted sheer, alternatively it may be desirable to adjust the attitude of approach. Control is best achieved by applying full rudder and utilizing a short but substantial burst of engine power. This is the "Kick Ahead" technique.

There are however, several pitfalls to avoid, which can all lead to an excessive increase in speed, thus ruining all the previous efforts to control it.

Kick Ahead. Rudder Angle.

(Parte 1 de 6)

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