principles of shiphandling_ pivot point

8/18/2019 Principles of Shiphandling_ Pivot Point

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4/1/2016 Principles of shiphandling: Pivot point

http://principlesofshiphandling.blogspot.in/p/pivot-point.html

Defines Shiphandling and Maneuvering. Also describes the Basic movement of a ship and talks about

the factors and forces affecting shiphandling.

Principles of shiphandling

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Ship handling Pivot point Basic Propeller Types

Pivot point

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

Pivot point of a vessel stopped

Making Headway

Two forces now come into play, firstly the forward momentum of the shipand secondly, longitudinal resistance to forward momentum, created bythe water ahead of the ship. These two forces must ultimately strike abalance and the pivot point moves forward. As a rough guide it can beassumed that 25% of the ships forward momentum, at constant speed, isspent in overcoming longitudinal resistance and the pivot point will beapproximately 1/4L from forward.

Pivot point moves forward as vessel gathers headway

Making Sternway

The situation is now totally reversed, the momentum of sternway mustbalance longitudinal resistance, this time created by the water astern of

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the ship. The pivot point moves aft and establishes itself approximately1/4L from the stern.

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

Pivot points aft as vessel moves astern

Turning Levers and Moments

More important perhaps, than the position of the pivot point, is the effectits shifting nature has upon the many turning forces that can influence aship. 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 lengthoverall of 160 metres. It is stopped in the water and two tugs are securefore and aft, on long lines, through centre leads.If the tugs apply the same bollard pull of say 15 tonnes each, it is to aposition 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.

Making Headway

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

This results in a swing of the bow to starboard.

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Principles of ship Handling

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Turning Levers: Making headway

Making Sternway

The efficiency of the tugs will change totally when the ship by contrastmakes sternway. Now the pivot point has moved aft to a position 40mfrom 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 areduced turning lever of 40m x 15t(600tm).

This now results in a swing of the bow to port. (See illustration below)

Vessel Turning

When making a turn the whole vessel does not pass on the curved pathformed by the turning ship. The pivot point, in this case located about onefourth of the length from the bow, is the one that traces the curvature of this path so that the stern of the ship swings in a wide area outside of the

curved path while the bow swings inside and needs a smaller area. Thisbehaviour results in a certain length of the ship’s turning radius thatdepends on the ship’s load condition, trim and speed.

A smaller turning radius can be attained if the pivot point is shifted moreforward closer to the bow. This maneuver can be achieved through theanchor dredging technique where the anchor at the side of the desiredturn. This way the forward movement as the vessel turns is reducedbecause the ships turns about the pivot point that is now near the bowwhere the anchor hawse pipe is located. See illustration below.


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Anchor dredging technique: Smaller turning radius, pivot point movesforward.

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