1. PIVOT POINT
A PIVOT POINT IS A CENTRAL POINT ON A VESSEL WHICH REMAINS FIXED AS THE BOW AND STERN SWING AROUND IT. FIXED IS NOT REALLY CORRECT, BECAUSE IT MOVES FORWARD OR AFT AS THE VESSEL MOVES THROUGH THE WATER AS STATED BELOW.
PIVOT POINT MEANS THE CENTER OF ANY ROTATIONAL SYSTEM. IT IS VERY VITAL TO KNOW THE LOCATION OF THE PIVOT POINT AS THE SHIP HANDLING DEPENDS GREATLY ON KNOWING THE LOCATION OF THE SAME.
1) STATIC VESSEL
THE PIVOT POINT IS NOT A FIXED POINT. IT CHANGES THE LOCATION DEPENDING ON THE BELOW FACTORS;
WHEN THE VESSEL IS AT REST OR STATIC, THE PIVOT POINT IS ALMOST THE SAME AS THAT OF THE CENTER OF GRAVITY, WHICH IS DENOTED BY G.
2) WHEN THE VESSEL MOVES FORWARD
WHEN THE VESSEL MOVES FORWARD, THE POSITION OF PIVOT POINT SHIFTS FORWARD.THE NEW PIVOT POINT WILL BE ABOUT 1/4TH OF THE LENGTH OF THE VESSEL FROM THE FORWARD
3) WHEN THE VESSEL MOVES ASTERN
WHEN THE VESSEL MOVES ASTERN, THE POSITION OF THE PIVOT POINT SHIFTS TOWARDS THE STERN.THE NEW PIVOT POINT WILL BE ABOUT 1/4TH OF THE LENGTH OF THE VESSEL FROM THE STERN.
4) WHILE THE VESSEL IS BERTHING WITH TUGS F & A AND HAVING STERNWAY
AS SEEN FROM THE PREVIOUS EXPLANATION, WHILE THE VESSEL MOVES ASTERN, THE PIVOT POINT MOVES TOWARDS THE STERN. THIS SHIFT OF THE PIVOT POINT CAN BE MADE TO ADVANTAGE.
LET’S ASSUME THAT BOTH THE TUGS ARE PULLING WITH THE SAME FORCE. SINCE THE PIVOT POINT HAS SHIFTED MORE TOWARDS THE STERN, THE EFFECT OF THE FORWARD TUG WILL BE INCREASED AUTOMATICALLY. THE REASON BEING THAT THE TURNING LEVER FOR THE FORWARD TUG HAS BEEN INCREASED, BECAUSE OF THE SHIFT OF THE PIVOT POINT.
THEREFORE THE ACTION OF THE FORWARD TUG WILL BE DOMINANT OVER THE STERN TUG. THEREFORE THE BOW WILL MOVE TO PORT.
2. TRANSVERSE TRUST OF THE PROPELLER
AHEAD - HIGHER PRESSURE ON THE PORT SIDE TRANSVERSE THRUST CAUSES SMALL SWING TO PORT HARDLY NOTICEABLE.
ASTERN - HIGHER PRESSURE ON THE STARBOARD SIDE MUCH MORE SIGNIFICANT TRANSVERSE THRUST CAUSES SWING TO STARBOARD.
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