Sailed4seas的部落格

WIND

THE WIND IS ACTING ON THE SUPERSTRUCTURE OF THE VESSEL ABOVE THE WATER. THE SUPERSTRUCTURE AREA BEFORE THE PP COMPARE WITH THE AREA AFT WILL DETERMINE THE VESSEL’S TENDENCY TO TURN WINDWARD OR NOT. THE PP IS LOCATED ONE FOURTH SHIP’S LENGTH FROM THE BOW. MOST VESSEL SUPERTRUCTUR AFT THE PP IS MORE THAN SUPERSTRUCTURE BEFORE THE PP. THIS KIND OF VESSEL HAS THE TENDENCY TO TURN UPWIND.

THE WIND PRESSURE ON THE VESSEL IS CARRING ALL THE WAY FROM OPEN SEA TO THE BERTH. IN EVERY SINGLE COURSE AND SPEED, WE HAVE TO TAKE THE WIND PRESSURE INTO CONSIDERATION. THE WIND FORCE IS PROPORTIONAL TO THE SQUARE OF WIND SPEED. FOR A CONSTANT WIND SPEED, THE WIND FORCE WORKING ON THE VESSEL IS MORE PROMINIENT IN A SLOWER VESSEL SPEED. EACH VESSEL HAS DIFFERENT RESPONSE TO THE WIND FORCE. USUALLY THE WIND SPEED IS NOT CONSTANT.      

WHEN VESSEL IS NAVIGATING STRAIGHT ALONG A WATERWAY AND UNDER THE INFLUENCE OF WIND, THE VESSEL MAY HAVE TO CARRY A LEE SIDE RUDDER TO COUNTER THE TENDENCY TO TURN UPWIND. THIS LEE SIDE RUDDER IS TO TRY TO BRING THE VESSEL BACK TO ORIGINAL COURSE. EVEN THE LEE SIDE RUDDER BEEN USED, THE VESSEL MAY STILL DRIFT OUT OF HER ORIGINAL COURSE LINE BY THE WIND FORCE WORKING ON THE SHIP’S HULL ABOVE THE WATER. SOME COURSE CHANGE TO UPWIND IS NECESSARY TO BRING HER BACK TO TRACK. THIS COURSE ADJUSTMENT IS CALLED LEEWAY.

THE AMOUNT OF THE LEE SIDE RUDDER WE USED TO BRING THE VESSEL BACK TO ORIGINAL COURSE IS RELATIVE TO THE VESSEL SPEED. THE LEE SIDE RUDDER NEEDED IN HIGHER SPEED WILL BE LESS THAN SLOWER SPEED.

THE WIND PRESSURE IS PROPORTIONAL TO THE WINDAGE. THE WINDAGE IS NOT A CONSTANT, BUT RELATED TO THE CROSSING ANGLE OF THE SHIP’S HEADING AND WIND DIRECTION. WHEN THE SHIP HEADING IS IN RICEPROCAL DIRECTION WITH THE WIND, THE WINDAGE IS THE CROSS SECTION AREA ABOVE THE WATER WHICH IS THE SMALLEST. WHEN THE SHIP HEADING IS RIGHT ANGLE TO THE WIND, THE WINDAGE IS LONGITUDINAL SECTION AREA ABOVE THE WATER WHICH IS THE LARGEST. THE WINDAGE IS THE PROJECTION AREA OF THE VESSEL’S SILHOUETTE PERPENDICULAR TO THE WIND DIRECTION WHICH IS VARIED AS SHIP HEADING CHANGE.

FOR THE WIND DIRECTION COMES AHEAD OF SHIP’S BEAM, THE WINDAGE IS INCREASING WHEN SHIP TURN TO DOWNWIND DIRECTION. WHEN THE WIND COMES FROM SHIP’S QUARTER DIRECTION, THE WINDAGE DECREASE WHEN SHIP TURN TO DOWNWIND DIRECTION.    

WHENEVER THE VESSEL CARRIED A LEE SIDE RUDDER UNDER THE WIND FORCE, THE VESSEL CAN MAKE AN EASY TURN TO UPWIND DIRECTION. THE PROBLEM IS WHEN THE VESSEL IS NEEDED TO TURN DOWNWIND. THE RESERVED RUDDER EFFECT HAS TO INITIAL THE TURN AND COUNTER THE INCREASING WIND PRESSURE BY THE CHANGE OF THE WINDAGE. IF THE WIND IS VERY STRONG, THE VESSEL MAY HAVE USED MOST OF ITS RUDDER EFFECT TO COUNTER THE WIND PRESSURE TO MAINTAIN ORIGINAL COURSE LINE. THESE MAY CONSIST OF LARGE RUDDER ANGLE AND RELATIVE HIGHER SPEED.

OTHER KIND OF ASSISTANT SHOULD BE TAKING INTO CONSIDERATION FOR SAFETY PURPOSE, LIKE ANCHOR, BOW THRUSTER OR TUG.

IN AN UNSTABLE WEATHER PATTERN, THE WIND MAY INCREASE IN A SHORT INTERVAL THEN DIES OUT TO THE AVERAGE SPEED. THIS SUDDEN WIND SPEED CHANGE IS CALLED THE GUST. WHEN THE WIND IS STRONG, THERE IS LOTS OF CHANCE ALSO HAS THE GUST TOGETHER WITH IT. THE GUST MAY VARIED FROM THE AVERAGE SPEED FROM 5 Kts TO 25 Kts. THE WIND FORCE IS PROPORTIONAL TO THE SQUARE OF WIND SPEED. THUS, THE GUST WILL BE EVEN MORE WORSEN FOR THE VESSEL TO INITIAL THE TURN OR MAITAIN THE ORIGINAL COURSE LINE.

CURRENT

THE CURRENT FORCE WORKS ON THE VESSEL IN THE SIMILAR MANNER LIKE THE WIND. ONLY THE CURRENT IS IN CONSTANTLY CHANGING DIRECTION AND STRENGTH, ESPECIALLY IN THE SHALLOW WATER REA. THAT’S WHY ALL SORT OF NAVIGATIONAL AIDS SET UP FOR. THESE BUOYS AND BEACONS ARE NOT ONLY MAKED THE NAVIGABLE CHANNEL, BUT ALSO SERVE AS A QUICK REFERENCE OF THE CURRENT STATUS OF THE REGION AT THE MOMENT. THE STRONGEST CURRENT USUALLY EXPERIENCED AT THE HARBOUR/BREAKWATER ENTRANCE. THE LONGER VESSEL STAY IN THE SHALLOW AREA, THE LONGER THE CURRENT WILL HAVE THE EFFECT ON THE VESSEL.AS WE LEARN FROM THE WINDAGE, THE BEAM CURRENT HAS THE STRONGEST EFFECT ON THE VESSEL DRIFTING OUT OF COURSE LINE AND MAITAIN ORIGIANL COURSE..

FOR REDUCING THE TIME UNDER THE CURRENT INFLUENCE AND MORE RUDDER EFFECT TO COUNTER THE CURRENT FORCE, THE VESSEL WILL PREFER USE HIGHER SPEED IN THE ENTRANCE. SOMETIMES, THIS HIGHER SPEED IS A MUST-BE.  

WIND AND CURRENT

THESE TWO FORCES MAY WORK ON DIFFERENT DIRECTION AND AMPLITUDE. THE VESSEL MAY HAVE TO CARRY SOME LEE RUDDER TO AGAINST THE RESULTANT VECTOR (FORCE AND DIRECTION) WORKING ON SHIP’S BODY. FROM THE LEE RUDDER WE CARRIED TO THE HARD OVER RUDDER AND THE CURRENT SHIP SPEED TO THE FULL ENGINE SPEED IS THE RESERVED RUDDER EFFECT WE HAVE WHEN CONDUCTING A TURN. ONLY THE EXPERIENCED NAVIGATER KNOWS “DOES THESE RESERVED RUDDER EFFECT IS ENOUGH FOR MAKING THE TURN TO THE DOWN WIND COURSE?”.

FOR THIS REASON, WE SHOULD AVOID THE UNNECESSARY COURSE CHANGE AT THE STRONG CURRENT AREA, LIKE BREAKWATER ENTREANCE OR BUOYED CHANNEL. WE SHOULD HAVE DONE ALL THE COURSE CHANGE BEFORE APPROACHING. A BREAKWATER ENTRANCE, THE CURRENT DIRECTION USUALLY IS COMING FROM THE BEAM. IF THE LEE SIDE RUDDER ALREADY IN USE, THE CURRENT EFFECT WILL EXAGERATE WHEN VESSEL’S PP PASSED THE BREAKWATER ENTRANCE. BY THEN THE CURRENT WILL ACT ONLY ON THE AFT PART OF THE VESSEL. COMBINED WITH THE FACT THAT THE CURRENT FLOW RESTRICTED BY THE BREAKWATER SHAPE WILL ALL CONCENTRATE AT THE BREAKWATER ENTRANCE, THESE TWO FACTOR PUT TOGETHER MAKE THE VESSEL’S COURSE HARD TO MAINTAIN WHEN ENTERING THE BREAKWATER.. 

THE VESSEL MAY NOT PROCEED IN THE SAME HEADING WITH THE COURSE LINE. FOR THE VESSEL MAY HAVE TO CHANGE THE HEADING SOME DEGREES WINDWARD TO COMPENSATTE THE DRIFTING CAUSING BY THE WIND OR/AND CURRENT. THE EFEEECTNESS OF THESE LEEWAY USED SHOULD BE MONITERED

https://www.youtube.com/watch?v=AeOV3YIK1PE

Maneuverability of pure car carriers wind effects