Sailed4seas的部落格

1.  VISUAL TURNING

HOW TO TURN A VESSEL IN A WATERWAY? IF YOU ARE IN THE NAVY, THERE WILL A SET OF SOP NEEDS TO FOLLOW. FIRST YOU WILL AVE TO READ THE MANEUVERING BOARD VERY CAREFULLY TO IDENTIFY THE TACTICAL DIAMETER. IN THE MANEUVERING BOARD, THEY WILL INVARIABLE BE SHOWN IN METERS WHICH IS INPRATICAL FOR MARINE USAGE EVEN WE USE THE RADAR TO MEASURE THE DISTANCE AND THE RUDDER USED TO TURN THE VESSEL FOR MEASURING THE TACTICAL DIAMETER IS ALWAYS IN FIXED RATE.

THEN YOU HAVE TO DRAW THE WHEEL OVER POINT IN THE SEA CHART AND RECOMMEND THE BEST RUDDER TO USE. THIS IS THE VOYAGE PLANNING.

EVENIF WE HAVE DONE ALL THE CALCULATION, MEASUREMENT AND PLANNING FROM THE MANEUVERING BOARD VERY CAREFULLY, THERE WILL BE STILL LOT OF UNCERTAIN FACTOR TO HAVE INFLUENCE IN THE SHIP’S TUNING RESPONSE, LIKE WIND DIRECTION/FORCE, CURRENT DIRECTION/FORCE, WATER DEPTH/WIDTH......

IT IS IN THIS NATURE THAT MAKES THE SHIP HANDLING STILL A SKILL RATHER THAN A ROUTINE.

IN THE OPEN SEA, THE SHIP TURNING IS UNRESTRICTED UNLESS FOR COLLISION AVOIDANCE PURPOSE. BUT, IN THE CONFINED WATER, THERE NEED TO BE SOME GUIDANCE TO FOLLOW. NAVIGATABLE WATERWAY ALWAYS HAS SOME NAVIGTION MARKS TO INDICATE LIKE BEACON, LIGHT BUOY OR LEADING LINE. AT THE TURN, THERE ARE ALWAYS SHOWN IN DISTIGUISH CHARACTERISTIC TO MARK. WITH THESE TURNING MARKS, WE WILL CONDUCT THE VISUAL TURNING SKILL TO TURN OVER IN A RELATIVE SAFE MANNER.

BEFORE WE PROCEED, WE SHOULD LOOK INTO SOME BASIC FACTS ABOUT THE MACHENIC OF THE TURNING.

1. THE PIVOT POINT (PP) IS VARIED AS THE SPEED CHANGED. IF A VESSEL IS PROCEEDING IN FULL SEA SPEED, THE PP IS LOCATE ONE FOURTH SHIP’S LENGTH FROM THE BOW. AS THE SHIP REDUCING THE SPEED, THE PP WILL MOVE FURTHER AHEAD. AT THE VERY SLOW AHEAD SPEED LIKE 0.5 KNOTS, THE PP WILL LOCATE AT ABOUT ONE EIGHTH SHIP’S LENGTH FROM THE BOW.

• PP (slow) = 7/8 SHIP’S LENGTH FROM THE RUDDER.
• PP (full) = 3/4 SHIP’S LENGTH FROM THE RUDDER.
• VESSEL CAN MAKE A MORE EASY TURN TO UPWIND DIRECTION THAN DOWNWIND. FOR THE AFTER PART OF SHIP BODY BEHIND THE PP IS MUCH MORE THE FORWARD PART.

2. THE RUDDER EFFECT (Re) IS BASED ON RUDDER AREA, THE SPEED OF WATER PASSING THROUGH RUDDER PLATE AND THE RUDDER ANGLE BEEN USED.  

• Re IS PROPORTIONAL TO THE RUDDER AREA.
• Re IS PROPORTIONAL TO THE SQUARE OF WATER SPEED.
• Re IS PROPORTIONAL TO THE SINE OF RUDDER ANGLE
• 

6. A SUCCESSFUL TURN MAY CONSIST OF THREE PARTS :
   1. THE VESSEL HAS TURNED TO THE DESIRED ANGLE.
   2. THE VESSEL HAS REACHED THE DESIRED SPEED.
   3. THE VESSEL HAS PLACED IN THE DESIRED POSITION.

TO MAKE A SUCCESSFUL TURNING, WE HAVE TO DERIVE SOME CONCEPT FROM THE FACTS ABOVE.

1. THE RUDDER AREA IS A CONSTANT. SO, WE HAVE FOUR VARIANTS: RUDDER ANGLE, VESSEL SPEED, ENGINE SPEED AND RUDDER DISTANCE TO PP.
2. THE MORE RUDDER ANGLE WE USED, THE MORE RUDDER EFFECT.
3. THE VESSEL CAN MAKE MORE EASY TURNING WITH SLOWER SHIP’S SPEED (Vs) WITH SAME RUDDER ANGLE. FOR THE PP IS MOVING MORE FAR AWAY FROM THE RUDDER WHICH MAKE THE DISTANCE TO THE PP LONGER. HOWEVER, THE DIFFERNCE BETWEEN FULL SPEED AND DEAD SLOW SPEED IS ONLY 1/8 SHIP’S LENGTH.
4. THE SPEED OF WATER THAT PASSING THROUGH THE RUDDER PLATE HAS THE DETERMINATELY EFFECT ON THE TURNING MOMENT(Tm). THE MORE WATER SPEED WORKING ON THE RUDDER PLATE, THE MORE QUICKLY THE VESSEL CAN MAKE THE TURN. . Vs IS IRERELAVENT TO THE Vp. FOR EXAMPLE: ONE VESSEL MAY PROCEED IN DEAD SLOW AHEAD (5 Kts). BUT, THE PROPELLER MAY MAKE A FULL SPEED (12 Kts). THE WATER SPEED WORKING ON RUDDER PLATE IS SOMEWHERE NEARING 12 Kts. IF WE CHANGE THE ENGINE ORDER TO HALF AHEAD(10 Kts), THE WATER SPEED WORKING ON THE RUDDER IS NEARING 10 Kts. IF WE STOP THE ENGINE( 0 Kts), THE WATER SPEED WORKING ON THE RUDDER PLATE IS NEARING 5 Kts, DUE TO THE VESSEL SPEED. THE SQUARE OF 12 IS 144 AND THE SQUARE OF 5 IS 25. ONE VESSEL HAS 5 KTS SPEED WITH ENGINE IN FULL AHEAD(12 Kts) CAN HAS ALMOST 5 TIMES OF RUDDER FORCE THAN OTHER VESSEL HAS 5 KTS SPEED WITH STOPPED ENGINE(0 Kts). OUR CONCLUSION IS: FOR MORE RUDDER EFFECT, WE NEED HIGHER ENGINE SPEED.
5. CONTROLLING THE VESSEL SPEED IS ALSO AN IMPORTANT CONCERN OF SHIP HANDLING. WHEN CONDUCTING A TURN IN THE WATERWAY OR HARBOUR AREA, THE INTIAL SHIP SPEED CAN HAVE DECISIVE INFLUENCE ON THE CONSEQUENCE OF TURNING. LET’S GO THROUGH COUNTER MEASURE WE TAKE, IF THE TURNING IS GETTING WRONG.  

• IF WE TURNING THE VESSEL TOO SLOWLY WHICH MAY IMPLY THE INITIAL SPEED TOO HIGH OR THE POSITION OF WHEEL OVER IS TOO CLOSE TO TURNING POINT, WE HAVE TO MAKE MORE RUDDER EFFECT  TO SPEED UP THE TURN. FOR MORE RUDDER EFFECT, WE HAVE THREE CHOICES (Vs,Vp,SIN θ).
  1. REDUCE THE SHIP SPEED (Vs) TO MAKE MORE DISTANCE FROM THE RUDDER TO PP TO INCREASE THE Tm. THE SHIP SPEED CANNOT REDUCE BY ENGINE ORDER ONLY. IT DEPENDS ON THE INFLUENCE OF INITIAL SPEED, VESSEL’S DEADWEIGHT WHICH CARRIES THE AHEAD MOMENTUM, WIND, CURRENT, AND WATER DEPTH. IT IS NOT AN INSTANT CONTROLLABLE FACTOR FOR EMERGENCY PURPOSE. EVEN IF THE SHIP SPEED CAN BE REDUCED IMMEDIATELY, THE PP POSITION CAN BE VARIED ONLY 1/8 SHIP’S LENGTH WHICH CAN ONLY INCREASE 1/6 TIMES OF ORIGINAL RUDDER EFFECT (ORIGINAL PP POSITION IS 6/8 SHIP’S LENGTH FROM THE RUDDER. AFTER REDUCING SHIP SPEED, THE PP CAN MOVE AHEAD TO 7/8 SHIP’S LENGTH).  
  2. THE RUDDER EFFECT IS PROPORTIONAL TO SINE OF RUDDER ANGLE (SIN θ). THE MORE RUDDER ANGLE WE USED, THE MORE RUDDER EFFECT.
  3. THE RUDDER EFFECT IS PROPORTIONAL TO THE SQUARE OF PROPELLER SPEED (Vp). INCREASING THE PROPELLER SPEED CAN HAVE INSTANT AND POWERFUL RESPONSE IN THE TURNING. IF THE PROPELLER SPEED USED IN A PROLONG INTERVAL, THE VESSEL SPEED MAY ALSO UNWILLINGLY INCREASED.

   THE COUNTER MEASURE FOR A VESSEL TURNING TOO SLOW WILL BE

1. USE MORE RUDDER ORDER. IF STILL TURNING TOO SLOWLY,
2. PUT THE RUDDER HARD OVER. IF STILL TOO SLOWLY,
3. INCREASING ENGINE SPEED. IF STILL TOO SLOWLY,
4. ENGINE FULL AHEAD.

THE REASON FOR TAKING THESE COUNTER MEASURES IN THIS SEQUENCE IS

1. THE RUDDER ANGLE IS EASILY ADJUSTED AND WE DON’T WANT INCREASE THE SHIP SPEED BY ENGINE WHICH MAY CAUSE THE LOST CONTROL OF THE SHIP’S POSITION AND SHORTEN THE REACTION TIME THAT IS NEEDED TO OBSERVE THE CORRECT TURNING RATE IS ACHIEVED. SO, WE PUT MORE RUDDER ANGLE FIRST TO INCREASE THE TURNING FORCE.
2. IF THE TURNING IS STILL TOO SLOW, THE RUDDER NEED TO BE PLACED IN HARD OVER POSITION TO GET THE MAXIMUM RUDDER EFFECT OUT FROM EFFECTIVE RUDDER AREA().
3. AFTER THE RUDDER IS PUT HARD OVER AND THE TURNING IS STILL SLOW, WE HAVE TO INCREASE THE ENGINE EXPELLED WATER SPEED TO GAIN MORE RUDDER EFFECT.
4. CONSEQUENTLY, IF VESSEL CANNOT RESPONSE TO THE TURNING RATE WE NEED WE HAVE TO USE FULL ENGINE SPEED TO GET THE MAXIMUM RUDDER EFFECT()

WIND AND CURRENT

WHEN VESSEL IS NAVIGATING ALONG A WATERWAY WITHOUT TURNING UNDER THE INFLUENCE OF WIND AND/OR 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 PILOT KNOWS “DOES THESE RESERVED RUDDER EFFECT IS ENOUGH FOR MAKING THE TURN TO THE LEE SIDE?”.

THE VESSEL MAY NOT PROCEED IN THE SAME DIRECTION 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.