Understanding Maneuverability in Maritime Operations

 
9.2 Maneuverability
 
Important when:
   Station keeping
   UNREP
   Docking
   “Dodging incoming...”
 
 
Predicted by:
   Equations of Motion
   Tank Models
 
 
Verified by Sea Trials
             (Same procedure for aircraft)
 
The maneuverability performance of the rudder
can be described by three broad categories:
1.
Directional Stability
2.
Response
3.
Slow Speed Maneuverability
 
Maneuverability
 
The ability to continue to travel in a straight line
 
- With rudder at midships
- With no external pressure acting on the vessel or rudder
 
Controls Fixed Straight Line Stability
 
- A condition rarely achieved
-
Any condition other than heading directly into the seas
  will alter the ability to continue straight
 
Directional Stability
 
The ability to continue to travel in a straight line
 
-
Longer ships are more likely to possess straight line
  stability
-
Short “beamy” ships, like tugs, small sport craft, have
  poor straight line stability
 
- To improve this, can increase “deadwood” of the ship
 
- This is the part of the hull that exists in front of
                the rudder, an extension of the ship
 
- Acts like the feathers on an arrow
 
Directional Stability
 
Directional Stability
 
   
Straight Line Stability
 - The ship responds to the
 
disturbance by steadying on some new course.
 
The ability to turn the ship when the rudder is applied, and
to return the ship to the desired heading with minimal
overshoot
 
-
When applied, the rudder must be able to change the orientation
  of the ship in a minimum set time.
 
-
The ship must be able to return on course without going beyond
  the desired heading.
 
Turn Response
 
- Responsiveness is determined by the ship’s mission
 
- A combatant needs high maneuverability
 
- A merchant ship needs much less than a combatant
 
- Can quantify responsiveness by the Rudder Area Ratio:
 
Rudder Area Ratio  = Rudder Area
     
   
     L
pp
 T
 
A cargo ship = 0.017,… a destroyer has about 0.025 ratio...
 
Turn Response
 
Turn Response
 
   
We want quick response time to helm commands with
 
minimum course overshoot.
 
   Rudder response depends on rudder dimensions,
 
rudder angle, and flow speed.
 
   Directly conflicts with “controls fixed straight line
 
stability”.
 
   Determined during sea trials and tank tests.
 
 
 
 
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Rudder angle
 
level of response depends on
        standard rudder ordered and available range.
 
   
Ship speed
 
determines level of water flow past control
 
surface. Bernoulli’s!
 
    
Coxswain Ability
 
Factors in Turn Response:
 
Turn Response
 
The ability to maneuver at slow speeds < 5 kts
 
- A ship requires some level of maneuverability at low speeds
 
- In canals
 
- Approaching harbor entries
 
- But as speed drops, so too does rudder control!
-
Typically requires some additional methods to aid turning
 
and positioning in slow speeds
 
Slow Speed Maneuverability
 
Slow Speed Maneuverability
 
   Must be able to maintain steerageway even at slow speeds.
 
   Directional control systems used at slower speeds.
   Position rudder behind prop (thrust directly on rudder).
-
Increases water flow over the rudder
 
   Twin screws (twist ship).
 
   Lateral/bow thrusters (research vessels, tugs,
 
merchants and some
 
amphibs).
 
   Rotational thrusters (specialized platforms only).
 
Maneuverability Requirements
 
Maneuverability Trade-Off
Stability (tendency to stay in a straight line) &
maneuverability (ability to easily depart from a straight
line) oppose one another
Large rudders can help both but increase drag
 
It is not possible to independently optimize
each (e.g. good response conflicts with
straightline directional stability)!
 
9.3 Rudders
 
Hull
 
Rudders
 
Chord:
Horizontal distance from leading to trailing edge
Limited by propeller and edge of stern
Span:
Vertical distance from stock to tip
Limited by local hull bottom and ship baseline
 
Chord
 
Span
 
Semi-Balanced Spade Rudder
 
Rudders
 
Rudder Balance
 
Center of Pressure vs. Position of Rudder
Stock
Vertically aligned: “Fully Balanced”
Rudder Stock at leading edge: “Unbalanced”
Semi-Balanced
Less operating torque than unbalanced
Returns to centerline on failure
 
 
 
1.  
Balanced Rudder
  The rudder stock is positioned toward the
center of the rudder, requiring less force to turn the it
 
Rudder Balance
 
2.  
Unbalanced Rudder
   The rudder stock is at the leading edge of
the rudder
 
Rudder Balance
 
3.  
Semi Balanced
  The rudder mounts on a “horn” protruding from
the hull
 
- The top can be considered “unbalanced”
 
- The bottom can be considered “balanced”
 
Rudder Balance
 
Semi-Balanced Spade Rudder
 
Rudders
 
 
 
 
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Rudder Performance
 
Stages of a ships turn:
 
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Rudder Performance
IT DOES NOT MAKE THE SHIP TURN!
 
What it DOES do is orient the ship at an angle to the
direction of travel…
 
The pressure on the side of the hull causes the ship
to turn (it acts like a flap on an aircraft wing)
 
Rudder Performance
 
Rudder Action:
“Kicks” stern in opposite to desired direction
Ship’s angle to flow drives ship in desired
 
 
   Lift produced by force imbalance acts
 
perpendicular to the flow stream.
 
   Lift and drag act at the center of pressure.
 
Rudder Performance
 
Rudder Performance
 
Rudder Stall
- Just like an aircraft wing, if the
angle of the rudder is too great, the
high and lower pressure areas on the
rudder will disrupt water flow over
the surface
 
-
Beyond 45
o
, the rudder will produce
no lift, and so will not effectively
orient the ship for turning
 
-
 Rudder will create turbulence and
drag with no effect on ability to turn
 
 
- Navy ships typically limit the
angle range to about 35
o
 
  Keep Rudder angle  
  
 35 or STALL likely.
 
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Rudder Performance
 
9.4 Slow Speed Maneuverability
 
Rudder Pressures or Forces
Rudder position relative to propeller
Twin propellers
Stbd: right handed; port: left handed
Twist ship by operating engines in opposite
directions
Lateral/Bow Thrusters
Rotational Thrusters (SPM/Outboard)
 
MANEUVERABILITY
 
The Bottom Line
 
 
 
 
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   Increased rudder area improves response and usually
 
improves directional stability.
 
   Theory and design use many assumptions so empirical
 
testing with models is required.
 
   
True test
 of ship’s maneuverability characteristics is at
 
Sea Trials
.
 
Example Problems
 
Of what is a ship’s directional stability a measure?
Without touching the throttles, the ship slows when we
commence a turn; why?
Which of the following helps an operator maneuver the
ship at slow speeds (<5kts)?
Fin stabilizers
  
Deadwood
  
2 Prop Shafts
Rotational thrusters
 
Bilge keels
  
Bow thruster
Small rudder
  
Large rudder
 
Example Answers
 
Of what is a ship’s directional stability a measure?
Ability to steam in a straight line with the rudder amidships
Without touching the throttles, the ship slows when we
commence a turn; why?
First, when the rudder leaves amidships, it presents a larger cross-section
to the flow, increasing drag and resistance, reducing speed for the same
EHP.
Second, when the ship starts to turn, the whole hull now presents a larger
cross-section to the flow, amplifying the rudder effect as it “slides”
through the turn.
Which of the following helps an operator maneuver the ship at
slow speeds (<5kts)?
Fin stabilizers
  
Deadwood
  
2 Prop Shafts
Rotational thrusters
  
Bilge keels
  
Bow thruster
Small rudder
   
Large rudder
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Maneuverability in maritime operations plays a crucial role in scenarios such as station-keeping, docking, and dodging incoming obstacles. It is predicted through equations of motion and tank models, verified by sea trials. The performance is categorized into directional stability, response, and slow-speed maneuverability, each essential for maintaining control and safety at sea.


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  1. 9.2 Maneuverability Important when: Station keeping UNREP Docking Dodging incoming... Predicted by: Equations of Motion Tank Models Verified by Sea Trials (Same procedure for aircraft)

  2. Maneuverability The maneuverability performance of the rudder can be described by three broad categories: 1. Directional Stability 2. Response 3. Slow Speed Maneuverability

  3. Directional Stability The ability to continue to travel in a straight line - With rudder at midships - With no external pressure acting on the vessel or rudder Controls Fixed Straight Line Stability - A condition rarely achieved -Any condition other than heading directly into the seas will alter the ability to continue straight

  4. Directional Stability The ability to continue to travel in a straight line -Longer ships are more likely to possess straight line stability -Short beamy ships, like tugs, small sport craft, have poor straight line stability - To improve this, can increase deadwood of the ship - This is the part of the hull that exists in front of the rudder, an extension of the ship - Acts like the feathers on an arrow

  5. Directional Stability Straight Line Stability - The ship responds to the disturbance by steadying on some new course. DISTURBANCE ORIGINAL STRAIGHT LINE PATH STRAIGHT LINE STABILITY -FINAL PATH IS STRAIGHT BUT DIRECTION HAS CHANGED.

  6. Turn Response The ability to turn the ship when the rudder is applied, and to return the ship to the desired heading with minimal overshoot -When applied, the rudder must be able to change the orientation of the ship in a minimum set time. -The ship must be able to return on course without going beyond the desired heading.

  7. Turn Response - Responsiveness is determined by the ship s mission - A combatant needs high maneuverability - A merchant ship needs much less than a combatant - Can quantify responsiveness by the Rudder Area Ratio: Rudder Area Ratio = Rudder Area Lpp T A cargo ship = 0.017, a destroyer has about 0.025 ratio...

  8. Turn Response We want quick response time to helm commands with minimum course overshoot. Rudder response depends on rudder dimensions, rudder angle, and flow speed. Directly conflicts with controls fixed straight line stability . Determined during sea trials and tank tests.

  9. Turn Response Factors in Turn Response: Rudder dimensions limited by space. Larger rudder area means more maneuverability, but more drag. Rudder angle level of response depends on standard rudder ordered and available range. Ship speed determines level of water flow past control surface. Bernoulli s! Coxswain Ability

  10. Slow Speed Maneuverability The ability to maneuver at slow speeds < 5 kts - A ship requires some level of maneuverability at low speeds - In canals - Approaching harbor entries - But as speed drops, so too does rudder control! -Typically requires some additional methods to aid turning and positioning in slow speeds

  11. Slow Speed Maneuverability Must be able to maintain steerageway even at slow speeds. Directional control systems used at slower speeds. Position rudder behind prop (thrust directly on rudder). -Increases water flow over the rudder Twin screws (twist ship). Lateral/bow thrusters (research vessels, tugs, merchants and some amphibs). Rotational thrusters (specialized platforms only).

  12. Maneuverability Requirements Maneuverability Trade-Off Stability (tendency to stay in a straight line) & maneuverability (ability to easily depart from a straight line) oppose one another Large rudders can help both but increase drag It is not possible to independently optimize each (e.g. good response conflicts with straightline directional stability)!

  13. 9.3 Rudders

  14. Rudders Root Chord Stock Span Water Flow Water Flow Trailing Edge Leading Edge Tip Chord

  15. Rudders Chord: Horizontal distance from leading to trailing edge Limited by propeller and edge of stern Span: Vertical distance from stock to tip Limited by local hull bottom and ship baseline Semi-Balanced Spade Rudder Span Chord

  16. Rudder Balance Center of Pressure vs. Position of Rudder Stock Vertically aligned: Fully Balanced Rudder Stock at leading edge: Unbalanced Semi-Balanced Less operating torque than unbalanced Returns to centerline on failure

  17. Rudder Balance 1. Balanced Rudder The rudder stock is positioned toward the center of the rudder, requiring less force to turn the it

  18. Rudder Balance 2. Unbalanced Rudder The rudder stock is at the leading edge of the rudder

  19. Rudder Balance 3. Semi Balanced The rudder mounts on a horn protruding from the hull - The top can be considered unbalanced - The bottom can be considered balanced

  20. Rudders Semi-Balanced Spade Rudder

  21. Rudder Performance Rudder doesn t turn ship, hydrodynamics of water flow past ship is reason for it turning. Rudder flow provides LIFT. Ship turns by moment produced about the LCP (not LCG) Center of Pressure

  22. Rudder Performance Stages of a ships turn: Rudder midships Water Flow Rudder is turned Ship orients itself at the desired angle to oncoming seas Hull Lift

  23. Rudder Performance IT DOES NOT MAKE THE SHIP TURN! What it DOES do is orient the ship at an angle to the direction of travel The pressure on the side of the hull causes the ship to turn (it acts like a flap on an aircraft wing) Rudder Action: Kicks stern in opposite to desired direction Ship s angle to flow drives ship in desired

  24. Rudder Performance Lift produced by force imbalance acts perpendicular to the flow stream. Lift and drag act at the center of pressure.

  25. Rudder Performance Rudder Stall - Just like an aircraft wing, if the angle of the rudder is too great, the high and lower pressure areas on the rudder will disrupt water flow over the surface -Beyond 45o, the rudder will produce no lift, and so will not effectively orient the ship for turning - Rudder will create turbulence and drag with no effect on ability to turn angle range to about 35o - Navy ships typically limit the

  26. Rudder Performance Keep Rudder angle 35 or STALL likely. Max Lift Point

  27. 9.4 Slow Speed Maneuverability Rudder Pressures or Forces V Rudder position relative to propeller Twin propellers Stbd: right handed; port: left handed Twist ship by operating engines in opposite directions Lateral/Bow Thrusters Rotational Thrusters (SPM/Outboard)

  28. MANEUVERABILITY The Bottom Line Good directional stability and minimum ship response conflict, so compromise involved. Increased rudder area improves response and usually improves directional stability. Theory and design use many assumptions so empirical testing with models is required. True test of ship s maneuverability characteristics is at Sea Trials.

  29. Example Problems Of what is a ship s directional stability a measure? Without touching the throttles, the ship slows when we commence a turn; why? Which of the following helps an operator maneuver the ship at slow speeds (<5kts)? Fin stabilizers Deadwood Rotational thrusters Bilge keels Small rudder Large rudder 2 Prop Shafts Bow thruster

  30. Example Answers Of what is a ship s directional stability a measure? Ability to steam in a straight line with the rudder amidships Without touching the throttles, the ship slows when we commence a turn; why? First, when the rudder leaves amidships, it presents a larger cross-section to the flow, increasing drag and resistance, reducing speed for the same EHP. Second, when the ship starts to turn, the whole hull now presents a larger cross-section to the flow, amplifying the rudder effect as it slides through the turn. Which of the following helps an operator maneuver the ship at slow speeds (<5kts)? Fin stabilizers Deadwood Rotational thrusters Bilge keels Small rudder Large rudder 2 Prop Shafts Bow thruster

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