Chapter 9: Why you need maneuverability!

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Chapter 9Why you need maneuverability!
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MANEUVERABILITY
Introduction (9.1)

• Important when
• Station keeping
• UNREP
• Docking
• Dodging incoming...
• Predicted by
• Equations of Motion (which motions?)
• Tank Models
• Verified by Sea Trials
• (Same procedure for aircraft)

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MANEUVERABILITY
Maneuvering Requirements (9.2)

• Maneuverability Categories
• Directional Stability
• Turning Response
• Slow Speed Maneuverability
• It is not possible to independently optimize
  each (e.g. good response conflicts with
  straightline directional stability)!

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MANEUVERABILITY
Directional Stability (9.2.1)

• Controls fixed straightline stability means
  when rudder is amidships, a straight course
  should be maintained.
• Hull form dependent streamlined hull shapes
  with deadwood have increased directional
  stability. (Think of an arrow or a dart.)
• Level of controls fixed straightline
  stability is determined during sea trials and
  tank tests.

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MANEUVERABILITY
Directional Stability (9.2.1)

• Straight Line Stability - The ship responds to
  the disturbance by steadying on some new course.

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MANEUVERABILITY
Turn Response (9.2.2)

• 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
  straightline stability.
• Determined during sea trials and tank tests.

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MANEUVERABILITY
Turn Response (9.2.2)
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. Bernoullis!

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MANEUVERABILITY
Rudder Types (9.3.1)
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MANEUVERABILITY
Spade Rudder
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MANEUVERABILITY
Rudder Performance (9.3.3)

• Rudder doesnt turn ship, hydrodynamics of
  water flow past ship is reason for it turning.
  Water flow past the rudder provides LIFT just
  like an airplane wing!
• Ship turns by moment produced about the LCP
  (not LCG)
• ( Ignore what you learned in Physics! )

Center of Pressure
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MANEUVERABILITY
Rudder/Airfoil Performance (9.3.3)

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

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MANEUVERABILITY
Rudder/Airfoil Performance (9.3.3)

• Keep Rudder angle ? ? 35 or STALL likely.

Max Lift Point
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MANEUVERABILITY
Low Speed Maneuverability (9.4)
Bernoullis Lift½ ? (Velocity)2 S Cl

• 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).
• Twin screws (twist ship).
• Lateral/bow thrusters (research vessels, tugs,
  merchants and some amphibs).
• Rotational thrusters (specialized platforms
  only).

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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 ships maneuverability
  characteristics is at Sea Trials.

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Not enough rudder area!