Chapter 9:
Why you need maneuverability!
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)
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)!
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.
MANEUVERABILITY
Directional Stability (9.2.1)
• Straight Line Stability - The ship responds to the
disturbance by steadying on some new course.
ORIGINAL STRAIGHT LINE PATH
STRAIGHT LINE STABILITY -FINAL
PATH IS STRAIGHT BUT
DIRECTION HAS CHANGED.
DISTURBANCE
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.
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. Bernoulli’s!
MANEUVERABILITY
Rudder Types (9.3.1)
MANEUVERABILITY
Spade Rudder
MANEUVERABILITY
Rudder Performance (9.3.3)
• Rudder doesn’t 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
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.
MANEUVERABILITY
Rudder/Airfoil Performance (9.3.3)
• Keep Rudder angle   35 or STALL likely.
Max Lift Point
MANEUVERABILITY
Low Speed Maneuverability (9.4)
Bernoulli’s Lift=½  (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).
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.
Not enough rudder area!