Chapter 9: Why you need maneuverability!. MANEUVERABILITY Introduction (9.1) Important when: – Station keeping – UNREP – Docking – “Dodging incoming...”

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Presentation transcript:

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.

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) 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! Factors in Turn Response

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) 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). Bernoulli’s Lift=½  (Velocity) 2 S C l

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!