Chapter 7 : Trials. Ch7. Sea trials / Manoeuvring characteristics of ships. IMO Recommendations MSC 137(76) The manoeuvrability of ships can be evaluated from the characteristics of conventional trial manoeuvres. Two methods can be used:
Ch7. Sea trials / Manoeuvring characteristics of ships IMO Recommendations MSC 137(76) • The manoeuvrability of ships can be evaluated from the characteristics of conventional trial manoeuvres. • Two methods can be used: • Scale model tests or computer predictions using mathematical models at the design stage / full scale trials must be conducted to validate these results • Full scale trials • Test speed = at least 90% of full speed = 85% of full engine power
Ch7. Sea trials / Manoeuvring characteristics of ships Imo Manoeuvring Standards • By resolution A.751(18) in 1993 IMO adopted Manoeuvring Standards • The standards apply to: • All ships of 100m in lenght and over • All chemical tankers and gas carriers • They consist of: • Turning circles to Port and starboard • Stopping Test • Zig-Zag Test
Ch7. Sea trials / Manoeuvring characteristics of ships Conditions at which the standards apply In order to evaluate the performance of a ship, manoeuvring trials should be conducted to both port and starboard and at conditions specified below: .1 deep, unrestricted water (> 4xmean draft) .2 calm environment (Wind< 5Bft / Sea< 4) .3 full load (summer load line draught), even keel condition .4 steady approach at the test speed(min90% full).
Ch7. Sea trials / Manoeuvring characteristics of ships • Manoeuvring performance has traditionally received little attention during the design stages of a commercial ship. • Consequently some ships have been built with very poor manoeuvring qualities, resulting in marine casualties / pollution. • Designers have relied on shiphandling abilities of human operators to compensate for deficiencies in inherent manoeuvring qualities of the hull. • The implementation of manoeuvring standards will ensure that ships are designed to a uniform standard, so that an undue burden is not imposed on shiphandlers in trying to compensate for deficiencies in inherent ship manoeuvrability. (Extract of IMO MSC/Circ1053)
Ch7. Sea trials / Preliminary Forces and motions in manoeuvrability Definition of the Pivot Point: • the point around which the ship rotates • The centre of the hydrodynamic forces acting on the ship’s hull Position of the Pivot Point: • Depends on the shape of the hull • With no forward speed: pivot point at midship • At speed: pivot point shifts forward
Ch7. Sea trials /Preliminary The Pivot Point at forward speed
Ch7. Sea trials / Manoeuvring characteristics of ships 1. Course keeping ability and dynamic stability • Dynamically stable ship moves along a new straight course without using rudder after a small disturbance • Dynamically unstable ship performs turning circle with rudder amidship • More difficult to handle dynamically unstable ships • Infos on course keeping and dynamic stability: obtained from « Initial turning test »
Ch7. Sea trials / Manoeuvring characteristics of ships Dynamic stability:dynamically stable ships maintain A straight course with zero rudder Dynamically unstable ships can only maintain a straight course by repeated use of rudder control
Ch7. Sea trials / Manoeuvring characteristics of ships Factors determining the Directional stability of vessels • Increase with the depth of the water • Increase with the lenght of the ship • Increase with Trim by the stern • Decrease with big blockage factor • Decrease for large vessel (ratio L/B) • Decrease when cross sectional area fwd larger than cross sectional area after (pivot point moves forward)
Ro-Ro ships are directionally unstable They need more rudder to stop a swing than to start a swing
Ch7. Sea trials / Manoeuvring characteristics of ships Change of trim • Ship by the stern has a better course keeping ability • Ship by the head: • Slow to start a swing • Difficult to stop a swing • In shallow water, a ship gets trim bythe head and looses directional stability
TURNING CIRCLE Turning circle: measure of turning ability of vessel
TURNING CIRCLE To determine the turning ability - The measure of the ability of a ship using hard-over rudder - The result is a minimum « advance at 90° change of heading » and « tactical diameter » defined by the « transfer at 180° change of heading » - Tactical diameter is usually given as multiplacity of ship lenght • The advance should not exceed 4.5 ship lengths (L) • the tactical diameter should not exceed 5 lengths • Turning circle to be performed with 35°Rudder angle
Statendam Lenght:196m / beam:25m / 24300DWT / Steamship/ 2 propellers/ 19Knots
Advance: 426m Transfer: 99m Diameter: 263m Tact.Dia: 290m Advance: 426m Transfer: 94m Diameter: 258m Tact.Dia: 292m
Advance: the distance traveled in the direction of the original course by the midship point of a ship from the position at which the rudder order is given to the position at which the heading has changed 900 from the original course.
Tactical diameter : the distance traveled by the midship point of a ship from the position at which the rudder order is given to the position at which the heading has changed 1800 from the original course. It is measured in a direction perpendicular to the original heading of the ship.
TURNING CIRCLE Comments • Advance of the ship smaller than the distance ahead with an emergency stop manœuvre • Request sufficient searoom on the beam (tactical diameter) • Test are carried out at sea and not in shallow waters: parameters are bigger in shallow water because rudder effect decreases in shallow water due to the reduced waterflow • Parameters of the turning circle do not change for different speeds of the ship
TURNING CIRCLE • Drift angle and Pivot point • The pivot point (D) is at the intersection of the longitudinal • axis of the vessel with the radius of the turning circle • The drift angle at the pivot point is zero • The drift angle at the centre of gravity (G)
TURNING CIRCLE In shallow waters, the drift angle is smaller : the water resistance decreases and the turning circle is larger
Crablike motion of the ship: Water resistance reduces the speed and the diameter of turning circle
TURNING CIRCLE Forces acting on a ship when turning
TURNING CIRCLE The turning circle is affected by the effects of wind and current
Turning characteristics of full and slender ships
TURNING CIRCLE Comparison of turning characteristics of full and slender ships: • Two ships of the same lenght have nearly the same transfer • Tactical diameters almost the same • Radius of turning circle smaller for tanker • Drift angle much larger for tanker • Pivot point closer to the bow in tanker
TURNING CIRCLE Water resistance on starboard Beam during turning circle
ZIG-ZAG TEST (Kempf) • Yaw checking ability a measure of : • the response to counter-rudder (Overshoot angle and overshoot time) • Measure of the ability to initiate and check course changes Two tests are included: the 10°/10° and 20°/20° tests 10°/10° zig-zag test: rudder is turned alternately by 10° to either side following a heading deviation of 10° from original heading
ZIG-ZAG TEST (Kempf) 10°/10° Zig-Zag Test
ZIG-ZAG TEST/ Procedure • after a steady approach, rudder is put over to 10° to starboard (port) (first execute) • when heading has changed to 10° off original heading, rudder reversed to 10° to port (starboard) (second execute) • after the rudder has been turned to port/starboard, the ship continues turning in original direction with decreasing turning rate. • In response to rudder, ship should then turn to port/starboard. • When ship has reached a heading of 10° to port/starboard of the original course the rudder is again reversed to 10° to starboard/port (third execute). • The first overshoot angle is the additional heading deviation experienced in the zig-zag test following second execute
Recommendations of IMO The value of the first overshoot angle in the 10°/10° zig-zag test should not exceed: . 10° if L/V is less than 10 s; . 20° if L/V is 30 s or more; and . (5 + 1/2(L/V)) degrees if L/V is 10 s or more, but less than 30s where L and V are expressed in m and m/s, respectively. The value of the second overshoot angle in the 10°/10° zig-zag test should not exceed: . 25°, if L/V is less than 10 s; . 40°, if L/V is 30 s or more; and . (17.5 + 0.75(L/V))°, if L/V is 10 s or more, but less than 30 s.
ZIG-ZAG TEST • The 20°/20° zig-zag test is performed using the same procedure using 20° rudder angles and 20° change of heading, instead of 10° rudder angles and 10° change of heading, respectively. • The value of the first overshoot angle in the 20°/20° Zig-Zag test should not exceed 25° Recommendation of IMO MSC 137(76)
STOPPING TEST • The "crash-stop" or "crash-astern" manoeuvre is mainly a test of engine functioning and propeller reversal. The stopping distance is a function of the ratio of astern power to ship displacement. Procedure 1. ship brought to a steady course and speed 2. The recording of data starts. 3. The manoeuvre is started by giving a stop order. The full astern engine order is applied with rudder amidship. 4. Data recording stops and the manoeuvre is terminated when the ship is stopped dead
STOPPING TEST • Parameters: • track reach • head reach • lateral deviation • time to dead in water
STOPPING TEST Measure of the ability to stop while maintaining control • Full astern stopping test determines the track reach of a ship from the time an order for full astern is given until the ship stops in the water. • Track reach is the distance along the path described by the midship point of a ship measured from the position at which an order for full astern is given to the position at which the ship stops in the water • Track reach must not exceed 15 ship’s lenghts excepted for very large vessels: maximum 20 Ship’s L.
Comparison between different manœuvres for stopping a ship
ADDITIONAL TESTS FOR UNSTABLE SHIPS • Where standard manoeuvres indicate dynamic instability, alternative tests may be conducted to define the degree of instability : « Initial turning test » • Guidelines for alternative tests such as a « spiral test » or « pull-out manœuvre » are included in the Explanatory notes to the Standards for ship manoeuvrability, referred to in paragraph 6.1 above.∗ • ∗ Refer to MSC/Circ.1053 on Explanatory notes to the Standards for ship manoeuvrability
INITIAL TURNING TEST Initial Turning ability • Measure of change of the heading in response to a moderate helm • Expressed in : distance covered before course change of 10° when 10° of rudder is applied (also with 20° rudder angle) • Assessed by the « Initial Turning Test »: Test to be performed for unstable ships (IMO Recommandations)