2Ch7. 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 resultsFull scale trialsTest speed = at least 90% of full speed = 85% of full engine power
3Ch7. Sea trials / Manoeuvring characteristics of ships Imo Manoeuvring StandardsBy resolution A.751(18) in 1993 IMO adopted Manoeuvring StandardsThe standards apply to:All ships of 100m in lenght and overAll chemical tankers and gas carriersThey consist of:Turning circles to Port and starboardStopping TestZig-Zag Test
4Ch7. Sea trials / Manoeuvring characteristics of ships Conditions at which the standards applyIn 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).
5Ch7. 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)
6Ch7. Sea trials / Preliminary Forces and motions in manoeuvrabilityDefinition of the Pivot Point:the point around which the ship rotatesThe centre of the hydrodynamic forces acting on the ship’s hullPosition of the Pivot Point:Depends on the shape of the hullWith no forward speed: pivot point at midshipAt speed: pivot point shifts forward
7Ch7. Sea trials /Preliminary The Pivot Point at forward speed
8Ch7. Sea trials / Manoeuvring characteristics of ships 1. Course keeping ability and dynamic stabilityDynamically stable ship moves along a new straight course without using rudder after a small disturbanceDynamically unstable ship performs turning circle with rudder amidshipMore difficult to handle dynamically unstable shipsInfos on course keeping and dynamic stability: obtained from « Initial turning test »
9Ch7. Sea trials / Manoeuvring characteristics of ships Dynamic stability: dynamically stable ships maintainA straight course with zero rudderDynamically unstable ships can onlymaintain a straight course by repeateduse of rudder control
11Ch7. Sea trials / Manoeuvring characteristics of ships Factors determining the Directional stability of vesselsIncrease with the depth of the waterIncrease with the lenght of the shipIncrease with Trim by the sternDecrease with big blockage factorDecrease for large vessel (ratio L/B)Decrease when cross sectional area fwd larger than cross sectional area after (pivot point moves forward)
12Ro-Ro ships are directionally unstable They need more rudder to stop a swing than to start a swing
13Ch7. Sea trials / Manoeuvring characteristics of ships Change of trimShip by the stern has a better course keeping abilityShip by the head:Slow to start a swingDifficult to stop a swingIn shallow water, a ship gets trim by the head and looses directional stability
17TURNING CIRCLETo 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 lenghtThe advance should not exceed 4.5 ship lengths (L)the tactical diameter should not exceed 5 lengthsTurning circle to be performed with 35°Rudder angle
20Advance: 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.
21Tactical 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 fromthe original course.It is measured in a direction perpendicular to the original heading of the ship.
22TURNING CIRCLECommentsAdvance of the ship smaller than the distance ahead with an emergency stop manœuvreRequest 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 waterflowParameters of the turning circle do not change for different speeds of the ship
23Drift angle and Pivot point TURNING CIRCLEDrift angle and Pivot pointThe pivot point (D) is at the intersection of the longitudinalaxis of the vessel with the radius of the turning circleThe drift angle at the pivot point is zeroThe drift angle at the centre of gravity (G)
24TURNING CIRCLEIn shallow waters, the drift angle is smaller : the waterresistance decreases and the turning circle is larger
25Crablike motion of the ship: Water resistance reduces the speedand the diameter of turning circle
26Forces acting on a ship when turning TURNING CIRCLEForces acting on a ship when turning
28TURNING CIRCLEThe turning circle is affected by the effects of wind and current
29Turning characteristics of full and slender ships
30TURNING CIRCLEComparison of turning characteristics of full and slender ships:Two ships of the same lenght have nearly the same transferTactical diameters almost the sameRadius of turning circle smaller for tankerDrift angle much larger for tankerPivot point closer to the bow in tanker
31TURNING CIRCLEWater resistance on starboardBeam during turning circle
33ZIG-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 changesTwo tests are included: the 10°/10° and 20°/20° tests10°/10° zig-zag test: rudder is turned alternately by 10° to either side following a heading deviation of 10° from original heading
35ZIG-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
36Recommendations 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 30swhere 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. ( (L/V))°, if L/V is 10 s or more, but less than 30 s.
38ZIG-ZAG TESTThe 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)
43STOPPING TESTThe "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.Procedure1. ship brought to a steady course and speed2. 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
44STOPPING TESTParameters:track reachhead reachlateral deviationtime to dead in water
45STOPPING TESTMeasure of the ability to stop while maintaining controlFull 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 waterTrack reach must not exceed 15 ship’s lenghts excepted for very large vessels: maximum 20 Ship’s L.
47Comparison betweendifferent manœuvresfor stopping a ship
48ADDITIONAL 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
50Initial Turning ability INITIAL TURNING TESTInitial Turning abilityMeasure of change of the heading in response to a moderate helmExpressed 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)
51Initial Turning Test directional stability Ability to control yaw Measure of nonlineardirectional stabilityAbility to control yawmotion with small rudderanglesWith 10° rudder angle to port/starboard, the ship should not have travelled more than 2.5 lengths by the time the heading has changed 10° from original heading
52PULL-OUT TEST Additional test for ships with unsatisfactory manoeuvringstandardsMeasure of coursekeeping ability anddynamic stability ofa ship
53PULL-OUT TESTThe ship is first made to turn with a certain rate of turnThe rudder is returned to midship positionWith a stable ship: rate of turn decays to zeroUnstable ship: rate of turn reduces but residual rate of turn will remain
56SPIRAL TESTThe Standard Manoeuvres are used to evaluatecourse-keeping ability based on the overshootangles resulting from the 10°/10° zig-zag manoeuvre.The zig-zag manoeuvre was chosen for reasons ofsimplicity and expediency in conducting trials.However, where more detailed analysis of dynamicstability is required some form of spiral manœuvre(direct or reverse) should be conducted as anadditional measure.
58DIRECT SPIRAL TESTThe direct spiral is a turning circle manoeuvre in which various steady state yaw rate/rudder angle values are measured by making incremental rudder changes throughout a circling manoeuvre.In the case where dynamic instability is detected with other trials or is expected, a direct spiral test can provide more detailed information about the degree of instability.In cases where the ship is dynamically unstable it will appear that it is still turning steadily in the original direction although the rudder is now slightly deflected to the opposite side.
59DIRECT SPIRAL TESTsteady course and speedrecording of data startsrudder turned 15 degrees and held until yaw rate remains constant for one minuterudder angle is then decreased in 5 degree increments. At each increment the rudder is held fixed until a steady yaw rate is obtained, measured and then decreased againthis is repeated for different rudder angles starting from large angles to both port and starboardwhen a sufficient number of points is defined, data recording stops.
60REVERSE SPIRAL MANOEUVRE In the reverse spiral test the ship is steered to obtain a constant yaw rate, the mean rudder angle required to produce this yaw rate is measured.the yaw rate versus rudder angle plot is created.
63DIEUDONNE SPIRAL MANOEUVRE the vessel path follows a growing spiral, and then a contracting spiral in the opposite direction.Suppose that:the first 15° rudder deflection (Sb) causes the vessel to turn rightAt zero rudder, the yaw rate is still to the right: the vessel has gotten “stuck” here, and will require a negative rudder action to pull out of the turn.the rudder in this case has to be used excessively driving the vessel back and forth.We say that the vessel is unstable, and clearly a poor design.
65Comments to IMO Standards For deep water and service/design speed onlyGive no indication of the handling characteristics in wind, waves and currentDo not look at manoeuvres normally carried out by most merchant shipsFull astern stopping test results in extreme termal loads on the engineCriteria derived from databases heavily biased towards (old) tankers and bulk carriers
66Comments to IMO Standards From operational aspects additional requirements should be developed:Manoeuvrability in shallow waterLow speed manoeuvring capabilitiesMaximum tolerable wind forces in harbour manoeuvresLimited heel anglesSteering in wavesSteering with special devices