Chapter 7 : Trials

Ch7. Sea trials / Manoeuvring characteristics of shipsIMO 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 trials Test speed = at least 90% of full speed = 85% of full engine power

Ch7. Sea trials / Manoeuvring characteristics of shipsImo 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

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).

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 / Manoeuvring characteristics of ships

Ch7. Sea trials / PreliminaryForces and motions in manoeuvrability

Definition of the Pivot Point: the point around which the ship rotatesThe centre of the hydrodynamic forces acting on the ships hull

Position of the Pivot Point:Depends on the shape of the hullWith no forward speed: pivot point at midshipAt speed: pivot point shifts forward

Ch7. Sea trials /PreliminaryThe Pivot Point at forward speed

Ch7. Sea trials / Manoeuvring characteristics of ships1. 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 shipsDynamic stability: dynamically stable ships maintainA straight course with zero rudderDynamically unstable ships can only maintain a straight course by repeated use of rudder control

Ch7. Sea trials / Manoeuvring characteristics of shipsFactors 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)

Ro-Ro ships are directionally unstableThey need more rudder to stop a swing than to start a swing

Ch7. Sea trials / Manoeuvring characteristics of shipsChange 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

3 STANDARD MANOEUVRES

TURNING CIRCLETurning circle: measure of turning ability of vessel

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 35Rudder angle

TURNING CIRCLE

Lenght:196m / beam:25m / 24300DWT / Steamship/ 2 propellers/ 19KnotsStatendam

Advance: 426mTransfer: 99mDiameter: 263mTact.Dia: 290mAdvance: 426mTransfer: 94mDiameter: 258mTact.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 fromthe original course.

It is measured in a direction perpendicular to the original heading of the ship.

CommentsAdvance of the ship smaller than the distance ahead with an emergency stop manuvreRequest 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

TURNING CIRCLE

Drift 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)TURNING CIRCLE

In shallow waters, the drift angle is smaller : the waterresistance decreases and the turning circle is largerTURNING CIRCLE

Crablike motion of the ship:Water resistance reduces the speedand the diameter of turning circle

Forces acting on a ship when turningTURNING CIRCLE

TURNING CIRCLE

The turning circle is affected by the effects of wind and currentTURNING CIRCLE

Turning characteristics of fulland slender ships

Comparison 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 tankerTURNING CIRCLE

TURNING CIRCLEWater resistance on starboardBeam during turning circle

ZIG-ZAG TEST

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 tests10/10 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

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 executeZIG-ZAG TEST/ Procedure

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.Recommendations of IMO

ZIG-ZAG TEST

ZIG-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)

20/20 Zig-Zag Test

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.

STOPPING TESTProcedure1. 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

STOPPING TESTParameters:

track reach

head reach

lateral deviation

time to dead in water

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 ships lenghts excepted for very large vessels: maximum 20 Ships L.STOPPING TEST

Comparison betweendifferent manuvresfor stopping a ship

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 manuvre 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

ADDITIONAL TESTS FOR UNSTABLE SHIPS

INITIAL TURNING TEST

INITIAL TURNING TESTInitial 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)

Initial Turning Test

Measure of nonlinear directional stability

Ability to control yaw motion 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

PULL-OUT TEST

Additional test forships with unsatisfactorymanoeuvring standards

Measure of coursekeeping ability anddynamic stability ofa ship

PULL-OUT TEST

The ship is first made to turn with a certain rate of turn

The rudder is returned to midship position

With a stable ship: rate of turn decays to zero

Unstable ship: rate of turn reduces but residual rate of turn will remain

SPIRAL TEST

SPIRAL TESTThe Standard Manoeuvres are used to evaluate course-keeping ability based on the overshoot angles resulting from the 10/10 zig-zag manoeuvre.

The zig-zag manoeuvre was chosen for reasons of simplicity and expediency in conducting trials.

However, where more detailed analysis of dynamic stability is required some form of spiral manuvre (direct or reverse) should be conducted as an additional measure.

SPIRAL TEST

DIRECT 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.

DIRECT SPIRAL TESTsteady course and speed recording 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.

REVERSE SPIRAL MANOEUVREIn 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.

RESULT OF SPIRAL TEST FOR STABLE SHIP

RESULT OF SPIRAL TEST FOR UNSTABLE SHIP

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.

DIEUDONNE SPIRAL MANOEUVRE

Comments 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

Comments 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 angles Steering in waves Steering with special devices