Chapter 7 : Trials

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Chapter 7 Trials
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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

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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

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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 (gt 4xmean draft)
• .2 calm environment (Windlt 5Bft / Sealt 4)
• .3 full load (summer load line draught), even
  keel condition
• .4 steady approach at the test speed(min90
  full).

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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)

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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 ships 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

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Ch7. Sea trials /Preliminary
The Pivot Point at forward speed
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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 

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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
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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)

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Ro-Ro ships are directionally unstable
They need more rudder to stop a swing than to
start a swing
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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 by the head
  and looses directional stability

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3 STANDARD MANOEUVRES
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TURNING CIRCLE
Turning circle measure of turning ability of
vessel
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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 35Rudder
  angle

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Statendam
Lenght196m / beam25m / 24300DWT / Steamship/ 2
propellers/ 19Knots
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Advance 426m Transfer 99m Diameter
263m Tact.Dia 290m
Advance 426m Transfer 94m Diameter
258m Tact.Dia 292m
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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.
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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.
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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

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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)

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TURNING CIRCLE
In shallow waters, the drift angle is smaller
the water resistance decreases and the turning
circle is larger
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Crablike motion of the ship Water resistance
reduces the speed and the diameter of turning
circle
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TURNING CIRCLE
Forces acting on a ship when turning
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TURNING CIRCLE
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TURNING CIRCLE

• The turning circle is affected by the effects of
  wind and current

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Turning characteristics of full and slender ships
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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

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TURNING CIRCLE
Water resistance on starboard Beam during turning
circle
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ZIG-ZAG TEST
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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
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ZIG-ZAG TEST (Kempf)
10/10 Zig-Zag Test
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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

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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.
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ZIG-ZAG TEST
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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)

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20/20 Zig-Zag Test
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STOP
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STOPPING TEST
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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
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STOPPING TEST

• Parameters
• track reach
• head reach
• lateral deviation
• time to dead in water

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

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Comparison between different manœuvres for
stopping a ship
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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

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INITIAL TURNING TEST
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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)

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• Initial Turning Test
• Measure of nonlinear
• directional stability
• Ability to control yaw
• motion with small rudder
• angles

With 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
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PULL-OUT TEST
Additional test for ships with
unsatisfactory manoeuvring standards Measure
of course keeping ability and dynamic stability
of a ship
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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

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SPIRAL TEST
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SPIRAL TEST

• The 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
  manœuvre
• (direct or reverse) should be conducted as an
• additional measure.

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SPIRAL TEST
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DIRECT SPIRAL TEST

• The 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.

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DIRECT SPIRAL TEST

• steady course and speed
• recording of data starts
• rudder turned 15 degrees and held until yaw rate
  remains constant for one minute
• rudder 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 again
• this is repeated for different rudder angles
  starting from large angles to both port and
  starboard
• when a sufficient number of points is defined,
  data recording stops.

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REVERSE 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.

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RESULT OF SPIRAL TEST FOR STABLE SHIP
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RESULT OF SPIRAL TEST FOR UNSTABLE SHIP
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DIEUDONNE 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 right
• At 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.

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Comments to IMO Standards

• For deep water and service/design speed only
• Give no indication of the handling
  characteristics in wind, waves and current
• Do not look at manoeuvres normally carried out by
  most merchant ships
• Full astern stopping test results in extreme
  termal loads on the engine
• Criteria derived from databases heavily biased
  towards (old) tankers and bulk carriers

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Comments to IMO Standards

• From operational aspects additional requirements
  should be developed
• Manoeuvrability in shallow water
• Low speed manoeuvring capabilities
• Maximum tolerable wind forces in harbour
  manoeuvres
• Limited heel angles
• Steering in waves
• Steering with special devices

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