EXPLANATORY NOTES TO THE STANDARDS FOR
SHIP MANOEUVRABILITY
(adopted
on 5 December 2002)
1. The Maritime Safety Committee, at its seventy-sixth session
(2-13 December 2002), adopted resolution MSC.137(76) on Standards for Ship
Manoeuvrability. In adopting the standards, the Committee recognized the
necessity of developing appropriate explanatory notes for the uniform
interpretation, application and consistent evaluation of the manoeuvring
performance of ships.
2. To this end, the Maritime Safety Committee, at its
seventy-sixth session (2—13 December 2002), approved the Explanatory Notes to
the Standards for Ship Manoeuvrability, set out in the Annex to the present
circular, as prepared by the Sub-Committee on Ship Design and Equipment at its
forty-fifth session.
3. The Explanatory Notes are intended to provide Administrations
with specific guidance to assist in the uniform interpretation and application
of the Standards for Ship Manoeuvrability and to provide the information
necessary to assist those responsible for the design, construction, repair and
operation of ships to evaluate the manoeuvrability of such ships.
4. Member Governments are invited to:
.1 use the
Explanatory Notes when applying the Standards contained in resolution
MSC.137(76); and
.2 use the form
contained in appendix 6 of the annex to the present circular if submitting
manoeuvring data to the Organization for consideration, as appropriate.
5. This circular supersedes MSC/Circ.644.
ANNEX.
EXPLANATORY NOTES TO THE STANDARDS FOR SHIP MANOEUVRABILITY
The purpose of this section is to provide guidance for the
application of the Standards for Ship Manoeuvrability (resolution MSC.137(76))
along with the general philosophy and background for the Standards.
Manoeuvring performance has traditionally received little
attention during the design stages of a commercial ship. A primary reason has
been the lack of manoeuvring performance standards for the ship designer to
design to, and/or regulatory authorities to enforce. Consequently some ships
have been built with very poor manoeuvring qualities that have resulted in
marine casualties and pollution. Designers have relied on the shiphandling
abilities of human operators to compensate for any 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.
IMO has been concerned with the safety implications of ships with
poor manoeuvring characteristics since the meeting of the Sub-Committee on Ship
Design and Equipment (DE) in 1968. MSC/Circ.389 titled "Interim Guidelines
for Estimating Manoeuvring Performance in Ship Design", dated 10 January
1985, encourages the integration of manoeuvrability requirements into the ship
design process through the collection and systematic evaluation of ship
manoeuvring data. Subsequently, the Assembly, at its fifteenth session in
November 1987, adopted resolution A.601(15), entitled "Provision and
Display of Manoeuvring Information on board Ships". This process
culminated at the eighteenth Assembly in November 1993, where "Jnterim
Standards for Ship Manoeuvrability" were adopted by resolution A.751(18).
After the adoption of resolution A.751(18), the Maritime Safety
Committee, at its sixty-third session, approved MSC/Circ.644 titled
"Explanatory Notes to the Interim Standards for Ship
Manoeuvrability", dated 6 June 1994, to provide Administrations with
specific guidance so that adequate data could be collected by the Organization
on the manoeuvrability of ships with a view to amending the aforementioned
Interim Standards. This process culminated at the seventy-sixth session of the
Maritime Safety Committee in December 2002, where "Standards for Ship
Manoeuvrability" were adopted by resolution MSC.137(76).
The Standards were selected so that they are simple, practical and
do not require a significant increase in trials time or complexity over that in
current trials practice. The Standards are based on the premise that the
manoeuvrability of ships can be adequately judged from the results of typical ship
trials manoeuvres. It is intended that the manoeuvring performance of a ship be
designed to comply with the Standards during the design stage, and that the
actual manoeuvring characteristics of the ship be verified for compliance by
trials. Alternatively, the compliance with the Standards can be demonstrated
based on the results of full-scale trials, although the Administration may
require remedial action if the ship is found in substantial disagreement with
the Standards. Upon completion of ship trials, the shipbuilder should examine
the validity of the manoeuvrability prediction methods used during the design
stage.
1.2 MANOEUVRING CHARACTERISTICS
The "manoeuvring characteristics" addressed by the IMO
Standards for Ship Manoeuvrability are typical measures of performance quality
and handling ability that are of direct nautical interest. Each can be
reasonably well predicted at the design stage and measured or evaluated from
simple trialtype manoeuvres.
1.2.1 Manoeuvring characteristics: general
In the following discussion, the assumption is made that the ship
has normal actuators for the control of forward speed and heading (i.e., a
stern propeller and a stern rudder). However, most of the definitions and
conclusions also apply to ships with other types of control actuators.
In accepted terminology, questions concerning the manoeuvrability
of a ship include the stability of steady-state motion with "fixed
controls" as well as the time-dependent responses that result from the
control actions used to maintain or modify steady motion, make the ship follow
a prescribed path or initiate an emergency manoeuvre, etc. Some of these
actions are considered to be especially characteristic of ship manoeuvring
performance and therefore should be required to meet a certain minimum
standard. A ship operator may choose to ask for a higher standard in some
respect, in which case it should be remembered that some requirements may be
mutually incompatible within conventional designs. For similar reasons the
formulation of the IMO Standards for Ship Manoeuvrability has involved certain
compromises.
1.2.2 Manoeuvring characteristics: some
fundamentals
At a given engine output and rudder angle d, the ship may
take up a certain steady motion. In general, this will be a turning motion with
constant yaw rate y, speed V and drift angle β(3 (bow-in). The radius of
the turn is then defined by the following relationship, expressed in consistent
units:
R = V/y
This particular ship-rudder angle configuration is said to be
"dynamically stable in a turn of radius R". Thus, a straight course
may be viewed as part of a very wide circle with an infinite radius,
corresponding to zero yaw rate.
Most ships, perhaps, are "dynamically stable on a straight
course" (usually referred to as simply "dynamically stable")
with the rudder in a neutral position close to midship. In the case of a single
screw ship with a right-handed propeller, this neutral helm is typically of the
order d0 = —1° (i.e., 1° to starboard). Other ships which are dynamically
unstable, however, can only maintain a straight course by repeated use of
rudder control. While some instability is fully acceptable, large instabilities
should be avoided by suitable design of ship proportions and stern shape.
The motion of the ship is governed mainly by the propeller thrust
and the hydrodynamic and mass forces acting on the hull. During a manoeuvre,
the side force due to the rudder is often small compared to the other lateral
forces. However, the introduced controlling moment is mostly sufficient to
balance or overcome the resultant moment of these other forces. In a steady
turn there is complete balance between all the forces and moments acting on the
hull. Some of these forces seeming to "stabilize" and others to
"destabilize" the motion. Thus the damping moment due to yaw, which
always resists the turning, is stabilizing and the moment associated with the
side force due to sway is destabilizing. Any small disturbance of the
equilibrium attitude in the steady turn causes a change of the force and moment
balance. If the ship is dynamically stable in the turn (or on a straight
course) the net effect of this change will strive to restore the original
turning (or straight) motion.
The general analytical criterion for dynamic stability may be
formulated and evaluated with the appropriate coefficients of the mathematical
model that describes the ship's motion. The criterion for dynamic stability on
a straight course includes only four "linear stability derivatives"
which, together with the centre-of-gravity position, may be used to express the
"dynamic stability lever". This lever denotes the longitudinal
distance from the centre-of-pressure of the side force due to pure sway (or
sideslip) to the position of the resultant side force due to pure turning,
including the mass force, for small deviations from the straight-line motion.
If this distance is positive (in the direction of positive x, i.e. towards the
bow) the ship is stable. Obviously "captive tests" with a ship model
in oblique towing and under the rotating arm will furnish results of immediate
interest.
It is understood that a change of trim will have a marked effect
mainly on the location of the centre-of-pressure of the side force resulting
from sway. This is easily seen that a ship with a stern trim, a common
situation in ballast trial condition, is likely to be much more stable than it
would be on an even draught.
Figure 1 gives an example of the equilibrium yaw-rate/helm
relation for a ship which is inherently dynamically unstable on a straight
course. The yaw rate is shown in the non-dimensional form for turn path
curvature discussed above. This diagram is often referred to as "the
spiral loop curve" because it may be obtained from spiral tests with a
ship or model. The dotted part of the curve can only be obtained from some kind
of reverse spiral test. Wherever the slope is positive, which is indicated by a
tangent sloping down to the right in the diagram, the equilibrium balance is
unstable. A ship which is unstable on a straight course will be stable in a
turn despite the rudder being fixed in the midship or neutral position. The
curvature of this stable turn is called "the loop height" and may be
obtained from the pullout manoeuvre. Loop height, width and slope at the origin
may all be regarded as a measure of the instability.
If motion is not in an equilibrium turn, which is the general case
of motion, there are not only unbalanced damping forces but also hydrodynamic
forces associated with the added inertia in the flow of water around the hull.
Therefore, if the rudder is left in a position the ship will search for a new
stable equilibrium. If the rudder is shifted (put over "to the other
side") the direction of the ship on the equilibrium turning curve is
reversed and the original yaw tendency will be checked. By use of early
counter-rudder it is fully possible to control the ship on a straight course
with helm angles and yaw rates well within the loop.
Figure
1.
The equilibrium yaw rate/rudder angle relation
The course-keeping ability or "directional stability"
obviously depends on the performance of the closed loop system including not
only the ship and rudder but also the course error sensor and control system.
Therefore, the acceptable amount of inherent dynamic instability decreases as
ship speed increases, covering more ship lengths in a given period of time.
This results because a human helmsman will face a certain limit of conceptual
capacity and response time. This fact is reflected in the IMO Standards for
Ship Manoeuvrability where the criterion for the acceptable first overshoot in
a zig-zag test includes a dependence on the ratio L/V, a factor characterizing
the ship "time constant" and the time history of the process.
In terms of control engineering, the acceptable inherent
instability may be expressed by the "phase margin" available in the
open loop. If the rudder is oscillated with a given amplitude, ship heading
also oscillates at the same frequency with a certain amplitude. Due to the
inertia and damping in the ship dynamics and time delays in the steering
engine, this amplitude will be smaller with increasing frequency, meaning the
open loop response will lag further and further behind the rudder input. At
some certain frequency, the "unit gain" frequency, the response to
the counter-rudder is still large enough to check the heading swing before the
oscillation diverges (i.e., the phase lag of the response must then be less
than 180°). If a manual helmsman takes over the heading control, closing the
steering process loop, a further steering lag could result but, in fact, he
will be able to anticipate the swing of the ship and thus introduce a certain
"phase advance". Various studies suggest that this phase advance may
be of the order of 10" to 20°. At present there is no straightforward
method available for evaluating the phase margin from routine trial manoeuvres.
Obviously the course-keeping ability will depend not only upon the
counter-rudder timing but also on how effectively the rudder can produce a yaw
checking moment large enough to prevent excessive heading error amplitudes. The
magnitude of the overshoot angle alone is a poor measure for separating the
opposing effects of instability and rudder effectiveness, additional
characteristics should therefore be observed. So, for instance, "time to
reach second execute", which is a measure of "initial turning
ability", is shortened by both large instability and high rudder
effectiveness.
It follows from the above that a large dynamic instability will
favour a high "turning ability" whereas the large yaw damping, which
contributes to a stable ship, will normally be accompanied by a larger turning
radius. This is noted by the thin full-drawn curve for a stable ship included
in figure 1.
Hard-over turning ability is mainly an asset when manoeuvring at
slow speed in confined waters. However, a small advance and tactical diameter
will be of value in case emergency collision avoidance manoeuvres at normal
service speeds are required.
The "crash-stop" or "crash-astern" manoeuvre
is mainly a test of engine functioning and propeller reversal. The stopping
distance is essentially a function of the ratio of astern power to ship
displacement. A test for the stopping distance from full speed has been
included in the Standards in order to allow a comparison with hard-over turning
results in terms of initial speed drop and lateral deviations.
1.2.3 Manoeuvring characteristics: selected
quality measures
The IMO Standards for Ship Manoeuvrability identify significant
qualities for the evaluation of ship manoeuvring characteristics. Each has been
discussed above and is briefly defined below:
.1 Inherent dynamic
stability: A ship is dynamically stable on a straight course if it, after a
small disturbance, soon will settle on a new straight course without any
corrective rudder. The resultant deviation from the original heading will
depend on the degree of inherent stability and on the
magnitude and duration of the disturbance.
.2
Course-keeping ability: The course-keeping quality is a measure of the ability
of the steered ship to maintain a straight path in a predetermined course
direction without excessive oscillations of rudder or heading. In most cases,
reasonable course control is still possible where there exists an inherent
dynamic instability of limited magnitude.
.3 Initial
turning/course-changing ability: The initial turning ability is defined by the
change-of-heading response to a moderate helm, in terms of heading deviation
per unit distance sailed (the P number) or in terms of the distance covered
before realizing a certain heading deviation (such as the "time to second
execute" demonstrated when entering the zig-zag manoeuvre).
.4 Yaw checking
ability: The yaw checking ability of the ship is a measure of the response to
counter-rudder applied in a certain state of turning, such as the heading
overshoot reached before the yawing tendency has been cancelled by the
counter-rudder in a standard zig-zag manoeuvre.
.5 Turning
ability: Turning ability is the measure of the ability to turn the ship using
hard-over rudder. The result being a minimum "advance at 90° change of
heading" and "tactical diameter" defined by the "transfer
at 180° change of heading". Analysis of the final turning diameter is of
additional interest.
.6 Stopping
ability: Stopping ability is measured by the "track reach" and
"time to dead in water" realized in a stop engine-full astern
manoeuvre performed after a steady approach at full test speed. Lateral
deviations are also of interest, but they are very sensitive to initial
conditions and wind disturbances.
1.3 TESTS REQUIRED BY THE STANDARDS
A turning circle manoeuvre is to be performed to both starboard
and port with 35° rudder angle or the maximum design rudder angle permissible
at the test speed. The rudder angle is executed following a steady approach
with zero yaw rate. The essential information to be obtained from this
manoeuvre is tactical diameter, advance, and transfer (see figure 2).
A zig-zag test should be initiated to both starboard and port and
begins by applying a specified amount of rudder angle to an initially straight
approach ("first execute"). The rudder angle is then alternately
shifted to either side after a specified deviation from the ship's original
heading is reached ("second execute" and following) (see figure 3).
Two kinds of zig-zag tests are included in the Standards, the
10°/100 and 200/200 zig-zag tests. The 10°/100 zig-zag test uses rudder angles of 10° to
either side following a heading deviation of 10° from the original course. The
20°/20° zig-zag test uses 20° rudder angles coupled with a 20° change of
heading from the original course. The essential information to be obtained from
these tests is the overshoot angles, initial turning time to second execute and
the time to check yaw.
A full astern stopping test is used to determine the track reach
of a ship from the time an order for full astern is given until the ship is
stopped dead in the water (see figure 4).
Figure
2.
Definitions used on turning circle test