Q. What are the various characteristics? Where will you find the relevant criteria?
The following criteria may be considered as defining the maneuverability of ship:
- Inherent dynamic stability
- Course-keeping ability
- Initial turning/course-changing ability
- Yaw checking ability
- Turning ability
- Stopping ability
The IMO resolution MSC.137(76) “Standards for Ship Maneuverability” identify these characteristics and provide criteria to regulate them.
Q. What is Inherent dynamic stability? How is it different from course-keeping ability?
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 action. Though, the new course followed is not the original course, the resultant deviation from the original heading will depend on the degree of inherent stability and on the magnitude and duration of the disturbance.
The course-keeping quality is a measure of the ability of the steered ship to maintain a straight path in a original direction, approximately maintaining low fluctuations of rudder or heading.
Q. What is Initial turning/course-changing ability and Yaw checking? How are these evident in the zig zag test?
The initial turning ability is defined by the change-of-heading response to a given helm It is represented as deviation per unit distance covered. In a zig zag test its effect is see at second execute.
The yaw checking ability of the ship is a measure of the response to counter-rudder applied in a certain state of turning. In a zig zag test the heading overshoot reached before the yawing tendency is made nil by the counter-rudder would describe this.
Q. What is turning ability? What is the result of good turning ability?
Turning ability is the measure of the ability to turn the ship using hardover rudder. A good turning ability means a low “advance at 90° change of heading” and a small “tactical diameter”.
Q. What is stopping ability of a ship?
The stopping ability is measured by the “track reach” and “time to stop wrt water” witnessed in a stop engine-full astern manoeuvre, performed after a steady approach at full test speed.
Q. What is a 100/100 zig zag test?
Zig-zag test is the manoeuvre where a known amount of helm is applied alternately to either side when a known heading deviation from the original heading is reached. The 10°/10° zig-zag test is performed by turning the rudder alternately by 10° to either side following a heading deviation of 10° from the original heading in accordance with the following procedure:
- after a steady approach with zero yaw rate, the rudder is put over to 10° to starboard or port (first execute);
- when the heading has changed to 10° off the original heading, the rudder is reversed to 10° to port or starboard (second execute); and
- after the rudder has been turned to port/starboard, the ship will continue turning in the original direction with decreasing turning rate. In response to the rudder, the ship should then turn to port/starboard. When the 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).
Q. What is first overshoot angle?
The first overshoot angle is the additional heading deviation experienced in the zig-zag test following the second execute.
Q. What is a 200/200 zig zag test?
The 20°/20° zig-zag test is performed using the procedure given in paragraph above using 20° rudder angles and 20° change of heading, instead of 10° rudder angles and 10° change of heading, respectively;
Q. What is full astern stopping test?
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.
Q. What is track reach?
It 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, (subsequent to stopping engines while going at full ahead), to the position at which the ship stops in the water.
Q. What is Head Reach?
Head Reach is defined as a distance along the direction of the course at the moment when the “full astern” command was given. The distance is measured from the moment when the “full astern” command is given until the vessel is stopped dead in the water.
Q. What is the criteria for turning ability?
For the turning ability to be satisfactory, the advance should not exceed 4.5 ship lengths (L) and the tactical diameter should not exceed 5 ship lengths in the turning circle maneuver;
Q. What is the satisfactory ‘initial turning ability’?
The application of 10° rudder angle to port/starboard, the ship should not have travelled more than 2.5 ship lengths by the time the heading has changed by 10° from the original heading;
Q. What are the criteria of ‘yaw-checking and course-keeping abilities’?
The following are the criteria:
1. 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 + ]/2(L/V)) degrees if L/V is 10 s or more, but less than 30 s,
where L and V are expressed in m and m/s, respectively;
2. 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; and
3. the value of the first overshoot angle in the 20°/20° zig-zag test should not exceed 25°.
Q. What are the criteria for stopping ability?
The track reach in the full astern stopping test should not exceed:
15 ships lengths, if (Δ/MCR) VmcrFn2 is less than 1; or
(5 + 10 (Δ/MCR) Vmcr Fn2, if (Δ/MCR) VmcrFn2 is 1 or more, but not to exceed 20 ship lengths.
Q. What is MCR?
MCR is the maximum continuous rating for propulsion power of the ship, expressed in horsepower. Vmcr is the ship’s speed at MCR. Fn is Froude number at the test speed in the trial.
Q. What is Froude number?
Froude number (Fr) is a dimensionless number defined as the ratio of the flow inertia to the external field. The external field in many applications, is simply due to gravity. It is named after Willium Froude. The Froude number is based on the speed–length ratio which he defined as: where U is the local flow velocity, g is the local external field, and L is a characteristic length.
Q. What is rudder cycling?
Rudder Cycling works on the principle of diverting the fore and aft thrust sideways but not really taking the vessel sideways. Alternately, the similar maneuver is done on other side. Rudder Cycling is found extremely effective in reducing the vessel’s speed in a given period and takes the vessel through minimum distance in original direction. In the Rudder Cycling the heading of the vessel does not changes much. A master must be well versed with this procedure as an effective tool to prevent the vessel from running in to danger.
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