Radar Plotting & Arpa

Question 1

What is the OAW triangle in radar plotting?

Answer 1

The OAW triangle is a tool used in radar plotting to determine CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach).

Question 2

What does OA represent in the triangle?

Answer 2

OA represents the Relative motion line.

Question 3

What does WO represent?

Answer 3

WO represents the Way of Own ship.

Question 4

What does WA represent?

Answer 4

WA represents the Way of Another ship.

Question 5

How do you determine CPA and TCPA manually?

Answer 5

CPA and TCPA can be determined by analyzing the relative motion of vessels using the OAW triangle.

Question 6

What actions must you take if CPA is unacceptable?

Answer 6

If CPA is unacceptable, action is required under COLREGS, especially Rule 19 in restricted visibility.

Question 7

Why must you state ‘overtaking’ when plotting?

Answer 7

Stating ‘overtaking’ is essential for proper navigation and adherence to COLREGS.

Question 8

What is the significance of the relative motion line?

Answer 8

The relative motion line indicates the CPA and TCPA, which are critical for collision avoidance.

Question 9

What does Rule 19 say about course alterations to port?

Answer 9

Rule 19 advises against course alterations to port for a vessel forward of the beam unless overtaking.

Question 10

How do you use radar plotting to confirm risk of collision?

Answer 10

Radar plotting is used to assess CPA and TCPA, which helps confirm the risk of collision.

Question 11

When does Rule 19 apply?

Answer 11

Rule 19 applies when vessels are not in sight of one another.

Question 12

What are OOW duties under Rule 19?

Answer 12

OOW duties include maintaining safe speed, having engines ready, and using all available means for navigation.

Question 13

What must a power-driven vessel have ready under Rule 19?

Answer 13

A power-driven vessel must have engines ready for immediate maneuvering.

Question 14

What actions are taken if fog signal heard forward of beam?

Answer 14

If a fog signal is heard forward of the beam, the vessel must reduce to minimum speed and navigate with extreme caution.

Question 15

Why is there no stand-on vessel under Rule 19?

Answer 15

Under Rule 19, there is no stand-on vessel; both vessels must take avoiding action.

Question 16

What does ARPA stand for?

Answer 16

ARPA stands for Automatic Radar Plotting Aid.

Question 17

What information does ARPA provide for each target?

Answer 17

ARPA provides range, bearing, predicted CPA, TCPA, and calculated true course and speed.

Question 18

Why is input accuracy vital for ARPA?

Answer 18

Input accuracy is vital for ARPA to ensure reliable target data and effective collision avoidance.

Question 19

What errors can arise from inaccurate gyro or log inputs?

Answer 19

Inaccurate gyro or log inputs can lead to significant errors in target data and collision risk assessment.

Question 20

Why are reciprocal courses risky on ARPA?

Answer 20

Reciprocal courses are especially sensitive to errors, increasing the risk of collision.

Question 21

If two radars are fitted, how should they be used?

Answer 21

One radar should be used for collision avoidance (preferably ARPA) and the other for navigation.

Question 22

Which radar should be used for anti-collision work?

Answer 22

The radar used for anti-collision work should preferably be ARPA.

Question 23

How do you verify ARPA target accuracy?

Answer 23

ARPA target accuracy can be verified by cross-referencing with manual plotting and other navigational aids.

Question 24

How do you cross-check ARPA predictions?

Answer 24

Cross-check ARPA predictions by comparing them with manual calculations and visual observations.

Question 25

Why must digital ARPA readouts be treated with caution?

Answer 25

Digital ARPA readouts must be treated with caution due to potential inaccuracies and reliance on input data.

Question 26

What are the limitations of ARPA?

Answer 26

ARPA has limitations such as dependency on accurate inputs and cannot replace manual plotting entirely.

Question 27

Can ARPA replace manual plotting? Why/why not?

Answer 27

ARPA cannot fully replace manual plotting because it relies on accurate inputs and may not account for all navigational factors.

Question 28

What must you do if ARPA and manual plot give different results?

Answer 28

If ARPA and manual plot give different results, further investigation and verification of data are required.

Question 29

How would you explain CPA/TCPA to an examiner in an oral?

Answer 29

CPA is the Closest Point of Approach, and TCPA is the Time to Closest Point of Approach, both crucial for collision assessment.

Question 30

What are your immediate actions as OOW if ARPA shows CPA = 0.3 NM in 6 mins?

Answer 30

Immediate actions include assessing the situation, preparing to take avoiding action, and notifying the bridge team.