Maneuvering Boards Review

• AGENDA:– Definition of Relative Motion– True Bearings vs. Relative Bearings– The Maneuvering Board– The Relative Plot– Application: Example of Collision

Avoidance (CPA)

Maneuvering Boards Review

True vs. Relative Motion

True vs Relative Motion: Understanding the difference between true and relative motion is essential in excelling in moboards.

– True Motion: the movement of an object with respect to the surface of the earth.

– Relative Motion: the apparent movement of an object with respect to another moving object.

Relative Movement

• Represented on maneuvering board– Consider yourself onboard ship monitoring

actions of other ships– Find actual course and speed required to

bring about desired change in relative position in maneuvering a ship at sea

True vs. Relative Bearings

• Line of Sight (LOS) = straight line between an observer and an object.

• True Bearings (T) = direction measured with respect to true of geographic north.

• Ship’s heading (°T)

• Relative Bearings (R) = direction measured with reference to the ship’s longitudinal axis.

Line of Sight

• Each line of sight has two directions which differ by 180 degrees

• Heading of either ship does not affect the LOS

True Bearings

• True Bearing - bearing measured clockwise from true north in degrees to line of sight

• heading of ship does not affect bearing

• reference line true north remains same regardless of movement

Ship’s course = 045T

135T

NN

EEWW

SS

TRUE BEARINGSTRUE BEARINGS

000T

090T

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Relative Bearings

• Relative Bearing - measured in degrees from ships heading (course) clockwise to the observed object

– Remember - course of reference ship will always affect relative bearing because it is reference line for relative bearing

Remember…

• Never plot relative bearings on a maneuvering board

• Must convert relative bearings to true bearings before plotting

NN

EE

SS

WW

090R

180R

RELATIVE BEARINGRELATIVE BEARING

270R

000R

090T

180 T

Ship’s course = 045T

135T

??

NN

EEWW

SS

RELATIVE RELATIVE

BEARINGSBEARINGS

?

Ship’s course = 045T

135T

090R

180R

NN

EEWW

SS

RELATIVE RELATIVE

BEARINGSBEARINGS

000R

• Converting relative bearing to true is accomplished by using the following formula:

Converting True and Relative Bearings

TB = SH + RB

• Example: Contact bears 270°R, ship is on course 330°T. What is the true bearing to the contact?

TB = SH + RB

TB = 330° + 270°

TB = 600° 240°T

The Maneuvering Board

• Purpose: a tool to plot the position of a ship relative to the position of another ship.

• Applications:– Collision Avoidance … CPA’s– Formation Steaming … Station Keeping– RDVU/Intercept … UNREP’s– Wind Problems … Desired wind

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aneu

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rd• Description:

– Ten concentric circles

– Bearing lines radiating from the center.

– Five scales provided.

– Nomogram provided.

The Relative Plot

• Relative Plot: Defined as a polar representation of the successive positions of one or more moving objects with respect to a reference position. This reference position will normally be your own vessel; the bearings and ranges are taken from your radar.

• The maneuvering board (relative plot).

– E - reference ship (placed in the center)

– M1, M2 - any maneuvering ship

– DRM - Direction of Relative Motion

– MRM - Measure of Relative Motion

– SRM - Speed of Relative Motion

– CPA - Closest Point of Approach

The Relative Plot

Closest Point of Approach (CPA)

• Made up of three parts– Range/Distance

– Bearing

– Time

Setting up a MoBoard

• First Determine Scale– Speed– Distance

D S

Setting up a MoBoard

• Second plot yourself in the center of the MoBoard

• Then plot your ship’s– Speed– Distance

– For Example you are on a course of 050T at a speed of 15 kts.

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Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of

16,000 yards.

1. Plot M1(points on the moboard the correspond to the bearings and ranges of the contact)- note, use a 2:1 scale.

M1

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er

Example: CPA

Determining CPA: At 0815 the same contact bears 300ºT, at 10,000

yards. Find the CPA (bearing, range, and time):

1. Plot M1 and M2 (points on the moboard the correspond to the bearings and ranges of the contact)- note, use a 2:1 scale.

M1M2

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Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of

16,000 yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

1. Plot M1 and M2 (points on the moboard the correspond to the bearings and ranges of the contact)- note, use a 2:1 scale.

2. Find DRM - 073ºT

M1M2

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Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of 16,000

yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

1. Plot M1 and M2 (points on the moboard the correspond to the bearings and ranges of the contact)- note, use a 2:1 scale.

2. Find DRM - 073ºT3. Find MRM - 7,500 yards

M1M2

MRM

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Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of

16,000 yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

4. Enter nomogram with 7,500 yards and 15 minutes (elapsed time between interval - 0800 to 0815) to find SRM - 15 knots

M1M2

MRM

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er

Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of 16,000

yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

4. Enter nomogram with 7,500 yards and 15 minutes (elapsed time between interval - 0800 to 0815) to find SRM - 15 knots

5. CPA bearing (Add or subtract 90 to/from DRM to determine CPA Bearing) - in this case subtract 90: 073ºT - 090ºT = 343ºT

M1M2

MRM

90

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343T

Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of

16,000 yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

6. CPA Range - measure from the center of the maneuvering board to CPA point to determine CPA range - 7,500 yards

M1M2

MRM

CPA Range

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er

Example: CPA

Determining CPA: At 0800 a contact bears 280ºT, at a range of

16,000 yards. At 0815 the same contact bears 300ºT, at 10,000 yards. Find CPA bearing, range, and time:

7. CPA Time - measure the distance between M2 & CPA point. This distance is 6,900 yards. Enter the nomogram with 6,900 yards and an SRM of 15 knots - 14 minutes (add this to time of M2 to find CPA time)

M1M2

MRM

CPA Range

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Questions?