Radar
plotting tips, radar plotting made easy
Time
to C.P.A. = "A" to C.P.A. x Plotting Interval 

Distance
"O" to "A" 
"W"
to "O" = Own ships speed x Plotting interval 

60 
Course
Line = "W" though Centre of "A"

Targets
speed = Distance from "W" to "A"
x 60 

Minutes
from "O" to "A" 
Aspect
= The angle of the course your going and from the centre of the
screen to "A" (Draw a line from the centre of the plot
to "A" and measure the angle between this line and your
course) (red if you would see targets red sidelight and green
if you would see his green sidelight)
Tidal
problems using Radar


Drift
= Distance from "A" to "W" 


Set
= Direction from "A" to "W" 

Rate
of Tide = Distance from "A" to "W" x 60 

Minutes
from "O" to "A" 


Basic
Radar Plot 

Warning
!!!!!!!!!!!!!!!!!!! Watch to see if you are using Relative or
True Compass Bearings.
Using a relative bearing is how you would take a compass bearing
of another vessel. 
Yellow
Plot is the targets first plot 
Red
Plot is the targets last plot 








(1)
Find the course your going, and draw a line for your course
(2) Draw a line where the first plot is and mark the distance, call
this "O" (Drawing 1 above)
(3) Lay off 2nd bearing course and distance (Drawing 2 above)
(4) Lay off 3rd bearing course and distance, call this "A"
(Drawing 3 above)
(5) From "O" draw a line through "A" and carry
the line well past the centre line on the screen (Drawing 4 above)
(6) Using your course line, transfer this onto "O" and
pull the line down the sheet (Drawing 5 above)
(7) This line is where "W" goes
(8) To find the distance you would find "W" use this formula 

own
ships speed x plotting interval 



(9)
Measure this distance and make a mark, call this "W" (Drawing
6 above)
(10) From "W" draw a line through "A" (this
is the other vessels course and speed) (Drawing 7 above)
(11) From the centre of the screen  draw a line to meet the "O"
to "A" line so they meet at 90 degrees
(12) This is called C.P.A. (Drawing 8 above) 

Aspect 

This
is the angle between your vessel and the targets vessel and what
colour of sidelight you see
From the course line your steering draw a line onto plot "A",
the angle between the 2 lines is the aspect and if you saw a green
sidelight then it would be (E.g."Green 048 degrees) and if
a red sidelight then (E.g. Red 056 degrees) 

In
this case, the Aspect would be roughly "RED 015 degrees" 

Alteration
of Course 



(1)
Make Basic radar plot
(2) Find out the amount of degrees your going to alter course
(3) The "W" to "O" is the course your steering
just now, lets assume your going to alter 90 degrees to starboard,
using a protractor with the zero degrees of the protractor on the
"W" to "O" line  measure off 90 degrees to
starboard and make a mark
(4) From "W" draw a line towards the mark you just made
(5) With a set of compasses measure the original "W" to
"O" and draw an arc towards and past the new line
(6) Call this Mark "O one" (this is your new course) "W"
to "O one"
(7) From "O one" draw a line through and well past "A"
(8) Make a new C.P.A. by drawing a line from the centre of the screen
onto the new "O one" to "A" line so the line
joins at 90 degrees
(9) Measure C.P.A. line 

Change
of your vessels speed 



(1)
Make Basic radar plot
(2) Find your new speed
(3) On the "W" to "O" line if you reduce your
speed then this line will get shorter
(4) On the "W" to "O" line if you increase your
speed then this line will get bigger
(5) Use this formula to find the distance you need to measure back
from "O" to get "W" 


"W"
to "O" = Own ships speed x plotting interval 



(6)
From "W" measure this distance and call it "O one
"
(7) From "O one" draw a line through "A" and
go well past the centre line of your screen
(8) From the centre of the screen draw a line so it meets the new
"W one" to "A" line at 90 degrees
(9) This is your new C.P.A. 

Change
of course and speed 



(1)
Make Basic Radar Plot
(2) Find out your new speed
(3) Use this formula to find the distance you need to measure back
from "O" to get "W" 

"W"
to "O" = Own ships speed x plotting interval




(4)From
"W" measure this distance and call the new mark "O
one"
(5) Now find out the amount of degrees you have to alter course
(6) The "O one " to "W" is the course your
steering just now, lets assume your going to alter 90 degrees
to starboard, using a protractor with the zero degrees of the
protractor on the "O" to "W" line  measure
off 90 degrees to starboard and make a mark
(7) With a set of compasses measure "O one" to "W"
and draw an arc towards and past the new line
(8) Call this Mark "W one" (this is your new course)
"O one" to "W one"
(9) From "W one" draw a line through and well past "A"
(10) Make a new C.P.A. by drawing a line from the centre of the
screen onto the new "W one" to "A" line so
the line joins at 90 degrees
(11) Measure the new C.P.A.
(C)
Navigation Orals at it's Best 28/3/02 